Re: [PATCH v2 5/8] drm/lima: handle spurious timeouts due to high irq latency

[Erico Nunes <nunes.erico@gmail.com>]

On Wed, Jan 24, 2024 at 1:38 PM Qiang Yu <yuq825@gmail.com> wrote:
>
> On Wed, Jan 24, 2024 at 11:00 AM Erico Nunes <nunes.erico@gmail.com> wrote:
> >
> > There are several unexplained and unreproduced cases of rendering
> > timeouts with lima, for which one theory is high IRQ latency coming from
> > somewhere else in the system.
> > This kind of occurrence may cause applications to trigger unnecessary
> > resets of the GPU or even applications to hang if it hits an issue in
> > the recovery path.
> > Panfrost already does some special handling to account for such
> > "spurious timeouts", it makes sense to have this in lima too to reduce
> > the chance that it hit users.
> >
> > Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
> > ---
> >  drivers/gpu/drm/lima/lima_sched.c | 31 ++++++++++++++++++++++++++++---
> >  1 file changed, 28 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/lima/lima_sched.c b/drivers/gpu/drm/lima/lima_sched.c
> > index c3bf8cda8498..814428564637 100644
> > --- a/drivers/gpu/drm/lima/lima_sched.c
> > +++ b/drivers/gpu/drm/lima/lima_sched.c
> > @@ -1,6 +1,7 @@
> >  // SPDX-License-Identifier: GPL-2.0 OR MIT
> >  /* Copyright 2017-2019 Qiang Yu <yuq825@gmail.com> */
> >
> > +#include <linux/hardirq.h>
> >  #include <linux/iosys-map.h>
> >  #include <linux/kthread.h>
> >  #include <linux/slab.h>
> > @@ -401,9 +402,35 @@ static enum drm_gpu_sched_stat lima_sched_timedout_job(struct drm_sched_job *job
> >         struct lima_sched_pipe *pipe = to_lima_pipe(job->sched);
> >         struct lima_sched_task *task = to_lima_task(job);
> >         struct lima_device *ldev = pipe->ldev;
> > +       struct lima_ip *ip = pipe->processor[0];
> > +       int i;
> > +
> > +       /*
> > +        * If the GPU managed to complete this jobs fence, the timeout is
> > +        * spurious. Bail out.
> > +        */
> > +       if (dma_fence_is_signaled(task->fence)) {
> > +               DRM_WARN("%s spurious timeout\n", lima_ip_name(ip));
> > +               return DRM_GPU_SCHED_STAT_NOMINAL;
> > +       }
> > +
> > +       /*
> > +        * Lima IRQ handler may take a long time to process an interrupt
> > +        * if there is another IRQ handler hogging the processing.
> > +        * In order to catch such cases and not report spurious Lima job
> > +        * timeouts, synchronize the IRQ handler and re-check the fence
> > +        * status.
> > +        */
> > +       for (i = 0; i < pipe->num_processor; i++)
> > +               synchronize_irq(pipe->processor[i]->irq);
> > +
> I have a question, this timeout handler will be called when GP/PP error IRQ.
> If we call synchronize_irq() in the IRQ handler, will we block ourselves here?

If I understand correctly, this handler is only called by drm_sched in
a workqueue, not by gp or pp IRQ and it also does not run in any IRQ
context.
So I think this sort of lockup can't happen here.

I ran some additional tests with both timeouts and actual error IRQs
(locally modified Mesa to produce some errored jobs) and was not able
to cause any lockup related to this.

Erico