Re: [PATCH 00/12 v4] multiqueue for MMC/SD

[Linus Walleij]

On Fri, Oct 27, 2017 at 2:59 PM, Adrian Hunter  wrote:
> On 27/10/17 14:25, Linus Walleij wrote:

>> It is indeed tough to juggle this with the pressure to "upstream
>> first" the BFQ scheduler policy that we are working on in Linaro
>> to increase interactivity. We need to enable this on devices
>> pronto and that means migrating MMC/SD to MQ and MQ only.
>> I have shared this motivation since the start, so it should come
>> as no surprise.
>
> IMHO BFQ is just another example of unnecessary delay.

I do not see it as a delay to anything, it is a motivation for
my work. I am telling you why I am still working on my patch
set, what is driving and motivating it.

I guess CQE is driving and motivating your work?

>> So I also have some pressure to "Get This Feature In Now".
>
> It has nothing to do with pressure.  It is about what is reasonable.
> Features should go in as soon as they are ready.  Ideally queued up in the
> same release cycle they are submitted.  If the code doesn't work right, then
> it can't go in straight away, but fake reasons for delaying things needs to
> stop.

I don't understand who you are addressing or accusing.

Nobody wants to delay CQE if that is what you are implying,
I want to see it supported as much as you do.

I just prefer to see MQ happen first, and now you say your patch
set does that and that is great, so I just need to review the code
better I guess?

Yours,
Linus Walleij

Re: [PATCH 00/12 v4] multiqueue for MMC/SD

[Adrian Hunter]

On 27/10/17 14:25, Linus Walleij wrote:
> On Thu, Oct 26, 2017 at 9:27 PM, Hunter, Adrian  
> wrote:
>> On Thu, Oct 26, 2017 at 3:34 PM, Adrian Hunter 
> 
>>> What I mean is that the CQE code will likely look better on top of these
>>> refactorings.
>>>
>>> But as I say it is a matter of taste. I just love the looks of my own code 
>>> as
>>> much as the next programmer so I can't judge that. Let's see what the
>>> reviewers say.
>>
>> It doesn't look ready.  There seems still to be 2 task switches between
>> each transfer.
> 
> IIUC there is a task in blk-mq submitting the requests, and that goes
> all the way to starting it on the host. Then as an interrupt comes back
> I kick the worker and that reports back to the block layer. So that
> is one task switch per request and then one switch back to the MQ
> layer.
> 
> But I am experimenting as we speak to get rid of the worker
> for all simple cases that don't require retune or BKOPS and that's
> pretty cool if it can be made to work :)

Well the CQE patches already do that.

> 
>> mmc_blk_rw_done_error() is still using the messy error
>> handling and doesn’t handle retries e.g. 'retry' is a local variable so it 
>> can't
>> count the number of retries now that there is no loop.
> 
> Right! Nice catch.
> 
> I will update the patch and put that in the struct mmc_async_req so
> it survives the retry rounds.
> 
> It sounds simple but I bet this drives a truck through Adrians patch
> series. Sorry. :(

 I waited a long time for your patches but I had to give up waiting
 when Ulf belated insisted on blk-mq before CQE.  I am not sure what
 you are expecting now it seems too late.
>>>
>>> Too late for what? It's just a patch set, I don't really have a deadline 
>>> for this or
>>> anything. As I explained above I have been working on this all the time, the
>>> problem was that I was/am not smart enough to find that solution for host
>>> claiming by context.
>>
>> Too late to go before CQE.  All the blk-mq work is now in the CQE patchset.
> 
> You seem to have an either/or stance.
> 
> Mine if more of a both/and.
> 
> It is not even necessary to have one set of these patches on
> top of each other, they can also be mixed in some order.
> 
> A lot of factors influence this I think, like structure of code and
> maintainability, performance, block layer interaction semantics,
> etc etc.
> 
> We definately need input from Ulf and Bartlomiej (who was actually
> the first to work on MQ for MMC/SD).
> 
>>> The host claiming by context was merged a month ago and now I have
>>> understood that I can use that and rebased my patches on it. Slow learner, I
>>> guess.
>>>
>>> If you feel it is ungrateful that you have put in so much work and things 
>>> are
>>> not getting merged, and you feel your patches deserve to be merged first
>>> (because of human nature reasons) I can empathize with that. It's sad that
>>> your patches are at v12. Also I see that patch 4 bears the signoffs of a
>>> significant team at CodeAurora, so they are likely as impatient.
>>
>> It is important that you understand that this has nothing to do with
>> "human nature reasons".
> 
> You do come across as a bit hard-headed.
> 
> But I think it is better to focus on the code per se.
> 
> I would suggest we go and review each others patch series to
> learn from each codebase what was done in a good way for the
> MMC/SD stack and what was not, you tossed out a nice review
> comment above for example.

I can make a few more comments about what else is broken.  Have you tried
suspend / resume?  At a glance, it looks like you are trying to use
blk_stop_queue() which is not a blk-mq function.

> 
>>  Linux distributions use upstream kernels.
>> ChromeOS  has an "upstream first" policy.  Delaying features for long
>> periods has real-world consequences.  When people ask, what kernel
>> should they use, we expect to reply, just use mainline.
> 
> We are in violent agreement.
> 
> I take it that you are working on ChromeOS context and that since
> they have this policy, they, through their influence over Intel as a
> supplier is putting heavy pressure on you personally to get this
> merged.
> 
> Is that correctly understood?

No.  We just expect to base everything on mainline.

> 
> That would explain your increasing pushing to get this
> upstream pretty well, especially if you have tech leads and
> managers hovering over your shoulder every week asking how
> the CQE upstream work is progressing.
> 
> It is indeed tough to juggle this with the pressure to "upstream
> first" the BFQ scheduler policy that we are working on in Linaro
> to increase interactivity. We need to enable this on devices
> pronto and that means migrating MMC/SD to MQ and MQ only.
> I have shared this motivation since the start, so it should come
> as no surprise.

IMHO BFQ is just another example of unnecessary delay.

> 
> So I also have some pressure to "Get This 

Re: [PATCH 00/12 v4] multiqueue for MMC/SD

[Linus Walleij]

On Thu, Oct 26, 2017 at 9:27 PM, Hunter, Adrian  wrote:
> On Thu, Oct 26, 2017 at 3:34 PM, Adrian Hunter 

>> What I mean is that the CQE code will likely look better on top of these
>> refactorings.
>>
>> But as I say it is a matter of taste. I just love the looks of my own code as
>> much as the next programmer so I can't judge that. Let's see what the
>> reviewers say.
>
> It doesn't look ready.  There seems still to be 2 task switches between
> each transfer.

IIUC there is a task in blk-mq submitting the requests, and that goes
all the way to starting it on the host. Then as an interrupt comes back
I kick the worker and that reports back to the block layer. So that
is one task switch per request and then one switch back to the MQ
layer.

But I am experimenting as we speak to get rid of the worker
for all simple cases that don't require retune or BKOPS and that's
pretty cool if it can be made to work :)

> mmc_blk_rw_done_error() is still using the messy error
> handling and doesn’t handle retries e.g. 'retry' is a local variable so it 
> can't
> count the number of retries now that there is no loop.

Right! Nice catch.

I will update the patch and put that in the struct mmc_async_req so
it survives the retry rounds.

>> >> It sounds simple but I bet this drives a truck through Adrians patch
>> >> series. Sorry. :(
>> >
>> > I waited a long time for your patches but I had to give up waiting
>> > when Ulf belated insisted on blk-mq before CQE.  I am not sure what
>> > you are expecting now it seems too late.
>>
>> Too late for what? It's just a patch set, I don't really have a deadline for 
>> this or
>> anything. As I explained above I have been working on this all the time, the
>> problem was that I was/am not smart enough to find that solution for host
>> claiming by context.
>
> Too late to go before CQE.  All the blk-mq work is now in the CQE patchset.

You seem to have an either/or stance.

Mine if more of a both/and.

It is not even necessary to have one set of these patches on
top of each other, they can also be mixed in some order.

A lot of factors influence this I think, like structure of code and
maintainability, performance, block layer interaction semantics,
etc etc.

We definately need input from Ulf and Bartlomiej (who was actually
the first to work on MQ for MMC/SD).

>> The host claiming by context was merged a month ago and now I have
>> understood that I can use that and rebased my patches on it. Slow learner, I
>> guess.
>>
>> If you feel it is ungrateful that you have put in so much work and things are
>> not getting merged, and you feel your patches deserve to be merged first
>> (because of human nature reasons) I can empathize with that. It's sad that
>> your patches are at v12. Also I see that patch 4 bears the signoffs of a
>> significant team at CodeAurora, so they are likely as impatient.
>
> It is important that you understand that this has nothing to do with
> "human nature reasons".

You do come across as a bit hard-headed.

But I think it is better to focus on the code per se.

I would suggest we go and review each others patch series to
learn from each codebase what was done in a good way for the
MMC/SD stack and what was not, you tossed out a nice review
comment above for example.

>  Linux distributions use upstream kernels.
> ChromeOS  has an "upstream first" policy.  Delaying features for long
> periods has real-world consequences.  When people ask, what kernel
> should they use, we expect to reply, just use mainline.

We are in violent agreement.

I take it that you are working on ChromeOS context and that since
they have this policy, they, through their influence over Intel as a
supplier is putting heavy pressure on you personally to get this
merged.

Is that correctly understood?

That would explain your increasing pushing to get this
upstream pretty well, especially if you have tech leads and
managers hovering over your shoulder every week asking how
the CQE upstream work is progressing.

It is indeed tough to juggle this with the pressure to "upstream
first" the BFQ scheduler policy that we are working on in Linaro
to increase interactivity. We need to enable this on devices
pronto and that means migrating MMC/SD to MQ and MQ only.
I have shared this motivation since the start, so it should come
as no surprise.

So I also have some pressure to "Get This Feature In Now".

Yours,
Linus Walleij

RE: [PATCH 00/12 v4] multiqueue for MMC/SD

[Hunter, Adrian]

> -Original Message-
> From: Linus Walleij [mailto:linus.wall...@linaro.org]
> Sent: Thursday, October 26, 2017 5:20 PM
> To: Hunter, Adrian <adrian.hun...@intel.com>
> Cc: linux-...@vger.kernel.org; Ulf Hansson <ulf.hans...@linaro.org>;
> linux-block <linux-block@vger.kernel.org>; Jens Axboe <ax...@kernel.dk>;
> Christoph Hellwig <h...@lst.de>; Arnd Bergmann <a...@arndb.de>;
> Bartlomiej Zolnierkiewicz <b.zolnier...@samsung.com>; Paolo Valente
> <paolo.vale...@linaro.org>; Avri Altman <avri.alt...@sandisk.com>
> Subject: Re: [PATCH 00/12 v4] multiqueue for MMC/SD
> 
> On Thu, Oct 26, 2017 at 3:34 PM, Adrian Hunter <adrian.hun...@intel.com>
> wrote:
> > On 26/10/17 15:57, Linus Walleij wrote:
> >> In my opinion this is also a better fit for command queueuing.
> >
> > Not true.  CQE support worked perfectly before blk-mq and did not
> > depend on blk-mq in any way.  Obviously the current CQE patch set
> > actually implements the CQE requirements for blk-mq - which this patch set
> does not.
> 
> What I mean is that the CQE code will likely look better on top of these
> refactorings.
> 
> But as I say it is a matter of taste. I just love the looks of my own code as
> much as the next programmer so I can't judge that. Let's see what the
> reviewers say.

It doesn't look ready.  There seems still to be 2 task switches between
each transfer.  mmc_blk_rw_done_error() is still using the messy error
handling and doesn’t handle retries e.g. 'retry' is a local variable so it can't
count the number of retries now that there is no loop.

> >> It sounds simple but I bet this drives a truck through Adrians patch
> >> series. Sorry. :(
> >
> > I waited a long time for your patches but I had to give up waiting
> > when Ulf belated insisted on blk-mq before CQE.  I am not sure what
> > you are expecting now it seems too late.
> 
> Too late for what? It's just a patch set, I don't really have a deadline for 
> this or
> anything. As I explained above I have been working on this all the time, the
> problem was that I was/am not smart enough to find that solution for host
> claiming by context.

Too late to go before CQE.  All the blk-mq work is now in the CQE patchset.

> 
> The host claiming by context was merged a month ago and now I have
> understood that I can use that and rebased my patches on it. Slow learner, I
> guess.
> 
> If you feel it is ungrateful that you have put in so much work and things are
> not getting merged, and you feel your patches deserve to be merged first
> (because of human nature reasons) I can empathize with that. It's sad that
> your patches are at v12. Also I see that patch 4 bears the signoffs of a
> significant team at CodeAurora, so they are likely as impatient.

It is important that you understand that this has nothing to do with
"human nature reasons".Linux distributions use upstream kernels. 
ChromeOS  has an "upstream first" policy.  Delaying features for long
periods has real-world consequences.  When people ask, what kernel
should they use, we expect to reply, just use mainline.

Re: [PATCH 00/12 v4] multiqueue for MMC/SD

[Linus Walleij]

On Thu, Oct 26, 2017 at 3:34 PM, Adrian Hunter  wrote:
> On 26/10/17 15:57, Linus Walleij wrote:

>> I have now worked on it for more than a year. I was side
>> tracked to clean up some code, move request allocation to
>> be handled by the block layer, delete bounce buffer handling
>> and refactoring the RPMB support. With the changes to request
>> allocation, the patch set is a better fit and has shrunk
>> from 16 to 12 patches as a result.
>
> None of which was necessary for blk-mq support.

I was not smart enough to realize that it was possible to do what
you did in
commit d685f5d5fcf75c30ef009771d3067f7438cd8baf
"mmc: core: Introduce host claiming by context"
this simple and clever solution simply didn't occur to me
at all.

And now it uses that solution, as you can see :)

But since I didn't have that simple solution, the other
solution was to get rid of the lock altogether (which we should
anyways...) getting rid of the RPMB "partition" for example
removes some locks. (I guess I still will have to go on and
find a solution for the boot and generic partitions but it's no
blocker for MQ anymore.)

My patch set was dependent on solving that. As I already wrote
to you on sep 13:
https://marc.info/?l=linux-mmc=150607944524752=2

My patches for allocating the struct mmc_queue_req from the
block layer was actually originally a part of this series
so the old patches
  mmc: queue: issue struct mmc_queue_req items
  mmc: queue: get/put struct mmc_queue_req
was doing a stupid homebrewn solution to what the block
layer already can do, mea culpa. (Yeah I was educating
myself in the block layer too...)

Anyways, all of this happened in the context of moving
forward with my MQ patch set, not as random activity.

Now it looks like I'm defending myself from a project leader,
haha :D

Well for better or worse, this was how I was working.

>> We use the trick to set the queue depth to 2 to get two
>> parallel requests pushed down to the host. I tried to set this
>> to 4, the code survives it, the queue just have three items
>> waiting to be submitted all the time.
>
> The queue depth also sets the number of requests, so you are strangling the
> I/O scheduler.

Yup. Just did it to see if it survives.

>> In my opinion this is also a better fit for command queueuing.
>
> Not true.  CQE support worked perfectly before blk-mq and did not depend on
> blk-mq in any way.  Obviously the current CQE patch set actually implements
> the CQE requirements for blk-mq - which this patch set does not.

What I mean is that the CQE code will likely look better on top
of these refactorings.

But as I say it is a matter of taste. I just love the looks of my own code
as much as the next programmer so I can't judge that. Let's see what
the reviewers say.

>> Handling command queueing needs to happen in the asynchronous
>> submission codepath, so instead of waiting on a pending
>> areq, we just stack up requests in the command queue.
>
> That is how CQE has always worked.  It worked that way just fine without 
> blk-mq.

Okay nice.

>> It sounds simple but I bet this drives a truck through Adrians
>> patch series. Sorry. :(
>
> I waited a long time for your patches but I had to give up waiting when Ulf
> belated insisted on blk-mq before CQE.  I am not sure what you are expecting
> now it seems too late.

Too late for what? It's just a patch set, I don't really have a deadline
for this or anything. As I explained above I have
been working on this all the time, the problem was that I was/am not
smart enough to find that solution for host claiming by context.

The host claiming by context was merged a month ago and now I have
understood that I can use that and rebased my patches on it. Slow
learner, I guess.

If you feel it is ungrateful that you have put in so much work and things
are not getting merged, and you feel your patches deserve to be
merged first (because of human nature reasons) I can empathize with
that. It's sad that your patches are at v12. Also I see that patch 4 bears
the signoffs of a significant team at CodeAurora, so they are likely
as impatient.

I would just rebase my remaining work on top of the CQE patches if
they end up being merged first, no big deal, just work.

Yours,
Linus Walleij

Re: [PATCH 00/12 v4] multiqueue for MMC/SD

[Adrian Hunter]

On 26/10/17 15:57, Linus Walleij wrote:
> This switches the MMC/SD stack over to unconditionally
> using the multiqueue block interface for block access.
> This modernizes the MMC/SD stack and makes it possible
> to enable BFQ scheduling on these single-queue devices.
> 
> This is the v4 version of this v3 patch set from february:
> https://marc.info/?l=linux-mmc=148665788227015=2
> 
> The patches are available in a git branch:
> https://git.kernel.org/pub/scm/linux/kernel/git/linusw/linux-stericsson.git/log/?h=mmc-mq-v4.14-rc4
> 
> You can pull it to a clean kernel tree like this:
> git checkout -b mmc-test v4.14-rc4
> git pull 
> git://git.kernel.org/pub/scm/linux/kernel/git/linusw/linux-stericsson.git 
> mmc-mq-v4.14-rc4
> 
> I have now worked on it for more than a year. I was side
> tracked to clean up some code, move request allocation to
> be handled by the block layer, delete bounce buffer handling
> and refactoring the RPMB support. With the changes to request
> allocation, the patch set is a better fit and has shrunk
> from 16 to 12 patches as a result.

None of which was necessary for blk-mq support.

> 
> It is still quite invasive. Yet it is something I think would
> be nice to merge for v4.16...
> 
> The rationale for this approach was Arnd's suggestion to try to
> switch the MMC/SD stack around so as to complete requests as
> quickly as possible when they return from the device driver
> so that new requests can be issued. We are doing this now:
> the polling loop that was pulling NULL out of the request
> queue and driving the pipeline with a loop is gone with
> the next-to last patch ("block: issue requests in massive
> parallel"). This sets the stage for MQ to go in and hammer
> requests on the asynchronous issuing layer.
> 
> We use the trick to set the queue depth to 2 to get two
> parallel requests pushed down to the host. I tried to set this
> to 4, the code survives it, the queue just have three items
> waiting to be submitted all the time.

The queue depth also sets the number of requests, so you are strangling the
I/O scheduler.

> 
> In my opinion this is also a better fit for command queueuing.

Not true.  CQE support worked perfectly before blk-mq and did not depend on
blk-mq in any way.  Obviously the current CQE patch set actually implements
the CQE requirements for blk-mq - which this patch set does not.

> Handling command queueing needs to happen in the asynchronous
> submission codepath, so instead of waiting on a pending
> areq, we just stack up requests in the command queue.

That is how CQE has always worked.  It worked that way just fine without blk-mq.

> 
> It sounds simple but I bet this drives a truck through Adrians
> patch series. Sorry. :(

I waited a long time for your patches but I had to give up waiting when Ulf
belated insisted on blk-mq before CQE.  I am not sure what you are expecting
now it seems too late.