Re: [libvirt] [PATCH 1/1] qemu: host NUMA hugepage policy without guest NUMA

[Sam Bobroff]

On Tue, Oct 25, 2016 at 02:13:07PM +0200, Martin Kletzander wrote:
> On Tue, Oct 25, 2016 at 01:10:23PM +1100, Sam Bobroff wrote:
> >On Tue, Oct 18, 2016 at 10:43:31PM +0200, Martin Kletzander wrote:
> >>On Mon, Oct 17, 2016 at 03:45:09PM +1100, Sam Bobroff wrote:
> >>>On Fri, Oct 14, 2016 at 10:19:42AM +0200, Martin Kletzander wrote:
> On Fri, Oct 14, 2016 at 11:52:22AM +1100, Sam Bobroff wrote:
> >I did look at the libnuma and cgroups approaches, but I was concerned 
> >they
> >wouldn't work in this case, because of the way QEMU allocates memory when
> >mem-prealloc is used: the memory is allocated in the main process, 
> >before the
> >CPU threads are created. (This is based only on a bit of hacking and 
> >debugging
> >in QEMU, but it does seem explain the behaviour I've seen so far.)
> >
> 
> But we use numactl before QEMU is exec()'d.
> >>>
> >>>Sorry, I jumped ahead a bit. I'll try to explain what I mean:
> >>>
> >>>I think the problem with using this method would be that the NUMA policy is
> >>>applied to all allocations by QEMU, not just ones related to the memory
> >>>backing. I'm not sure if that would cause a serious problem but it seems 
> >>>untidy,
> >>>and it doesn't happen in other situations (i.e. with separate memory 
> >>>backend
> >>>objects, QEMU sets up the policy specifically for each one and other
> >>>allocations aren't affected, AFAIK).  Presumably, if memory were very
> >>>restricted it could prevent the guest from starting.
> >>>
> >>
> >>Yes, it is, that's what  does if you don't have any
> >>other () specifics set.
> >>
> >I think QEMU could be altered to move the preallocations into the VCPU
> >threads but it didn't seem trivial and I suspected the QEMU community 
> >would
> >point out that there was already a way to do it using backend objects.  
> >Another
> >option would be to add a -host-nodes parameter to QEMU so that the 
> >policy can
> >be given without adding a memory backend object. (That seems like a more
> >reasonable change to QEMU.)
> >
> 
> I think upstream won't like that, mostly because there is already a
> way.  And that is using memory-backend object.  I think we could just
> use that and disable changing it live.  But upstream will probably want
> that to be configurable or something.
> >>>
> >>>Right, but isn't this already an issue in the cases where libvirt is 
> >>>already
> >>>using memory backend objects and NUMA policy? (Or does libvirt already 
> >>>disable
> >>>changing it live in those situations?)
> >>>
> >>
> >>It is.  I'm not trying to say libvirt is perfect.  There are bugs,
> >>e.g. like this one.  The problem is that we tried to do *everything*,
> >>but it's not currently possible.  I'm trying to explain how stuff works
> >>now.  It definitely needs some fixing, though.
> >
> >OK :-)
> >
> >Well, given our discussion, do you think it's worth a v2 of my original patch
> >or would it be better to drop it in favour of some broader change?
> >
> 
> Honestly, I thought about the approaches so much I'm now not sure I'll
> make a good decision.  RFC could do.  If I were to pick, I would go with
> a new setting that would control whether we want the binding to be
> changeable throughout the domain's lifetime or not so that we can make
> better decisions (and don't feel bad about the bad ones).

I feel the same way.

OK, I'll try an RFC patch with a lot of description.

I'm specifically trying to address the issue I originally raised, which isn't
quite the same thing as the changeability of the bindings but I'll keep that in
mind. I think your point about changing the bindings will apply in the
same way whenever QEMU's memory-backend objects are used with their
"host-nodes" attribute (since they are what causes QEMU to apply policy), so I
don't think I'm suggesting any significant change there.

If you want to add the new setting you mention above, I'd be happy to base my
patch of top of that work. ;-)

Cheers,
Sam.

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Re: [libvirt] [PATCH 1/1] qemu: host NUMA hugepage policy without guest NUMA

[Martin Kletzander]

On Tue, Oct 25, 2016 at 01:10:23PM +1100, Sam Bobroff wrote:

On Tue, Oct 18, 2016 at 10:43:31PM +0200, Martin Kletzander wrote:

On Mon, Oct 17, 2016 at 03:45:09PM +1100, Sam Bobroff wrote:
>On Fri, Oct 14, 2016 at 10:19:42AM +0200, Martin Kletzander wrote:
>>On Fri, Oct 14, 2016 at 11:52:22AM +1100, Sam Bobroff wrote:
>>>I did look at the libnuma and cgroups approaches, but I was concerned they
>>>wouldn't work in this case, because of the way QEMU allocates memory when
>>>mem-prealloc is used: the memory is allocated in the main process, before the
>>>CPU threads are created. (This is based only on a bit of hacking and 
debugging
>>>in QEMU, but it does seem explain the behaviour I've seen so far.)
>>>
>>
>>But we use numactl before QEMU is exec()'d.
>
>Sorry, I jumped ahead a bit. I'll try to explain what I mean:
>
>I think the problem with using this method would be that the NUMA policy is
>applied to all allocations by QEMU, not just ones related to the memory
>backing. I'm not sure if that would cause a serious problem but it seems 
untidy,
>and it doesn't happen in other situations (i.e. with separate memory backend
>objects, QEMU sets up the policy specifically for each one and other
>allocations aren't affected, AFAIK).  Presumably, if memory were very
>restricted it could prevent the guest from starting.
>

Yes, it is, that's what  does if you don't have any
other () specifics set.

>>>I think QEMU could be altered to move the preallocations into the VCPU
>>>threads but it didn't seem trivial and I suspected the QEMU community would
>>>point out that there was already a way to do it using backend objects.  
Another
>>>option would be to add a -host-nodes parameter to QEMU so that the policy can
>>>be given without adding a memory backend object. (That seems like a more
>>>reasonable change to QEMU.)
>>>
>>
>>I think upstream won't like that, mostly because there is already a
>>way.  And that is using memory-backend object.  I think we could just
>>use that and disable changing it live.  But upstream will probably want
>>that to be configurable or something.
>
>Right, but isn't this already an issue in the cases where libvirt is already
>using memory backend objects and NUMA policy? (Or does libvirt already disable
>changing it live in those situations?)
>

It is.  I'm not trying to say libvirt is perfect.  There are bugs,
e.g. like this one.  The problem is that we tried to do *everything*,
but it's not currently possible.  I'm trying to explain how stuff works
now.  It definitely needs some fixing, though.


OK :-)

Well, given our discussion, do you think it's worth a v2 of my original patch
or would it be better to drop it in favour of some broader change?



Honestly, I thought about the approaches so much I'm now not sure I'll
make a good decision.  RFC could do.  If I were to pick, I would go with
a new setting that would control whether we want the binding to be
changeable throughout the domain's lifetime or not so that we can make
better decisions (and don't feel bad about the bad ones).


Cheers,
Sam.



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Re: [libvirt] [PATCH 1/1] qemu: host NUMA hugepage policy without guest NUMA

[Sam Bobroff]

On Tue, Oct 18, 2016 at 10:43:31PM +0200, Martin Kletzander wrote:
> On Mon, Oct 17, 2016 at 03:45:09PM +1100, Sam Bobroff wrote:
> >On Fri, Oct 14, 2016 at 10:19:42AM +0200, Martin Kletzander wrote:
> >>On Fri, Oct 14, 2016 at 11:52:22AM +1100, Sam Bobroff wrote:
> >>>I did look at the libnuma and cgroups approaches, but I was concerned they
> >>>wouldn't work in this case, because of the way QEMU allocates memory when
> >>>mem-prealloc is used: the memory is allocated in the main process, before 
> >>>the
> >>>CPU threads are created. (This is based only on a bit of hacking and 
> >>>debugging
> >>>in QEMU, but it does seem explain the behaviour I've seen so far.)
> >>>
> >>
> >>But we use numactl before QEMU is exec()'d.
> >
> >Sorry, I jumped ahead a bit. I'll try to explain what I mean:
> >
> >I think the problem with using this method would be that the NUMA policy is
> >applied to all allocations by QEMU, not just ones related to the memory
> >backing. I'm not sure if that would cause a serious problem but it seems 
> >untidy,
> >and it doesn't happen in other situations (i.e. with separate memory backend
> >objects, QEMU sets up the policy specifically for each one and other
> >allocations aren't affected, AFAIK).  Presumably, if memory were very
> >restricted it could prevent the guest from starting.
> >
> 
> Yes, it is, that's what  does if you don't have any
> other () specifics set.
> 
> >>>I think QEMU could be altered to move the preallocations into the VCPU
> >>>threads but it didn't seem trivial and I suspected the QEMU community would
> >>>point out that there was already a way to do it using backend objects.  
> >>>Another
> >>>option would be to add a -host-nodes parameter to QEMU so that the policy 
> >>>can
> >>>be given without adding a memory backend object. (That seems like a more
> >>>reasonable change to QEMU.)
> >>>
> >>
> >>I think upstream won't like that, mostly because there is already a
> >>way.  And that is using memory-backend object.  I think we could just
> >>use that and disable changing it live.  But upstream will probably want
> >>that to be configurable or something.
> >
> >Right, but isn't this already an issue in the cases where libvirt is already
> >using memory backend objects and NUMA policy? (Or does libvirt already 
> >disable
> >changing it live in those situations?)
> >
> 
> It is.  I'm not trying to say libvirt is perfect.  There are bugs,
> e.g. like this one.  The problem is that we tried to do *everything*,
> but it's not currently possible.  I'm trying to explain how stuff works
> now.  It definitely needs some fixing, though.

OK :-)

Well, given our discussion, do you think it's worth a v2 of my original patch
or would it be better to drop it in favour of some broader change?

Cheers,
Sam.

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Re: [libvirt] [PATCH 1/1] qemu: host NUMA hugepage policy without guest NUMA

[Martin Kletzander]

On Mon, Oct 17, 2016 at 03:45:09PM +1100, Sam Bobroff wrote:

On Fri, Oct 14, 2016 at 10:19:42AM +0200, Martin Kletzander wrote:

On Fri, Oct 14, 2016 at 11:52:22AM +1100, Sam Bobroff wrote:
>I did look at the libnuma and cgroups approaches, but I was concerned they
>wouldn't work in this case, because of the way QEMU allocates memory when
>mem-prealloc is used: the memory is allocated in the main process, before the
>CPU threads are created. (This is based only on a bit of hacking and debugging
>in QEMU, but it does seem explain the behaviour I've seen so far.)
>

But we use numactl before QEMU is exec()'d.


Sorry, I jumped ahead a bit. I'll try to explain what I mean:

I think the problem with using this method would be that the NUMA policy is
applied to all allocations by QEMU, not just ones related to the memory
backing. I'm not sure if that would cause a serious problem but it seems untidy,
and it doesn't happen in other situations (i.e. with separate memory backend
objects, QEMU sets up the policy specifically for each one and other
allocations aren't affected, AFAIK).  Presumably, if memory were very
restricted it could prevent the guest from starting.



Yes, it is, that's what  does if you don't have any
other () specifics set.


>I think QEMU could be altered to move the preallocations into the VCPU
>threads but it didn't seem trivial and I suspected the QEMU community would
>point out that there was already a way to do it using backend objects.  Another
>option would be to add a -host-nodes parameter to QEMU so that the policy can
>be given without adding a memory backend object. (That seems like a more
>reasonable change to QEMU.)
>

I think upstream won't like that, mostly because there is already a
way.  And that is using memory-backend object.  I think we could just
use that and disable changing it live.  But upstream will probably want
that to be configurable or something.


Right, but isn't this already an issue in the cases where libvirt is already
using memory backend objects and NUMA policy? (Or does libvirt already disable
changing it live in those situations?)



It is.  I'm not trying to say libvirt is perfect.  There are bugs,
e.g. like this one.  The problem is that we tried to do *everything*,
but it's not currently possible.  I'm trying to explain how stuff works
now.  It definitely needs some fixing, though.


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Re: [libvirt] [PATCH 1/1] qemu: host NUMA hugepage policy without guest NUMA

[Sam Bobroff]

On Fri, Oct 14, 2016 at 10:19:42AM +0200, Martin Kletzander wrote:
> On Fri, Oct 14, 2016 at 11:52:22AM +1100, Sam Bobroff wrote:
> >On Thu, Oct 13, 2016 at 11:34:43AM +0200, Martin Kletzander wrote:
> >>On Thu, Oct 13, 2016 at 11:34:16AM +1100, Sam Bobroff wrote:
> >>>On Wed, Oct 12, 2016 at 10:27:50AM +0200, Martin Kletzander wrote:
> On Wed, Oct 12, 2016 at 03:04:53PM +1100, Sam Bobroff wrote:
> >At the moment, guests that are backed by hugepages in the host are
> >only able to use policy to control the placement of those hugepages
> >on a per-(guest-)CPU basis. Policy applied globally is ignored.
> >
> >Such guests would use  and
> >a  block with  but no  >.../> elements.
> >
> >This patch corrects this by, in this specific case, changing the QEMU
> >command line from "-mem-prealloc -mem-path=..." (which cannot
> >specify NUMA policy) to "-object memory-backend-file ..." (which can).
> >
> >Note: This is not visible to the guest and does not appear to create
> >a migration incompatibility.
> >
> 
> It could make sense, I haven't tried yet, though.  However, I still
> don't see the point in using memory-backend-file.  Is it that when you
> don't have cpuset cgroup the allocation doesn't work well?  Because it
> certainly does work for me.
> >>>
> >>>Thanks for taking a look at this :-)
> >>>
> >>>The point of using a memory-backend-file is that with it, the NUMA policy 
> >>>can
> >>>be specified to QEMU, but with -mem-path it can't. It seems to be a way to 
> >>>tell
> >>>QEMU to apply NUMA policy in the right place. It does seem odd to me to use
> >>>memory-backend-file without attaching the backend to a guest NUMA node, 
> >>>but it
> >>>seems to do the right thing in this case. (If there are guest NUMA nodes, 
> >>>or if
> >>>hugepages aren't being used, policy is correctly applied.)
> >>>
> >>>I'll describe my test case in detail, perhaps there's something I don't 
> >>>understand
> >>>happening.
> >>>
> >>>* I set up a machine with two (fake) NUMA nodes (0 and 1), with 2G of 
> >>>hugepages
> >>> on node 1, and none on node 0.
> >>>
> >>>* I create a 2G guest using virt-install:
> >>>
> >>>virt-install --name ppc --memory=2048 --disk ~/tmp/tmp.qcow2 --cdrom 
> >>>~/tmp/ubuntu-16.04-server-ppc64el.iso --wait 0 --virt-type qemu 
> >>>--memorybacking hugepages=on --graphics vnc --arch ppc64le
> >>>
> >>>* I "virsh destroy" and then "virsh edit" to add this block to the guest 
> >>>XML:
> >>>
> >>> 
> >>>
> >>> 
> >>>
> >>>* "virsh start", and the machine starts (I believe it should fail due to 
> >>>insufficient memory satasfying the policy).
> >>>* "numastat -p $(pidof qemu-system-ppc64)" shows something like this:
> >>>
> >>>Per-node process memory usage (in MBs) for PID 8048 (qemu-system-ppc)
> >>>  Node 0  Node 1   Total
> >>> --- --- ---
> >>>Huge 0.00 2048.00 2048.00
> >>>Heap 8.120.008.12
> >>>Stack0.030.000.03
> >>>Private 35.806.10   41.90
> >>>  --- --- ---
> >>>Total   43.95 2054.10 2098.05
> >>>
> >>>So it looks like it's allocated hugepages from node 1, isn't this 
> >>>violating the
> >>>policy I set via numatune?
> >>>
> >>
> >>Oh, now I get it.  We are doing our best to apply that policy to qemu
> >>even when we don't have this option.  However, using this works even
> >>better (which is probably* what we want).  And that's the reasoning
> >>behind this.
> >>
> >>* I'm saying probably because when I was adding numactl binding to be
> >>  used together with cgroups, I was told that we couldn't change the
> >>  binding afterwards and it's bad.  I feel like we could do something
> >>  with that and it would help us in the future, but there needs to be a
> >>  discussion, I guess.  Because I might be one of the few =)
> >>
> >>So to recapitulate that, there are three options how to affect the
> >>allocation of qemu's memory:
> >>
> >>1) numactl (libnuma): it works as expected, but cannot be changed later
> >>
> >>2) cgroups: so strict it has to be applied after qemu started, due to
> >>   that it doesn't work right, especially for stuff that gets all
> >>   pre-allocated (like hugepages).  it can be changed later, but it
> >>   won't always mean the memory will migrate, so upon change there is
> >>   no guarantee.  If it's unavailable, we fallback to (1) anyway
> >>
> >>3) memory-backing-file's host-nodes=: this works as expected, but
> >>   cannot be used with older QEMUs, cannot be changed later and in some
> >>   cases (not your particular one) it might screw up migration if it
> >>   wasn't used before.
> >>
> >>Selecting the best option from 

Re: [libvirt] [PATCH 1/1] qemu: host NUMA hugepage policy without guest NUMA

[Martin Kletzander]

On Fri, Oct 14, 2016 at 11:52:22AM +1100, Sam Bobroff wrote:

On Thu, Oct 13, 2016 at 11:34:43AM +0200, Martin Kletzander wrote:

On Thu, Oct 13, 2016 at 11:34:16AM +1100, Sam Bobroff wrote:
>On Wed, Oct 12, 2016 at 10:27:50AM +0200, Martin Kletzander wrote:
>>On Wed, Oct 12, 2016 at 03:04:53PM +1100, Sam Bobroff wrote:
>>>At the moment, guests that are backed by hugepages in the host are
>>>only able to use policy to control the placement of those hugepages
>>>on a per-(guest-)CPU basis. Policy applied globally is ignored.
>>>
>>>Such guests would use  and
>>>a  block with  but no >>.../> elements.
>>>
>>>This patch corrects this by, in this specific case, changing the QEMU
>>>command line from "-mem-prealloc -mem-path=..." (which cannot
>>>specify NUMA policy) to "-object memory-backend-file ..." (which can).
>>>
>>>Note: This is not visible to the guest and does not appear to create
>>>a migration incompatibility.
>>>
>>
>>It could make sense, I haven't tried yet, though.  However, I still
>>don't see the point in using memory-backend-file.  Is it that when you
>>don't have cpuset cgroup the allocation doesn't work well?  Because it
>>certainly does work for me.
>
>Thanks for taking a look at this :-)
>
>The point of using a memory-backend-file is that with it, the NUMA policy can
>be specified to QEMU, but with -mem-path it can't. It seems to be a way to tell
>QEMU to apply NUMA policy in the right place. It does seem odd to me to use
>memory-backend-file without attaching the backend to a guest NUMA node, but it
>seems to do the right thing in this case. (If there are guest NUMA nodes, or if
>hugepages aren't being used, policy is correctly applied.)
>
>I'll describe my test case in detail, perhaps there's something I don't 
understand
>happening.
>
>* I set up a machine with two (fake) NUMA nodes (0 and 1), with 2G of hugepages
> on node 1, and none on node 0.
>
>* I create a 2G guest using virt-install:
>
>virt-install --name ppc --memory=2048 --disk ~/tmp/tmp.qcow2 --cdrom 
~/tmp/ubuntu-16.04-server-ppc64el.iso --wait 0 --virt-type qemu --memorybacking 
hugepages=on --graphics vnc --arch ppc64le
>
>* I "virsh destroy" and then "virsh edit" to add this block to the guest XML:
>
> 
>
> 
>
>* "virsh start", and the machine starts (I believe it should fail due to 
insufficient memory satasfying the policy).
>* "numastat -p $(pidof qemu-system-ppc64)" shows something like this:
>
>Per-node process memory usage (in MBs) for PID 8048 (qemu-system-ppc)
>  Node 0  Node 1   Total
> --- --- ---
>Huge 0.00 2048.00 2048.00
>Heap 8.120.008.12
>Stack0.030.000.03
>Private 35.806.10   41.90
>  --- --- ---
>Total   43.95 2054.10 2098.05
>
>So it looks like it's allocated hugepages from node 1, isn't this violating the
>policy I set via numatune?
>

Oh, now I get it.  We are doing our best to apply that policy to qemu
even when we don't have this option.  However, using this works even
better (which is probably* what we want).  And that's the reasoning
behind this.

* I'm saying probably because when I was adding numactl binding to be
  used together with cgroups, I was told that we couldn't change the
  binding afterwards and it's bad.  I feel like we could do something
  with that and it would help us in the future, but there needs to be a
  discussion, I guess.  Because I might be one of the few =)

So to recapitulate that, there are three options how to affect the
allocation of qemu's memory:

1) numactl (libnuma): it works as expected, but cannot be changed later

2) cgroups: so strict it has to be applied after qemu started, due to
   that it doesn't work right, especially for stuff that gets all
   pre-allocated (like hugepages).  it can be changed later, but it
   won't always mean the memory will migrate, so upon change there is
   no guarantee.  If it's unavailable, we fallback to (1) anyway

3) memory-backing-file's host-nodes=: this works as expected, but
   cannot be used with older QEMUs, cannot be changed later and in some
   cases (not your particular one) it might screw up migration if it
   wasn't used before.

Selecting the best option from these, plus making the code work with
every possibility (erroring out when you want to change the memory node
and we had to use (1) for example) is a pain.  We should really think
about that and reorganize these things for the better of the future.
Otherwise we're going to get overwhelm ourselves.  Cc'ing Peter to get
his thoughts as well as he worked on some parts of this as well.

Martin


Thanks for the explanation, and I agree (I'm already a bit overwhelmed!) :-)

What do you mean by "changed later"? Do 

Re: [libvirt] [PATCH 1/1] qemu: host NUMA hugepage policy without guest NUMA

[Sam Bobroff]

On Thu, Oct 13, 2016 at 11:34:43AM +0200, Martin Kletzander wrote:
> On Thu, Oct 13, 2016 at 11:34:16AM +1100, Sam Bobroff wrote:
> >On Wed, Oct 12, 2016 at 10:27:50AM +0200, Martin Kletzander wrote:
> >>On Wed, Oct 12, 2016 at 03:04:53PM +1100, Sam Bobroff wrote:
> >>>At the moment, guests that are backed by hugepages in the host are
> >>>only able to use policy to control the placement of those hugepages
> >>>on a per-(guest-)CPU basis. Policy applied globally is ignored.
> >>>
> >>>Such guests would use  and
> >>>a  block with  but no  >>>.../> elements.
> >>>
> >>>This patch corrects this by, in this specific case, changing the QEMU
> >>>command line from "-mem-prealloc -mem-path=..." (which cannot
> >>>specify NUMA policy) to "-object memory-backend-file ..." (which can).
> >>>
> >>>Note: This is not visible to the guest and does not appear to create
> >>>a migration incompatibility.
> >>>
> >>
> >>It could make sense, I haven't tried yet, though.  However, I still
> >>don't see the point in using memory-backend-file.  Is it that when you
> >>don't have cpuset cgroup the allocation doesn't work well?  Because it
> >>certainly does work for me.
> >
> >Thanks for taking a look at this :-)
> >
> >The point of using a memory-backend-file is that with it, the NUMA policy can
> >be specified to QEMU, but with -mem-path it can't. It seems to be a way to 
> >tell
> >QEMU to apply NUMA policy in the right place. It does seem odd to me to use
> >memory-backend-file without attaching the backend to a guest NUMA node, but 
> >it
> >seems to do the right thing in this case. (If there are guest NUMA nodes, or 
> >if
> >hugepages aren't being used, policy is correctly applied.)
> >
> >I'll describe my test case in detail, perhaps there's something I don't 
> >understand
> >happening.
> >
> >* I set up a machine with two (fake) NUMA nodes (0 and 1), with 2G of 
> >hugepages
> > on node 1, and none on node 0.
> >
> >* I create a 2G guest using virt-install:
> >
> >virt-install --name ppc --memory=2048 --disk ~/tmp/tmp.qcow2 --cdrom 
> >~/tmp/ubuntu-16.04-server-ppc64el.iso --wait 0 --virt-type qemu 
> >--memorybacking hugepages=on --graphics vnc --arch ppc64le
> >
> >* I "virsh destroy" and then "virsh edit" to add this block to the guest XML:
> >
> > 
> >
> > 
> >
> >* "virsh start", and the machine starts (I believe it should fail due to 
> >insufficient memory satasfying the policy).
> >* "numastat -p $(pidof qemu-system-ppc64)" shows something like this:
> >
> >Per-node process memory usage (in MBs) for PID 8048 (qemu-system-ppc)
> >  Node 0  Node 1   Total
> > --- --- ---
> >Huge 0.00 2048.00 2048.00
> >Heap 8.120.008.12
> >Stack0.030.000.03
> >Private 35.806.10   41.90
> >  --- --- ---
> >Total   43.95 2054.10 2098.05
> >
> >So it looks like it's allocated hugepages from node 1, isn't this violating 
> >the
> >policy I set via numatune?
> >
> 
> Oh, now I get it.  We are doing our best to apply that policy to qemu
> even when we don't have this option.  However, using this works even
> better (which is probably* what we want).  And that's the reasoning
> behind this.
> 
> * I'm saying probably because when I was adding numactl binding to be
>   used together with cgroups, I was told that we couldn't change the
>   binding afterwards and it's bad.  I feel like we could do something
>   with that and it would help us in the future, but there needs to be a
>   discussion, I guess.  Because I might be one of the few =)
> 
> So to recapitulate that, there are three options how to affect the
> allocation of qemu's memory:
> 
> 1) numactl (libnuma): it works as expected, but cannot be changed later
> 
> 2) cgroups: so strict it has to be applied after qemu started, due to
>that it doesn't work right, especially for stuff that gets all
>pre-allocated (like hugepages).  it can be changed later, but it
>won't always mean the memory will migrate, so upon change there is
>no guarantee.  If it's unavailable, we fallback to (1) anyway
> 
> 3) memory-backing-file's host-nodes=: this works as expected, but
>cannot be used with older QEMUs, cannot be changed later and in some
>cases (not your particular one) it might screw up migration if it
>wasn't used before.
> 
> Selecting the best option from these, plus making the code work with
> every possibility (erroring out when you want to change the memory node
> and we had to use (1) for example) is a pain.  We should really think
> about that and reorganize these things for the better of the future.
> Otherwise we're going to get overwhelm ourselves.  Cc'ing Peter to get
> his thoughts as well as he 

Re: [libvirt] [PATCH 1/1] qemu: host NUMA hugepage policy without guest NUMA

[Martin Kletzander]

On Thu, Oct 13, 2016 at 11:34:16AM +1100, Sam Bobroff wrote:

On Wed, Oct 12, 2016 at 10:27:50AM +0200, Martin Kletzander wrote:

On Wed, Oct 12, 2016 at 03:04:53PM +1100, Sam Bobroff wrote:
>At the moment, guests that are backed by hugepages in the host are
>only able to use policy to control the placement of those hugepages
>on a per-(guest-)CPU basis. Policy applied globally is ignored.
>
>Such guests would use  and
>a  block with  but no .../> elements.
>
>This patch corrects this by, in this specific case, changing the QEMU
>command line from "-mem-prealloc -mem-path=..." (which cannot
>specify NUMA policy) to "-object memory-backend-file ..." (which can).
>
>Note: This is not visible to the guest and does not appear to create
>a migration incompatibility.
>

It could make sense, I haven't tried yet, though.  However, I still
don't see the point in using memory-backend-file.  Is it that when you
don't have cpuset cgroup the allocation doesn't work well?  Because it
certainly does work for me.


Thanks for taking a look at this :-)

The point of using a memory-backend-file is that with it, the NUMA policy can
be specified to QEMU, but with -mem-path it can't. It seems to be a way to tell
QEMU to apply NUMA policy in the right place. It does seem odd to me to use
memory-backend-file without attaching the backend to a guest NUMA node, but it
seems to do the right thing in this case. (If there are guest NUMA nodes, or if
hugepages aren't being used, policy is correctly applied.)

I'll describe my test case in detail, perhaps there's something I don't 
understand
happening.

* I set up a machine with two (fake) NUMA nodes (0 and 1), with 2G of hugepages
 on node 1, and none on node 0.

* I create a 2G guest using virt-install:

virt-install --name ppc --memory=2048 --disk ~/tmp/tmp.qcow2 --cdrom 
~/tmp/ubuntu-16.04-server-ppc64el.iso --wait 0 --virt-type qemu --memorybacking 
hugepages=on --graphics vnc --arch ppc64le

* I "virsh destroy" and then "virsh edit" to add this block to the guest XML:

 

 

* "virsh start", and the machine starts (I believe it should fail due to 
insufficient memory satasfying the policy).
* "numastat -p $(pidof qemu-system-ppc64)" shows something like this:

Per-node process memory usage (in MBs) for PID 8048 (qemu-system-ppc)
  Node 0  Node 1   Total
 --- --- ---
Huge 0.00 2048.00 2048.00
Heap 8.120.008.12
Stack0.030.000.03
Private 35.806.10   41.90
  --- --- ---
Total   43.95 2054.10 2098.05

So it looks like it's allocated hugepages from node 1, isn't this violating the
policy I set via numatune?



Oh, now I get it.  We are doing our best to apply that policy to qemu
even when we don't have this option.  However, using this works even
better (which is probably* what we want).  And that's the reasoning
behind this.

* I'm saying probably because when I was adding numactl binding to be
  used together with cgroups, I was told that we couldn't change the
  binding afterwards and it's bad.  I feel like we could do something
  with that and it would help us in the future, but there needs to be a
  discussion, I guess.  Because I might be one of the few =)

So to recapitulate that, there are three options how to affect the
allocation of qemu's memory:

1) numactl (libnuma): it works as expected, but cannot be changed later

2) cgroups: so strict it has to be applied after qemu started, due to
   that it doesn't work right, especially for stuff that gets all
   pre-allocated (like hugepages).  it can be changed later, but it
   won't always mean the memory will migrate, so upon change there is
   no guarantee.  If it's unavailable, we fallback to (1) anyway

3) memory-backing-file's host-nodes=: this works as expected, but
   cannot be used with older QEMUs, cannot be changed later and in some
   cases (not your particular one) it might screw up migration if it
   wasn't used before.

Selecting the best option from these, plus making the code work with
every possibility (erroring out when you want to change the memory node
and we had to use (1) for example) is a pain.  We should really think
about that and reorganize these things for the better of the future.
Otherwise we're going to get overwhelm ourselves.  Cc'ing Peter to get
his thoughts as well as he worked on some parts of this as well.

Martin


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Re: [libvirt] [PATCH 1/1] qemu: host NUMA hugepage policy without guest NUMA

[Sam Bobroff]

On Wed, Oct 12, 2016 at 09:48:07AM +0200, Peter Krempa wrote:
> On Wed, Oct 12, 2016 at 15:04:53 +1100, Sam Bobroff wrote:
> > At the moment, guests that are backed by hugepages in the host are
> > only able to use policy to control the placement of those hugepages
> > on a per-(guest-)CPU basis. Policy applied globally is ignored.
> > 
> > Such guests would use  and
> > a  block with  but no  > .../> elements.
> > 
> > This patch corrects this by, in this specific case, changing the QEMU
> > command line from "-mem-prealloc -mem-path=..." (which cannot
> > specify NUMA policy) to "-object memory-backend-file ..." (which can).
> > 
> > Note: This is not visible to the guest and does not appear to create
> > a migration incompatibility.
> > 
> > Signed-off-by: Sam Bobroff 
> > ---
> > There was some discussion leading up to this patch, here:
> > 
> > https://www.redhat.com/archives/libvir-list/2016-October/msg00033.html
> > 
> > During that discussion it seemed that fixing this issue would cause
> > migration incompatibility but after some careful testing, it appears
> > that it only does when a memory-backend object is attached to a guest
> > NUMA node and that is not the case here. If only one is created, and
> > used globally (not attached via mem=), the migration data does not
> > seem to be changed and so it seems reasonable to try something like
> > this patch.
> > 
> > This patch does work for my test cases but I don't claim a deep
> > understanding of the libvirt code so this is at least partly a RFC.
> > Comments welcome :-)
> > 
> > Cheers,
> > Sam.
> > 
> >  src/qemu/qemu_command.c | 55 
> > -
> >  src/qemu/qemu_command.h |  2 +-
> >  2 files changed, 46 insertions(+), 11 deletions(-)
> > 
> > diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
> > index 0804133..c28c8f2 100644
> > --- a/src/qemu/qemu_command.c
> > +++ b/src/qemu/qemu_command.c
> > @@ -3143,7 +3143,7 @@ qemuBuildMemoryBackendStr(unsigned long long size,
> >int guestNode,
> >virBitmapPtr userNodeset,
> >virBitmapPtr autoNodeset,
> > -  virDomainDefPtr def,
> > +  const virDomainDefPtr def,
> 
> This does not work as expected. Using 'const' with types that hide the
> pointer makes the pointer const and not the structure that the pointer
> points to.

Ah, of course you're right.

> >virQEMUCapsPtr qemuCaps,
> >virQEMUDriverConfigPtr cfg,
> >const char **backendType,
> > @@ -7129,12 +7129,18 @@ qemuBuildSmpCommandLine(virCommandPtr cmd,
> >  
> >  static int
> >  qemuBuildMemPathStr(virQEMUDriverConfigPtr cfg,
> > -const virDomainDef *def,
> > +const virDomainDefPtr def,
> 
> Same as above. This change is wrong.

OK.

> >  virQEMUCapsPtr qemuCaps,
> > -virCommandPtr cmd)
> > +virCommandPtr cmd,
> > +virBitmapPtr auto_nodeset)
> >  {
> >  const long system_page_size = virGetSystemPageSizeKB();
> >  char *mem_path = NULL;
> > +virBitmapPtr nodemask = NULL;
> > +const char *backendType = NULL;
> > +char *backendStr = NULL;
> > +virJSONValuePtr props;
> > +int rv = -1;
> >  
> >  /*
> >   *  No-op if hugepages were not requested.
> > @@ -7159,18 +7165,47 @@ qemuBuildMemPathStr(virQEMUDriverConfigPtr cfg,
> >  if (qemuGetHupageMemPath(cfg, def->mem.hugepages[0].size, _path) < 
> > 0)
> >  return -1;
> >  
> > -virCommandAddArgList(cmd, "-mem-prealloc", "-mem-path", mem_path, 
> > NULL);
> > +if (virDomainNumatuneMaybeGetNodeset(def->numa, auto_nodeset,
> > + , -1) < 0)
> > +return -1;
> > +if (nodemask && virQEMUCapsGet(qemuCaps, 
> > QEMU_CAPS_OBJECT_MEMORY_FILE)) {
> > +props = virJSONValueNewObject();
> 
> This is leaked, qemuBuildMemoryBackendStr allocates one itself.

Ah of course, sorry. I think I can just remove that line.

> > +if (qemuBuildMemoryBackendStr(virDomainDefGetMemoryInitial(def),
> > +  0, -1, NULL, auto_nodeset,
> > +  def, qemuCaps, cfg, ,
> > +  , false) < 0)
> > +goto cleanup;
> > +if (!(backendStr = 
> > virQEMUBuildObjectCommandlineFromJSON(backendType,
> > + "mem",
> > + props)))
> > +goto cleanup;
> 
> So you create a memory object that does not get used ...

Yes, that is correct.

> > +virCommandAddArgList(cmd, "-object", backendStr, NULL);
> 
> So does qemu use memory objects that are not used magically 

Re: [libvirt] [PATCH 1/1] qemu: host NUMA hugepage policy without guest NUMA

[Sam Bobroff]

On Wed, Oct 12, 2016 at 10:27:50AM +0200, Martin Kletzander wrote:
> On Wed, Oct 12, 2016 at 03:04:53PM +1100, Sam Bobroff wrote:
> >At the moment, guests that are backed by hugepages in the host are
> >only able to use policy to control the placement of those hugepages
> >on a per-(guest-)CPU basis. Policy applied globally is ignored.
> >
> >Such guests would use  and
> >a  block with  but no  >.../> elements.
> >
> >This patch corrects this by, in this specific case, changing the QEMU
> >command line from "-mem-prealloc -mem-path=..." (which cannot
> >specify NUMA policy) to "-object memory-backend-file ..." (which can).
> >
> >Note: This is not visible to the guest and does not appear to create
> >a migration incompatibility.
> >
> 
> It could make sense, I haven't tried yet, though.  However, I still
> don't see the point in using memory-backend-file.  Is it that when you
> don't have cpuset cgroup the allocation doesn't work well?  Because it
> certainly does work for me.

Thanks for taking a look at this :-)

The point of using a memory-backend-file is that with it, the NUMA policy can
be specified to QEMU, but with -mem-path it can't. It seems to be a way to tell
QEMU to apply NUMA policy in the right place. It does seem odd to me to use
memory-backend-file without attaching the backend to a guest NUMA node, but it
seems to do the right thing in this case. (If there are guest NUMA nodes, or if
hugepages aren't being used, policy is correctly applied.)

I'll describe my test case in detail, perhaps there's something I don't 
understand
happening.

* I set up a machine with two (fake) NUMA nodes (0 and 1), with 2G of hugepages
  on node 1, and none on node 0.

* I create a 2G guest using virt-install:

virt-install --name ppc --memory=2048 --disk ~/tmp/tmp.qcow2 --cdrom 
~/tmp/ubuntu-16.04-server-ppc64el.iso --wait 0 --virt-type qemu --memorybacking 
hugepages=on --graphics vnc --arch ppc64le

* I "virsh destroy" and then "virsh edit" to add this block to the guest XML:

  
 
  

* "virsh start", and the machine starts (I believe it should fail due to 
insufficient memory satasfying the policy).
* "numastat -p $(pidof qemu-system-ppc64)" shows something like this:

Per-node process memory usage (in MBs) for PID 8048 (qemu-system-ppc)
   Node 0  Node 1   Total
  --- --- ---
Huge 0.00 2048.00 2048.00
Heap 8.120.008.12
Stack0.030.000.03
Private 35.806.10   41.90
  --- --- ---
Total   43.95 2054.10 2098.05

So it looks like it's allocated hugepages from node 1, isn't this violating the
policy I set via numatune?

> >Signed-off-by: Sam Bobroff 
> >---
> >There was some discussion leading up to this patch, here:
> >
> >https://www.redhat.com/archives/libvir-list/2016-October/msg00033.html
> >
> >During that discussion it seemed that fixing this issue would cause
> >migration incompatibility but after some careful testing, it appears
> >that it only does when a memory-backend object is attached to a guest
> >NUMA node and that is not the case here. If only one is created, and
> >used globally (not attached via mem=), the migration data does not
> >seem to be changed and so it seems reasonable to try something like
> >this patch.
> >
> >This patch does work for my test cases but I don't claim a deep
> >understanding of the libvirt code so this is at least partly a RFC.
> >Comments welcome :-)
> >
> >Cheers,
> >Sam.
> >
> >src/qemu/qemu_command.c | 55 
> >-
> >src/qemu/qemu_command.h |  2 +-
> >2 files changed, 46 insertions(+), 11 deletions(-)
> >
> >diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
> >index 0804133..c28c8f2 100644
> >--- a/src/qemu/qemu_command.c
> >+++ b/src/qemu/qemu_command.c
> >@@ -3143,7 +3143,7 @@ qemuBuildMemoryBackendStr(unsigned long long size,
> >  int guestNode,
> >  virBitmapPtr userNodeset,
> >  virBitmapPtr autoNodeset,
> >-  virDomainDefPtr def,
> >+  const virDomainDefPtr def,
> >  virQEMUCapsPtr qemuCaps,
> >  virQEMUDriverConfigPtr cfg,
> >  const char **backendType,
> >@@ -7129,12 +7129,18 @@ qemuBuildSmpCommandLine(virCommandPtr cmd,
> >
> >static int
> >qemuBuildMemPathStr(virQEMUDriverConfigPtr cfg,
> >-const virDomainDef *def,
> >+const virDomainDefPtr def,
> 
> These two hunks have nothing to do with the change.  If you want to do
> them for some reason, explain the reasoning and put it into separate
> 

Re: [libvirt] [PATCH 1/1] qemu: host NUMA hugepage policy without guest NUMA

[Martin Kletzander]

On Wed, Oct 12, 2016 at 03:04:53PM +1100, Sam Bobroff wrote:

At the moment, guests that are backed by hugepages in the host are
only able to use policy to control the placement of those hugepages
on a per-(guest-)CPU basis. Policy applied globally is ignored.

Such guests would use  and
a  block with  but no  elements.

This patch corrects this by, in this specific case, changing the QEMU
command line from "-mem-prealloc -mem-path=..." (which cannot
specify NUMA policy) to "-object memory-backend-file ..." (which can).

Note: This is not visible to the guest and does not appear to create
a migration incompatibility.



It could make sense, I haven't tried yet, though.  However, I still
don't see the point in using memory-backend-file.  Is it that when you
don't have cpuset cgroup the allocation doesn't work well?  Because it
certainly does work for me.


Signed-off-by: Sam Bobroff 
---
There was some discussion leading up to this patch, here:

https://www.redhat.com/archives/libvir-list/2016-October/msg00033.html

During that discussion it seemed that fixing this issue would cause
migration incompatibility but after some careful testing, it appears
that it only does when a memory-backend object is attached to a guest
NUMA node and that is not the case here. If only one is created, and
used globally (not attached via mem=), the migration data does not
seem to be changed and so it seems reasonable to try something like
this patch.

This patch does work for my test cases but I don't claim a deep
understanding of the libvirt code so this is at least partly a RFC.
Comments welcome :-)

Cheers,
Sam.

src/qemu/qemu_command.c | 55 -
src/qemu/qemu_command.h |  2 +-
2 files changed, 46 insertions(+), 11 deletions(-)

diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
index 0804133..c28c8f2 100644
--- a/src/qemu/qemu_command.c
+++ b/src/qemu/qemu_command.c
@@ -3143,7 +3143,7 @@ qemuBuildMemoryBackendStr(unsigned long long size,
  int guestNode,
  virBitmapPtr userNodeset,
  virBitmapPtr autoNodeset,
-  virDomainDefPtr def,
+  const virDomainDefPtr def,
  virQEMUCapsPtr qemuCaps,
  virQEMUDriverConfigPtr cfg,
  const char **backendType,
@@ -7129,12 +7129,18 @@ qemuBuildSmpCommandLine(virCommandPtr cmd,

static int
qemuBuildMemPathStr(virQEMUDriverConfigPtr cfg,
-const virDomainDef *def,
+const virDomainDefPtr def,


These two hunks have nothing to do with the change.  If you want to do
them for some reason, explain the reasoning and put it into separate
patch, please.


virQEMUCapsPtr qemuCaps,
-virCommandPtr cmd)
+virCommandPtr cmd,
+virBitmapPtr auto_nodeset)
{
const long system_page_size = virGetSystemPageSizeKB();
char *mem_path = NULL;
+virBitmapPtr nodemask = NULL;
+const char *backendType = NULL;
+char *backendStr = NULL;
+virJSONValuePtr props;
+int rv = -1;

/*
 *  No-op if hugepages were not requested.
@@ -7159,18 +7165,47 @@ qemuBuildMemPathStr(virQEMUDriverConfigPtr cfg,
if (qemuGetHupageMemPath(cfg, def->mem.hugepages[0].size, _path) < 0)
return -1;

-virCommandAddArgList(cmd, "-mem-prealloc", "-mem-path", mem_path, NULL);
+if (virDomainNumatuneMaybeGetNodeset(def->numa, auto_nodeset,
+ , -1) < 0)
+return -1;
+if (nodemask && virQEMUCapsGet(qemuCaps, QEMU_CAPS_OBJECT_MEMORY_FILE)) {
+props = virJSONValueNewObject();
+if (qemuBuildMemoryBackendStr(virDomainDefGetMemoryInitial(def),
+  0, -1, NULL, auto_nodeset,
+  def, qemuCaps, cfg, ,
+  , false) < 0)
+goto cleanup;
+if (!(backendStr = virQEMUBuildObjectCommandlineFromJSON(backendType,
+ "mem",
+ props)))
+goto cleanup;
+virCommandAddArgList(cmd, "-object", backendStr, NULL);


So now a function that's called BuildMemPathStr does more than that.  It
is also called when the memory is added in a different way in which case
this function would just add even more memory which is clearly wrong.


+rv = 0;
+cleanup:
+VIR_FREE(backendStr);
+VIR_FREE(props);
+}
+else {
+if (nodemask || 1)


" || 1" ?  Leftover from debugging?


+virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+   _("Memory file backend objects are "
+ "unsupported by QEMU binary. Global NUMA "
+   

Re: [libvirt] [PATCH 1/1] qemu: host NUMA hugepage policy without guest NUMA

[Peter Krempa]

On Wed, Oct 12, 2016 at 15:04:53 +1100, Sam Bobroff wrote:
> At the moment, guests that are backed by hugepages in the host are
> only able to use policy to control the placement of those hugepages
> on a per-(guest-)CPU basis. Policy applied globally is ignored.
> 
> Such guests would use  and
> a  block with  but no  .../> elements.
> 
> This patch corrects this by, in this specific case, changing the QEMU
> command line from "-mem-prealloc -mem-path=..." (which cannot
> specify NUMA policy) to "-object memory-backend-file ..." (which can).
> 
> Note: This is not visible to the guest and does not appear to create
> a migration incompatibility.
> 
> Signed-off-by: Sam Bobroff 
> ---
> There was some discussion leading up to this patch, here:
> 
> https://www.redhat.com/archives/libvir-list/2016-October/msg00033.html
> 
> During that discussion it seemed that fixing this issue would cause
> migration incompatibility but after some careful testing, it appears
> that it only does when a memory-backend object is attached to a guest
> NUMA node and that is not the case here. If only one is created, and
> used globally (not attached via mem=), the migration data does not
> seem to be changed and so it seems reasonable to try something like
> this patch.
> 
> This patch does work for my test cases but I don't claim a deep
> understanding of the libvirt code so this is at least partly a RFC.
> Comments welcome :-)
> 
> Cheers,
> Sam.
> 
>  src/qemu/qemu_command.c | 55 
> -
>  src/qemu/qemu_command.h |  2 +-
>  2 files changed, 46 insertions(+), 11 deletions(-)
> 
> diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
> index 0804133..c28c8f2 100644
> --- a/src/qemu/qemu_command.c
> +++ b/src/qemu/qemu_command.c
> @@ -3143,7 +3143,7 @@ qemuBuildMemoryBackendStr(unsigned long long size,
>int guestNode,
>virBitmapPtr userNodeset,
>virBitmapPtr autoNodeset,
> -  virDomainDefPtr def,
> +  const virDomainDefPtr def,

This does not work as expected. Using 'const' with types that hide the
pointer makes the pointer const and not the structure that the pointer
points to.

>virQEMUCapsPtr qemuCaps,
>virQEMUDriverConfigPtr cfg,
>const char **backendType,
> @@ -7129,12 +7129,18 @@ qemuBuildSmpCommandLine(virCommandPtr cmd,
>  
>  static int
>  qemuBuildMemPathStr(virQEMUDriverConfigPtr cfg,
> -const virDomainDef *def,
> +const virDomainDefPtr def,

Same as above. This change is wrong.

>  virQEMUCapsPtr qemuCaps,
> -virCommandPtr cmd)
> +virCommandPtr cmd,
> +virBitmapPtr auto_nodeset)
>  {
>  const long system_page_size = virGetSystemPageSizeKB();
>  char *mem_path = NULL;
> +virBitmapPtr nodemask = NULL;
> +const char *backendType = NULL;
> +char *backendStr = NULL;
> +virJSONValuePtr props;
> +int rv = -1;
>  
>  /*
>   *  No-op if hugepages were not requested.
> @@ -7159,18 +7165,47 @@ qemuBuildMemPathStr(virQEMUDriverConfigPtr cfg,
>  if (qemuGetHupageMemPath(cfg, def->mem.hugepages[0].size, _path) < 0)
>  return -1;
>  
> -virCommandAddArgList(cmd, "-mem-prealloc", "-mem-path", mem_path, NULL);
> +if (virDomainNumatuneMaybeGetNodeset(def->numa, auto_nodeset,
> + , -1) < 0)
> +return -1;
> +if (nodemask && virQEMUCapsGet(qemuCaps, QEMU_CAPS_OBJECT_MEMORY_FILE)) {
> +props = virJSONValueNewObject();

This is leaked, qemuBuildMemoryBackendStr allocates one itself.

> +if (qemuBuildMemoryBackendStr(virDomainDefGetMemoryInitial(def),
> +  0, -1, NULL, auto_nodeset,
> +  def, qemuCaps, cfg, ,
> +  , false) < 0)
> +goto cleanup;
> +if (!(backendStr = virQEMUBuildObjectCommandlineFromJSON(backendType,
> + "mem",
> + props)))
> +goto cleanup;

So you create a memory object that does not get used ...

> +virCommandAddArgList(cmd, "-object", backendStr, NULL);

So does qemu use memory objects that are not used magically as it's
memory? That seems odd. Could you please elaborate on this part and how
it's supposed to work? Without a proper elaboration how this is supposed
to work with qemu this patch is not acceptable.

> +rv = 0;
> +cleanup:

This won't pass syntax check. Labels have to be spaced out.

> +VIR_FREE(backendStr);
> +VIR_FREE(props);

This is a JSON value object, you need to free it with the