Re: [PATCH 1/3] sched/topology: Allow archs to populate distance map
* Peter Zijlstra [2021-05-28 10:43:23]:
> On Mon, May 24, 2021 at 09:48:29PM +0530, Srikar Dronamraju wrote:
> > * Valentin Schneider [2021-05-24 15:16:09]:
>
> > > I suppose one way to avoid the hook would be to write some "fake" distance
> > > values into your distance_lookup_table[] for offline nodes using your
> > > distance_ref_point_depth thing, i.e. ensure an iteration of
> > > node_distance(a, b) covers all distance values [1]. You can then keep
> > > patch
> > > 3 around, and that should roughly be it.
> > >
> >
> > Yes, this would suffice but to me its not very clean.
> > static int found[distance_ref_point_depth];
> >
> > for_each_node(node){
> > int i, nd, distance = LOCAL_DISTANCE;
> > goto out;
> >
> > nd = node_distance(node, first_online_node)
> > for (i=0; i < distance_ref_point_depth; i++, distance *= 2) {
> > if (node_online) {
> > if (distance != nd)
> > continue;
> > found[i] ++;
> > break;
> > }
> > if (found[i])
> > continue;
> > distance_lookup_table[node][i] =
> > distance_lookup_table[first_online_node][i];
> > found[i] ++;
> > break;
> > }
> > }
> >
> > But do note: We are setting a precedent for node distance between two nodes
> > to change.
>
> Not really; or rather not more than already is the case AFAICT. Because
> currently your distance table will have *something* in it
> (LOCAL_DISTANCE afaict) for nodes that have never been online, which is
> what triggered the whole problem to begin with.
>
> Only after the node has come online for the first time, will it contain
> the right value.
>
> So both before and after this proposal the actual distance value changes
> after the first time a node goes online.
>
> Yes that's unfortunate, but I don't see a problem with pre-filling it
> with something useful in order to avoid aditional arch hooks.
>
>
Okay,
Will post a v2 with prefilling.
Thanks for the update.
--
Thanks and Regards
Srikar Dronamraju
Re: [PATCH 1/3] sched/topology: Allow archs to populate distance map
On Mon, May 24, 2021 at 09:48:29PM +0530, Srikar Dronamraju wrote:
> * Valentin Schneider [2021-05-24 15:16:09]:
> > I suppose one way to avoid the hook would be to write some "fake" distance
> > values into your distance_lookup_table[] for offline nodes using your
> > distance_ref_point_depth thing, i.e. ensure an iteration of
> > node_distance(a, b) covers all distance values [1]. You can then keep patch
> > 3 around, and that should roughly be it.
> >
>
> Yes, this would suffice but to me its not very clean.
> static int found[distance_ref_point_depth];
>
> for_each_node(node){
> int i, nd, distance = LOCAL_DISTANCE;
> goto out;
>
> nd = node_distance(node, first_online_node)
> for (i=0; i < distance_ref_point_depth; i++, distance *= 2) {
> if (node_online) {
> if (distance != nd)
> continue;
> found[i] ++;
> break;
> }
> if (found[i])
> continue;
> distance_lookup_table[node][i] =
> distance_lookup_table[first_online_node][i];
> found[i] ++;
> break;
> }
> }
>
> But do note: We are setting a precedent for node distance between two nodes
> to change.
Not really; or rather not more than already is the case AFAICT. Because
currently your distance table will have *something* in it
(LOCAL_DISTANCE afaict) for nodes that have never been online, which is
what triggered the whole problem to begin with.
Only after the node has come online for the first time, will it contain
the right value.
So both before and after this proposal the actual distance value changes
after the first time a node goes online.
Yes that's unfortunate, but I don't see a problem with pre-filling it
with something useful in order to avoid aditional arch hooks.
Re: [PATCH 1/3] sched/topology: Allow archs to populate distance map
* Valentin Schneider [2021-05-25 11:21:02]:
> On 24/05/21 21:48, Srikar Dronamraju wrote:
> > * Valentin Schneider [2021-05-24 15:16:09]:
> >> Ok so from your arch you can figure out the *size* of the set of unique
> >> distances, but not the individual node_distance(a, b)... That's quite
> >> unfortunate.
> >
> > Yes, thats true.
> >
> >>
> >> I suppose one way to avoid the hook would be to write some "fake" distance
> >> values into your distance_lookup_table[] for offline nodes using your
> >> distance_ref_point_depth thing, i.e. ensure an iteration of
> >> node_distance(a, b) covers all distance values [1]. You can then keep patch
> >> 3 around, and that should roughly be it.
> >>
> >
> > Yes, this would suffice but to me its not very clean.
> > static int found[distance_ref_point_depth];
> >
> > for_each_node(node){
> > int i, nd, distance = LOCAL_DISTANCE;
> > goto out;
> >
> > nd = node_distance(node, first_online_node)
> > for (i=0; i < distance_ref_point_depth; i++, distance *= 2) {
> > if (node_online) {
> > if (distance != nd)
> > continue;
> > found[i] ++;
> > break;
> > }
> > if (found[i])
> > continue;
> > distance_lookup_table[node][i] =
> > distance_lookup_table[first_online_node][i];
> > found[i] ++;
> > break;
> > }
> > }
> >
> > But do note: We are setting a precedent for node distance between two nodes
> > to change.
> >
>
> Indeed. AFAICT it's that or the unique-distance-values hook :/
Peter, Valentin, Michael,
Can you please let me know which approach you would want me to follow.
Or do let me know any other alternative solutions that you would want me to
try.
--
Thanks and Regards
Srikar Dronamraju
Re: [PATCH 1/3] sched/topology: Allow archs to populate distance map
* Valentin Schneider [2021-05-25 11:21:02]:
> On 24/05/21 21:48, Srikar Dronamraju wrote:
> > * Valentin Schneider [2021-05-24 15:16:09]:
> >> Ok so from your arch you can figure out the *size* of the set of unique
> >> distances, but not the individual node_distance(a, b)... That's quite
> >> unfortunate.
> >
> > Yes, thats true.
> >
> >>
> >> I suppose one way to avoid the hook would be to write some "fake" distance
> >> values into your distance_lookup_table[] for offline nodes using your
> >> distance_ref_point_depth thing, i.e. ensure an iteration of
> >> node_distance(a, b) covers all distance values [1]. You can then keep patch
> >> 3 around, and that should roughly be it.
> >>
> >
> > Yes, this would suffice but to me its not very clean.
> > static int found[distance_ref_point_depth];
> >
> > for_each_node(node){
> > int i, nd, distance = LOCAL_DISTANCE;
> > goto out;
> >
> > nd = node_distance(node, first_online_node)
> > for (i=0; i < distance_ref_point_depth; i++, distance *= 2) {
> > if (node_online) {
> > if (distance != nd)
> > continue;
> > found[i] ++;
> > break;
> > }
> > if (found[i])
> > continue;
> > distance_lookup_table[node][i] =
> > distance_lookup_table[first_online_node][i];
> > found[i] ++;
> > break;
> > }
> > }
> >
> > But do note: We are setting a precedent for node distance between two nodes
> > to change.
> >
>
> Indeed. AFAICT it's that or the unique-distance-values hook :/
Peter,
Please let me know which approach would you prefer.
I am open to try any other approach too.
In my humble opinion, unique-distance-values hook is more cleaner.
Do you still have any concerns with the unique-distance-values hook?
--
Thanks and Regards
Srikar Dronamraju
Re: [PATCH 1/3] sched/topology: Allow archs to populate distance map
On 24/05/21 21:48, Srikar Dronamraju wrote:
> * Valentin Schneider [2021-05-24 15:16:09]:
>> Ok so from your arch you can figure out the *size* of the set of unique
>> distances, but not the individual node_distance(a, b)... That's quite
>> unfortunate.
>
> Yes, thats true.
>
>>
>> I suppose one way to avoid the hook would be to write some "fake" distance
>> values into your distance_lookup_table[] for offline nodes using your
>> distance_ref_point_depth thing, i.e. ensure an iteration of
>> node_distance(a, b) covers all distance values [1]. You can then keep patch
>> 3 around, and that should roughly be it.
>>
>
> Yes, this would suffice but to me its not very clean.
> static int found[distance_ref_point_depth];
>
> for_each_node(node){
> int i, nd, distance = LOCAL_DISTANCE;
> goto out;
>
> nd = node_distance(node, first_online_node)
> for (i=0; i < distance_ref_point_depth; i++, distance *= 2) {
> if (node_online) {
> if (distance != nd)
> continue;
> found[i] ++;
> break;
> }
> if (found[i])
> continue;
> distance_lookup_table[node][i] =
> distance_lookup_table[first_online_node][i];
> found[i] ++;
> break;
> }
> }
>
> But do note: We are setting a precedent for node distance between two nodes
> to change.
>
Indeed. AFAICT it's that or the unique-distance-values hook :/
Re: [PATCH 1/3] sched/topology: Allow archs to populate distance map
* Valentin Schneider [2021-05-24 15:16:09]: > On 21/05/21 14:58, Srikar Dronamraju wrote: > > * Peter Zijlstra [2021-05-21 10:14:10]: > > > >> On Fri, May 21, 2021 at 08:08:02AM +0530, Srikar Dronamraju wrote: > >> > * Peter Zijlstra [2021-05-20 20:56:31]: > >> > > >> > > On Thu, May 20, 2021 at 09:14:25PM +0530, Srikar Dronamraju wrote: > >> > > > Currently scheduler populates the distance map by looking at distance > >> > > > of each node from all other nodes. This should work for most > >> > > > architectures and platforms. > >> > > > > >> > > > However there are some architectures like POWER that may not expose > >> > > > the distance of nodes that are not yet onlined because those > >> > > > resources > >> > > > are not yet allocated to the OS instance. Such architectures have > >> > > > other means to provide valid distance data for the current platform. > >> > > > > >> > > > For example distance info from numactl from a fully populated 8 node > >> > > > system at boot may look like this. > >> > > > > >> > > > node distances: > >> > > > node 0 1 2 3 4 5 6 7 > >> > > > 0: 10 20 40 40 40 40 40 40 > >> > > > 1: 20 10 40 40 40 40 40 40 > >> > > > 2: 40 40 10 20 40 40 40 40 > >> > > > 3: 40 40 20 10 40 40 40 40 > >> > > > 4: 40 40 40 40 10 20 40 40 > >> > > > 5: 40 40 40 40 20 10 40 40 > >> > > > 6: 40 40 40 40 40 40 10 20 > >> > > > 7: 40 40 40 40 40 40 20 10 > >> > > > > >> > > > However the same system when only two nodes are online at boot, then > >> > > > the > >> > > > numa topology will look like > >> > > > node distances: > >> > > > node 0 1 > >> > > > 0: 10 20 > >> > > > 1: 20 10 > >> > > > > >> > > > It may be implementation dependent on what node_distance(0,3) where > >> > > > node 0 is online and node 3 is offline. In POWER case, it returns > >> > > > LOCAL_DISTANCE(10). Here at boot the scheduler would assume that the > >> > > > max > >> > > > distance between nodes is 20. However that would not be true. > >> > > > > >> > > > When Nodes are onlined and CPUs from those nodes are hotplugged, > >> > > > the max node distance would be 40. > >> > > > > >> > > > To handle such scenarios, let scheduler allow architectures to > >> > > > populate > >> > > > the distance map. Architectures that like to populate the distance > >> > > > map > >> > > > can overload arch_populate_distance_map(). > >> > > > >> > > Why? Why can't your node_distance() DTRT? The arch interface is > >> > > nr_node_ids and node_distance(), I don't see why we need something new > >> > > and then replace one special use of it. > >> > > > >> > > By virtue of you being able to actually implement this new hook, you > >> > > supposedly can actually do node_distance() right too. > >> > > >> > Since for an offline node, arch interface code doesn't have the info. > >> > As far as I know/understand, in POWER, unless there is an active memory > >> > or > >> > CPU that's getting onlined, arch can't fetch the correct node distance. > >> > > >> > Taking the above example: node 3 is offline, then node_distance of (3,X) > >> > where X is anything other than 3, is not reliable. The moment node 3 is > >> > onlined, the node distance is reliable. > >> > > >> > This problem will not happen even on POWER if all the nodes have either > >> > memory or CPUs active at the time of boot. > >> > >> But then how can you implement this new hook? Going by the fact that > >> both nr_node_ids and distance_ref_points_depth are fixed, how many > >> possible __node_distance() configurations are there left? > >> > > > > distance_ref_point_depth is provided as a different property and is readily > > available at boot. The new api will use just use that. So based on the > > distance_ref_point_depth, we know all possible node distances for that > > platform. > > > > For an offline node, we don't have that specific nodes distance_lookup_table > > array entries. Each array would be of distance_ref_point_depth entries. > > Without the distance_lookup_table for an array populated, we will not be > > able to tell how far the node is with respect to other nodes. > > > > We can lookup the correct distance_lookup_table for a node based on memory > > or the CPUs attached to that node. Since in an offline node, both of them > > would not be around, the distance_lookup_table will have stale values. > > > > Ok so from your arch you can figure out the *size* of the set of unique > distances, but not the individual node_distance(a, b)... That's quite > unfortunate. Yes, thats true. > > I suppose one way to avoid the hook would be to write some "fake" distance > values into your distance_lookup_table[] for offline nodes using your > distance_ref_point_depth thing, i.e. ensure an iteration of > node_distance(a, b) covers all distance values [1]. You can then keep patch > 3 around, and that should roughly be it. > Yes, this would suffice but to me its not very
Re: [PATCH 1/3] sched/topology: Allow archs to populate distance map
On 21/05/21 14:58, Srikar Dronamraju wrote: > * Peter Zijlstra [2021-05-21 10:14:10]: > >> On Fri, May 21, 2021 at 08:08:02AM +0530, Srikar Dronamraju wrote: >> > * Peter Zijlstra [2021-05-20 20:56:31]: >> > >> > > On Thu, May 20, 2021 at 09:14:25PM +0530, Srikar Dronamraju wrote: >> > > > Currently scheduler populates the distance map by looking at distance >> > > > of each node from all other nodes. This should work for most >> > > > architectures and platforms. >> > > > >> > > > However there are some architectures like POWER that may not expose >> > > > the distance of nodes that are not yet onlined because those resources >> > > > are not yet allocated to the OS instance. Such architectures have >> > > > other means to provide valid distance data for the current platform. >> > > > >> > > > For example distance info from numactl from a fully populated 8 node >> > > > system at boot may look like this. >> > > > >> > > > node distances: >> > > > node 0 1 2 3 4 5 6 7 >> > > > 0: 10 20 40 40 40 40 40 40 >> > > > 1: 20 10 40 40 40 40 40 40 >> > > > 2: 40 40 10 20 40 40 40 40 >> > > > 3: 40 40 20 10 40 40 40 40 >> > > > 4: 40 40 40 40 10 20 40 40 >> > > > 5: 40 40 40 40 20 10 40 40 >> > > > 6: 40 40 40 40 40 40 10 20 >> > > > 7: 40 40 40 40 40 40 20 10 >> > > > >> > > > However the same system when only two nodes are online at boot, then >> > > > the >> > > > numa topology will look like >> > > > node distances: >> > > > node 0 1 >> > > > 0: 10 20 >> > > > 1: 20 10 >> > > > >> > > > It may be implementation dependent on what node_distance(0,3) where >> > > > node 0 is online and node 3 is offline. In POWER case, it returns >> > > > LOCAL_DISTANCE(10). Here at boot the scheduler would assume that the >> > > > max >> > > > distance between nodes is 20. However that would not be true. >> > > > >> > > > When Nodes are onlined and CPUs from those nodes are hotplugged, >> > > > the max node distance would be 40. >> > > > >> > > > To handle such scenarios, let scheduler allow architectures to populate >> > > > the distance map. Architectures that like to populate the distance map >> > > > can overload arch_populate_distance_map(). >> > > >> > > Why? Why can't your node_distance() DTRT? The arch interface is >> > > nr_node_ids and node_distance(), I don't see why we need something new >> > > and then replace one special use of it. >> > > >> > > By virtue of you being able to actually implement this new hook, you >> > > supposedly can actually do node_distance() right too. >> > >> > Since for an offline node, arch interface code doesn't have the info. >> > As far as I know/understand, in POWER, unless there is an active memory or >> > CPU that's getting onlined, arch can't fetch the correct node distance. >> > >> > Taking the above example: node 3 is offline, then node_distance of (3,X) >> > where X is anything other than 3, is not reliable. The moment node 3 is >> > onlined, the node distance is reliable. >> > >> > This problem will not happen even on POWER if all the nodes have either >> > memory or CPUs active at the time of boot. >> >> But then how can you implement this new hook? Going by the fact that >> both nr_node_ids and distance_ref_points_depth are fixed, how many >> possible __node_distance() configurations are there left? >> > > distance_ref_point_depth is provided as a different property and is readily > available at boot. The new api will use just use that. So based on the > distance_ref_point_depth, we know all possible node distances for that > platform. > > For an offline node, we don't have that specific nodes distance_lookup_table > array entries. Each array would be of distance_ref_point_depth entries. > Without the distance_lookup_table for an array populated, we will not be > able to tell how far the node is with respect to other nodes. > > We can lookup the correct distance_lookup_table for a node based on memory > or the CPUs attached to that node. Since in an offline node, both of them > would not be around, the distance_lookup_table will have stale values. > Ok so from your arch you can figure out the *size* of the set of unique distances, but not the individual node_distance(a, b)... That's quite unfortunate. I suppose one way to avoid the hook would be to write some "fake" distance values into your distance_lookup_table[] for offline nodes using your distance_ref_point_depth thing, i.e. ensure an iteration of node_distance(a, b) covers all distance values [1]. You can then keep patch 3 around, and that should roughly be it. >> The example provided above does not suggest there's much room for >> alternatives, and hence for actual need of this new interface. >> > > -- > Thanks and Regards > Srikar Dronamraju
Re: [PATCH 1/3] sched/topology: Allow archs to populate distance map
* Peter Zijlstra [2021-05-21 10:14:10]: > On Fri, May 21, 2021 at 08:08:02AM +0530, Srikar Dronamraju wrote: > > * Peter Zijlstra [2021-05-20 20:56:31]: > > > > > On Thu, May 20, 2021 at 09:14:25PM +0530, Srikar Dronamraju wrote: > > > > Currently scheduler populates the distance map by looking at distance > > > > of each node from all other nodes. This should work for most > > > > architectures and platforms. > > > > > > > > However there are some architectures like POWER that may not expose > > > > the distance of nodes that are not yet onlined because those resources > > > > are not yet allocated to the OS instance. Such architectures have > > > > other means to provide valid distance data for the current platform. > > > > > > > > For example distance info from numactl from a fully populated 8 node > > > > system at boot may look like this. > > > > > > > > node distances: > > > > node 0 1 2 3 4 5 6 7 > > > > 0: 10 20 40 40 40 40 40 40 > > > > 1: 20 10 40 40 40 40 40 40 > > > > 2: 40 40 10 20 40 40 40 40 > > > > 3: 40 40 20 10 40 40 40 40 > > > > 4: 40 40 40 40 10 20 40 40 > > > > 5: 40 40 40 40 20 10 40 40 > > > > 6: 40 40 40 40 40 40 10 20 > > > > 7: 40 40 40 40 40 40 20 10 > > > > > > > > However the same system when only two nodes are online at boot, then the > > > > numa topology will look like > > > > node distances: > > > > node 0 1 > > > > 0: 10 20 > > > > 1: 20 10 > > > > > > > > It may be implementation dependent on what node_distance(0,3) where > > > > node 0 is online and node 3 is offline. In POWER case, it returns > > > > LOCAL_DISTANCE(10). Here at boot the scheduler would assume that the max > > > > distance between nodes is 20. However that would not be true. > > > > > > > > When Nodes are onlined and CPUs from those nodes are hotplugged, > > > > the max node distance would be 40. > > > > > > > > To handle such scenarios, let scheduler allow architectures to populate > > > > the distance map. Architectures that like to populate the distance map > > > > can overload arch_populate_distance_map(). > > > > > > Why? Why can't your node_distance() DTRT? The arch interface is > > > nr_node_ids and node_distance(), I don't see why we need something new > > > and then replace one special use of it. > > > > > > By virtue of you being able to actually implement this new hook, you > > > supposedly can actually do node_distance() right too. > > > > Since for an offline node, arch interface code doesn't have the info. > > As far as I know/understand, in POWER, unless there is an active memory or > > CPU that's getting onlined, arch can't fetch the correct node distance. > > > > Taking the above example: node 3 is offline, then node_distance of (3,X) > > where X is anything other than 3, is not reliable. The moment node 3 is > > onlined, the node distance is reliable. > > > > This problem will not happen even on POWER if all the nodes have either > > memory or CPUs active at the time of boot. > > But then how can you implement this new hook? Going by the fact that > both nr_node_ids and distance_ref_points_depth are fixed, how many > possible __node_distance() configurations are there left? > distance_ref_point_depth is provided as a different property and is readily available at boot. The new api will use just use that. So based on the distance_ref_point_depth, we know all possible node distances for that platform. For an offline node, we don't have that specific nodes distance_lookup_table array entries. Each array would be of distance_ref_point_depth entries. Without the distance_lookup_table for an array populated, we will not be able to tell how far the node is with respect to other nodes. We can lookup the correct distance_lookup_table for a node based on memory or the CPUs attached to that node. Since in an offline node, both of them would not be around, the distance_lookup_table will have stale values. > The example provided above does not suggest there's much room for > alternatives, and hence for actual need of this new interface. > -- Thanks and Regards Srikar Dronamraju
Re: [PATCH 1/3] sched/topology: Allow archs to populate distance map
On Fri, May 21, 2021 at 08:08:02AM +0530, Srikar Dronamraju wrote: > * Peter Zijlstra [2021-05-20 20:56:31]: > > > On Thu, May 20, 2021 at 09:14:25PM +0530, Srikar Dronamraju wrote: > > > Currently scheduler populates the distance map by looking at distance > > > of each node from all other nodes. This should work for most > > > architectures and platforms. > > > > > > However there are some architectures like POWER that may not expose > > > the distance of nodes that are not yet onlined because those resources > > > are not yet allocated to the OS instance. Such architectures have > > > other means to provide valid distance data for the current platform. > > > > > > For example distance info from numactl from a fully populated 8 node > > > system at boot may look like this. > > > > > > node distances: > > > node 0 1 2 3 4 5 6 7 > > > 0: 10 20 40 40 40 40 40 40 > > > 1: 20 10 40 40 40 40 40 40 > > > 2: 40 40 10 20 40 40 40 40 > > > 3: 40 40 20 10 40 40 40 40 > > > 4: 40 40 40 40 10 20 40 40 > > > 5: 40 40 40 40 20 10 40 40 > > > 6: 40 40 40 40 40 40 10 20 > > > 7: 40 40 40 40 40 40 20 10 > > > > > > However the same system when only two nodes are online at boot, then the > > > numa topology will look like > > > node distances: > > > node 0 1 > > > 0: 10 20 > > > 1: 20 10 > > > > > > It may be implementation dependent on what node_distance(0,3) where > > > node 0 is online and node 3 is offline. In POWER case, it returns > > > LOCAL_DISTANCE(10). Here at boot the scheduler would assume that the max > > > distance between nodes is 20. However that would not be true. > > > > > > When Nodes are onlined and CPUs from those nodes are hotplugged, > > > the max node distance would be 40. > > > > > > To handle such scenarios, let scheduler allow architectures to populate > > > the distance map. Architectures that like to populate the distance map > > > can overload arch_populate_distance_map(). > > > > Why? Why can't your node_distance() DTRT? The arch interface is > > nr_node_ids and node_distance(), I don't see why we need something new > > and then replace one special use of it. > > > > By virtue of you being able to actually implement this new hook, you > > supposedly can actually do node_distance() right too. > > Since for an offline node, arch interface code doesn't have the info. > As far as I know/understand, in POWER, unless there is an active memory or > CPU that's getting onlined, arch can't fetch the correct node distance. > > Taking the above example: node 3 is offline, then node_distance of (3,X) > where X is anything other than 3, is not reliable. The moment node 3 is > onlined, the node distance is reliable. > > This problem will not happen even on POWER if all the nodes have either > memory or CPUs active at the time of boot. But then how can you implement this new hook? Going by the fact that both nr_node_ids and distance_ref_points_depth are fixed, how many possible __node_distance() configurations are there left? The example provided above does not suggest there's much room for alternatives, and hence for actual need of this new interface.
Re: [PATCH 1/3] sched/topology: Allow archs to populate distance map
* Peter Zijlstra [2021-05-20 20:56:31]: > On Thu, May 20, 2021 at 09:14:25PM +0530, Srikar Dronamraju wrote: > > Currently scheduler populates the distance map by looking at distance > > of each node from all other nodes. This should work for most > > architectures and platforms. > > > > However there are some architectures like POWER that may not expose > > the distance of nodes that are not yet onlined because those resources > > are not yet allocated to the OS instance. Such architectures have > > other means to provide valid distance data for the current platform. > > > > For example distance info from numactl from a fully populated 8 node > > system at boot may look like this. > > > > node distances: > > node 0 1 2 3 4 5 6 7 > > 0: 10 20 40 40 40 40 40 40 > > 1: 20 10 40 40 40 40 40 40 > > 2: 40 40 10 20 40 40 40 40 > > 3: 40 40 20 10 40 40 40 40 > > 4: 40 40 40 40 10 20 40 40 > > 5: 40 40 40 40 20 10 40 40 > > 6: 40 40 40 40 40 40 10 20 > > 7: 40 40 40 40 40 40 20 10 > > > > However the same system when only two nodes are online at boot, then the > > numa topology will look like > > node distances: > > node 0 1 > > 0: 10 20 > > 1: 20 10 > > > > It may be implementation dependent on what node_distance(0,3) where > > node 0 is online and node 3 is offline. In POWER case, it returns > > LOCAL_DISTANCE(10). Here at boot the scheduler would assume that the max > > distance between nodes is 20. However that would not be true. > > > > When Nodes are onlined and CPUs from those nodes are hotplugged, > > the max node distance would be 40. > > > > To handle such scenarios, let scheduler allow architectures to populate > > the distance map. Architectures that like to populate the distance map > > can overload arch_populate_distance_map(). > > Why? Why can't your node_distance() DTRT? The arch interface is > nr_node_ids and node_distance(), I don't see why we need something new > and then replace one special use of it. > > By virtue of you being able to actually implement this new hook, you > supposedly can actually do node_distance() right too. Since for an offline node, arch interface code doesn't have the info. As far as I know/understand, in POWER, unless there is an active memory or CPU that's getting onlined, arch can't fetch the correct node distance. Taking the above example: node 3 is offline, then node_distance of (3,X) where X is anything other than 3, is not reliable. The moment node 3 is onlined, the node distance is reliable. This problem will not happen even on POWER if all the nodes have either memory or CPUs active at the time of boot. -- Thanks and Regards Srikar Dronamraju
Re: [PATCH 1/3] sched/topology: Allow archs to populate distance map
On Thu, May 20, 2021 at 09:14:25PM +0530, Srikar Dronamraju wrote: > Currently scheduler populates the distance map by looking at distance > of each node from all other nodes. This should work for most > architectures and platforms. > > However there are some architectures like POWER that may not expose > the distance of nodes that are not yet onlined because those resources > are not yet allocated to the OS instance. Such architectures have > other means to provide valid distance data for the current platform. > > For example distance info from numactl from a fully populated 8 node > system at boot may look like this. > > node distances: > node 0 1 2 3 4 5 6 7 > 0: 10 20 40 40 40 40 40 40 > 1: 20 10 40 40 40 40 40 40 > 2: 40 40 10 20 40 40 40 40 > 3: 40 40 20 10 40 40 40 40 > 4: 40 40 40 40 10 20 40 40 > 5: 40 40 40 40 20 10 40 40 > 6: 40 40 40 40 40 40 10 20 > 7: 40 40 40 40 40 40 20 10 > > However the same system when only two nodes are online at boot, then the > numa topology will look like > node distances: > node 0 1 > 0: 10 20 > 1: 20 10 > > It may be implementation dependent on what node_distance(0,3) where > node 0 is online and node 3 is offline. In POWER case, it returns > LOCAL_DISTANCE(10). Here at boot the scheduler would assume that the max > distance between nodes is 20. However that would not be true. > > When Nodes are onlined and CPUs from those nodes are hotplugged, > the max node distance would be 40. > > To handle such scenarios, let scheduler allow architectures to populate > the distance map. Architectures that like to populate the distance map > can overload arch_populate_distance_map(). Why? Why can't your node_distance() DTRT? The arch interface is nr_node_ids and node_distance(), I don't see why we need something new and then replace one special use of it. By virtue of you being able to actually implement this new hook, you supposedly can actually do node_distance() right too.
