Re: [Intel-gfx] [PATCH igt v2] tests/kms_cursor_legacy: Boost timing sensitive subtests to RT prio

[Imre Deak]

On ti, 2016-09-13 at 08:38 +0100, Chris Wilson wrote:
> On Mon, Sep 12, 2016 at 11:57:54PM +0300, Imre Deak wrote:
> > On Mon, 2016-09-12 at 21:04 +0100, Chris Wilson wrote:
> > > On Mon, Sep 12, 2016 at 05:47:57PM +0300, Imre Deak wrote:
> > > > Even in an otherwise quiescent system there may be user/kernel
> > > > threads
> > > > independent of the test that add enough latency to make timing
> > > > sensitive
> > > > subtests fail. Boost the priority of such subtests to avoid
> > > > these
> > > > failures.
> > > > 
> > > > This got rid of sporadic failures in basic-cursor-vs-flip-
> > > > legacy
> > > > and
> > > > basic-cursor-vs-flip-varying-size with 'missed 1 frame' error
> > > > message
> > > > APL and BSW.
> > > > 
> > > > v2:
> > > > - Boost the priority in flip_vs_cursor_crc() too.
> > > > 
> > > > CC: Chris Wilson 
> > > > CC: Maarten Lankhorst 
> > > > Signed-off-by: Imre Deak 
> > > 
> > > But we shouldn't need to. The basic test is:
> > > 
> > >   align to vblank
> > >   request non-blocking flip
> > >   update cursor
> > 
> > In these subtests we run these cursor updates in a loop.
> 
> Oh, those. Ok, for the purpose of bat we want:
> 
>   align to vblank
>   update cursor
>   request non-blocking flip
>   check vblank == vblank
>   check flip-completion == vblank + 1

That's basic_flip_vs_cursor, the ones failing are the cursor_vs_flip_*
running the cursor update in a separate thread. So are you suggesting
just removing these from bat or doing only a single cursor update
(target=1)? The latter would reduce the chance for failure, but
wouldn't eliminate it.

> > >   check vblank hasn't advanced
> > > 
> > > We are not doing any busy loops here and there should be nothing
> > > else
> > > running on the system. So what caused the context switch? Who are
> > > we
> > > fighting against?
> > 
> > The cursor thread is one source for the delay, other than that it
> > could
> > be anything running in the background. In my traces it looked like
> > something related to CI remote logging that caused >16ms delay for
> > both
> > the user flip thread and the subsequent MMIO work. Imo there is no
> > guarantee that such delays won't happen between threads running at
> > the
> > same priority, hence the need for higher priority for timing
> > sensitive
> > stuff. Note that we see this problem on BSW with with 2 CPUs.
> > 
> > > If the only thing that is causing the issue is the
> > > kernel thread used for the mmioflip (which won't be scheduled for
> > > another 16ms until the next vblank), we have another bug to track
> > > down.
> > 
> > The MMIO flip work is scheduled right after we request the flip
> > (since
> > we do the request after the previous flip completed) and I saw it
> > being
> > delayed >16ms for the above reasons. Besides this I also saw the
> > user
> > space flip thread being delayed the same way.
> > 
> > > Imo, this patch is just papering over an issue that as it stands
> > > will
> > > be
> > > present in real userspace (i.e. causing jerkiness in X, weston,
> > > cros
> > > etc).
> > 
> > I can't see any other way than adjusting priorities to guarantee
> > the
> > timely completion of some work. Otherwise you'll only get best
> > effort
> > scheduling and that doesn't seem to be enough in these subtests.
> 
> Our worker has multiple phases and waits of which only a small
> portion is
> timing crucial. We don't want to boost the priority of everything it
> does, only the reprogramming of the registers within the next vblank.
> The inputs to that crucial phase are irq driven (be they render
> completion on any dmabuf device, or delay until after vblank) and we
> could
> move the mmio into that irq context and that would avoid scheduling
> issues
> on all but the RT systems that want threaded irqs.

There are differences in how time critical the different phases are
(for example preparing vs. register updating for vblank evasion), but
the whole work of queuing the flip is time critical. There is one frame
time to complete that work when a single flip can be queued.

--Imre
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Re: [Intel-gfx] [PATCH igt v2] tests/kms_cursor_legacy: Boost timing sensitive subtests to RT prio

[Chris Wilson]

On Mon, Sep 12, 2016 at 11:57:54PM +0300, Imre Deak wrote:
> On Mon, 2016-09-12 at 21:04 +0100, Chris Wilson wrote:
> > On Mon, Sep 12, 2016 at 05:47:57PM +0300, Imre Deak wrote:
> > > Even in an otherwise quiescent system there may be user/kernel
> > > threads
> > > independent of the test that add enough latency to make timing
> > > sensitive
> > > subtests fail. Boost the priority of such subtests to avoid these
> > > failures.
> > > 
> > > This got rid of sporadic failures in basic-cursor-vs-flip-legacy
> > > and
> > > basic-cursor-vs-flip-varying-size with 'missed 1 frame' error
> > > message
> > > APL and BSW.
> > > 
> > > v2:
> > > - Boost the priority in flip_vs_cursor_crc() too.
> > > 
> > > CC: Chris Wilson 
> > > CC: Maarten Lankhorst 
> > > Signed-off-by: Imre Deak 
> > 
> > But we shouldn't need to. The basic test is:
> > 
> > align to vblank
> > request non-blocking flip
> > update cursor
> 
> In these subtests we run these cursor updates in a loop.

Oh, those. Ok, for the purpose of bat we want:

align to vblank
update cursor
request non-blocking flip
check vblank == vblank
check flip-completion == vblank + 1
> 
> > check vblank hasn't advanced
> > 
> > We are not doing any busy loops here and there should be nothing else
> > running on the system. So what caused the context switch? Who are we
> > fighting against?
> 
> The cursor thread is one source for the delay, other than that it could
> be anything running in the background. In my traces it looked like
> something related to CI remote logging that caused >16ms delay for both
> the user flip thread and the subsequent MMIO work. Imo there is no
> guarantee that such delays won't happen between threads running at the
> same priority, hence the need for higher priority for timing sensitive
> stuff. Note that we see this problem on BSW with with 2 CPUs.
> 
> > If the only thing that is causing the issue is the
> > kernel thread used for the mmioflip (which won't be scheduled for
> > another 16ms until the next vblank), we have another bug to track
> > down.
> 
> The MMIO flip work is scheduled right after we request the flip (since
> we do the request after the previous flip completed) and I saw it being
> delayed >16ms for the above reasons. Besides this I also saw the user
> space flip thread being delayed the same way.
> 
> > Imo, this patch is just papering over an issue that as it stands will
> > be
> > present in real userspace (i.e. causing jerkiness in X, weston, cros
> > etc).
> 
> I can't see any other way than adjusting priorities to guarantee the
> timely completion of some work. Otherwise you'll only get best effort
> scheduling and that doesn't seem to be enough in these subtests.

Our worker has multiple phases and waits of which only a small portion is
timing crucial. We don't want to boost the priority of everything it
does, only the reprogramming of the registers within the next vblank.
The inputs to that crucial phase are irq driven (be they render
completion on any dmabuf device, or delay until after vblank) and we could
move the mmio into that irq context and that would avoid scheduling issues
on all but the RT systems that want threaded irqs.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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Re: [Intel-gfx] [PATCH igt v2] tests/kms_cursor_legacy: Boost timing sensitive subtests to RT prio

[Imre Deak]

On Mon, 2016-09-12 at 21:04 +0100, Chris Wilson wrote:
> On Mon, Sep 12, 2016 at 05:47:57PM +0300, Imre Deak wrote:
> > Even in an otherwise quiescent system there may be user/kernel
> > threads
> > independent of the test that add enough latency to make timing
> > sensitive
> > subtests fail. Boost the priority of such subtests to avoid these
> > failures.
> > 
> > This got rid of sporadic failures in basic-cursor-vs-flip-legacy
> > and
> > basic-cursor-vs-flip-varying-size with 'missed 1 frame' error
> > message
> > APL and BSW.
> > 
> > v2:
> > - Boost the priority in flip_vs_cursor_crc() too.
> > 
> > CC: Chris Wilson 
> > CC: Maarten Lankhorst 
> > Signed-off-by: Imre Deak 
> 
> But we shouldn't need to. The basic test is:
> 
>   align to vblank
>   request non-blocking flip
>   update cursor

In these subtests we run these cursor updates in a loop.

>   check vblank hasn't advanced
> 
> We are not doing any busy loops here and there should be nothing else
> running on the system. So what caused the context switch? Who are we
> fighting against?

The cursor thread is one source for the delay, other than that it could
be anything running in the background. In my traces it looked like
something related to CI remote logging that caused >16ms delay for both
the user flip thread and the subsequent MMIO work. Imo there is no
guarantee that such delays won't happen between threads running at the
same priority, hence the need for higher priority for timing sensitive
stuff. Note that we see this problem on BSW with with 2 CPUs.

> If the only thing that is causing the issue is the
> kernel thread used for the mmioflip (which won't be scheduled for
> another 16ms until the next vblank), we have another bug to track
> down.

The MMIO flip work is scheduled right after we request the flip (since
we do the request after the previous flip completed) and I saw it being
delayed >16ms for the above reasons. Besides this I also saw the user
space flip thread being delayed the same way.

> Imo, this patch is just papering over an issue that as it stands will
> be
> present in real userspace (i.e. causing jerkiness in X, weston, cros
> etc).

I can't see any other way than adjusting priorities to guarantee the
timely completion of some work. Otherwise you'll only get best effort
scheduling and that doesn't seem to be enough in these subtests.

--Imre
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Re: [Intel-gfx] [PATCH igt v2] tests/kms_cursor_legacy: Boost timing sensitive subtests to RT prio

[Chris Wilson]

On Mon, Sep 12, 2016 at 05:47:57PM +0300, Imre Deak wrote:
> Even in an otherwise quiescent system there may be user/kernel threads
> independent of the test that add enough latency to make timing sensitive
> subtests fail. Boost the priority of such subtests to avoid these
> failures.
> 
> This got rid of sporadic failures in basic-cursor-vs-flip-legacy and
> basic-cursor-vs-flip-varying-size with 'missed 1 frame' error message
> APL and BSW.
> 
> v2:
> - Boost the priority in flip_vs_cursor_crc() too.
> 
> CC: Chris Wilson 
> CC: Maarten Lankhorst 
> Signed-off-by: Imre Deak 

But we shouldn't need to. The basic test is:

align to vblank
request non-blocking flip
update cursor
check vblank hasn't advanced

We are not doing any busy loops here and there should be nothing else
running on the system. So what caused the context switch? Who are we
fighting against? If the only thing that is causing the issue is the
kernel thread used for the mmioflip (which won't be scheduled for
another 16ms until the next vblank), we have another bug to track down.

Imo, this patch is just papering over an issue that as it stands will be
present in real userspace (i.e. causing jerkiness in X, weston, cros
etc).
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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