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

[Qiang Yu <yuq825@gmail.com>]

On Tue, Jan 30, 2024 at 6:55 AM Erico Nunes <nunes.erico@gmail.com> wrote:
>
> 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.
>
Oh, right. I miss understand the drm_sched_fault() which still call the timeout
handler in work queue instead of caller thread.

> 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