Re: [PATCH v5 02/20] drm/msm: Fix drm/sched point of no return rules

[Daniel Vetter <daniel.vetter@ffwll.ch>]

On Fri, Aug 6, 2021 at 8:57 PM Rob Clark <robdclark@gmail.com> wrote:
>
> On Fri, Aug 6, 2021 at 11:41 AM Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> >
> > On Fri, Aug 6, 2021 at 7:15 PM Rob Clark <robdclark@gmail.com> wrote:
> > >
> > > On Fri, Aug 6, 2021 at 9:42 AM Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> > > >
> > > > On Fri, Aug 6, 2021 at 12:58 AM Rob Clark <robdclark@gmail.com> wrote:
> > > > >
> > > > > On Thu, Aug 5, 2021 at 3:47 AM Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> > > > > >
> > > > > > Originally drm_sched_job_init was the point of no return, after which
> > > > > > drivers must submit a job. I've split that up, which allows us to fix
> > > > > > this issue pretty easily.
> > > > > >
> > > > > > Only thing we have to take care of is to not skip to error paths after
> > > > > > that. Other drivers do this the same for out-fence and similar things.
> > > > > >
> > > > > > Fixes: 1d8a5ca436ee ("drm/msm: Conversion to drm scheduler")
> > > > > > Cc: Rob Clark <robdclark@chromium.org>
> > > > > > Cc: Rob Clark <robdclark@gmail.com>
> > > > > > Cc: Sean Paul <sean@poorly.run>
> > > > > > Cc: Sumit Semwal <sumit.semwal@linaro.org>
> > > > > > Cc: "Christian König" <christian.koenig@amd.com>
> > > > > > Cc: linux-arm-msm@vger.kernel.org
> > > > > > Cc: dri-devel@lists.freedesktop.org
> > > > > > Cc: freedreno@lists.freedesktop.org
> > > > > > Cc: linux-media@vger.kernel.org
> > > > > > Cc: linaro-mm-sig@lists.linaro.org
> > > > > > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > > > > > ---
> > > > > >  drivers/gpu/drm/msm/msm_gem_submit.c | 15 +++++++--------
> > > > > >  1 file changed, 7 insertions(+), 8 deletions(-)
> > > > > >
> > > > > > diff --git a/drivers/gpu/drm/msm/msm_gem_submit.c b/drivers/gpu/drm/msm/msm_gem_submit.c
> > > > > > index 6d6c44f0e1f3..d0ed4ddc509e 100644
> > > > > > --- a/drivers/gpu/drm/msm/msm_gem_submit.c
> > > > > > +++ b/drivers/gpu/drm/msm/msm_gem_submit.c
> > > > > > @@ -52,9 +52,6 @@ static struct msm_gem_submit *submit_create(struct drm_device *dev,
> > > > > >                 return ERR_PTR(ret);
> > > > > >         }
> > > > > >
> > > > > > -       /* FIXME: this is way too early */
> > > > > > -       drm_sched_job_arm(&job->base);
> > > > > > -
> > > > > >         xa_init_flags(&submit->deps, XA_FLAGS_ALLOC);
> > > > > >
> > > > > >         kref_init(&submit->ref);
> > > > > > @@ -883,6 +880,9 @@ int msm_ioctl_gem_submit(struct drm_device *dev, void *data,
> > > > > >
> > > > > >         submit->user_fence = dma_fence_get(&submit->base.s_fence->finished);
> > > > > >
> > > > > > +       /* point of no return, we _have_ to submit no matter what */
> > > > > > +       drm_sched_job_arm(&submit->base);
> > > > > > +
> > > > > >         /*
> > > > > >          * Allocate an id which can be used by WAIT_FENCE ioctl to map back
> > > > > >          * to the underlying fence.
> > > > > > @@ -892,17 +892,16 @@ int msm_ioctl_gem_submit(struct drm_device *dev, void *data,
> > > > > >         if (submit->fence_id < 0) {
> > > > > >                 ret = submit->fence_id = 0;
> > > > > >                 submit->fence_id = 0;
> > > > > > -               goto out;
> > > > > >         }
> > > > > >
> > > > > > -       if (args->flags & MSM_SUBMIT_FENCE_FD_OUT) {
> > > > > > +       if (ret == 0 && args->flags & MSM_SUBMIT_FENCE_FD_OUT) {
> > > > > >                 struct sync_file *sync_file = sync_file_create(submit->user_fence);
> > > > > >                 if (!sync_file) {
> > > > > >                         ret = -ENOMEM;
> > > > > > -                       goto out;
> > > > > > +               } else {
> > > > > > +                       fd_install(out_fence_fd, sync_file->file);
> > > > > > +                       args->fence_fd = out_fence_fd;
> > > > > >                 }
> > > > > > -               fd_install(out_fence_fd, sync_file->file);
> > > > > > -               args->fence_fd = out_fence_fd;
> > > > >
> > > > > I wonder if instead we should (approximately) undo "drm/msm/submit:
> > > > > Simplify out-fence-fd handling" so that the point that it could fail
> > > > > is moved up ahead of the drm_sched_job_arm()?
> > > >
> > > > Hm yeah. Up to you how you want to paint this shed, I think either is fine.
> > > >
> > > > > Also, does the dma_fence_get() work before drm_sched_job_arm()?  From
> > > > > a quick look, it looks like it won't, but I'm still playing catchup
> > > > > and haven't had a chance to look at your entire series.  If it doesn't
> > > > > work before drm_sched_job_arm(), then there is really no way to
> > > > > prevent a error path between the fence-init and job-submit.
> > > >
> > > > Yes. I thought I've checked that I put the _arm() in the right spot,
> > > > but I guess I screwed up and you need the fence before the point where
> > > > I've put the job_arm()? And yes the error path cannot be avoided for
> > > > out-fences, that's what I tried to explain in the commit message.
> > > >
> > > > > But, prior to your series, wouldn't a failure after
> > > > > drm_sched_job_init() but before the job is submitted just burn a
> > > > > fence-id, and otherwise carry on it's merry way?
> > > >
> > > > Maybe? I'm not sure whether the scheduler gets confused about the gap
> > > > and freak out abou that. I'm fairly new to that code and learning
> > > > (which is part why I'm working on it). Since you look up in
> > > > fences/syncobj after job_init() it should be pretty easy to whip up a
> > > > testcase and see what happens. Also as long as nothing fails you won't
> > > > see an issue, that's for sure.
> > >
> > > fair.. I'll try to come up with a test case.. pre-scheduler-conversion
> > > it wasn't a problem to fail after the fence seqno was allocated (well,
> > > I guess you might have problems if you had 2^31 failures in a row)
> >
> > Yeah one thing drm/sched forces you to do is have a very clear notion
> > about the point of no return in your submit ioctl. Which I think is a
> > Very Good Thing, at least looking at i915 execbuf where the point of
> > no return is a multi-stage thing with such interesting intermediate
> > points like "we submit the ruquest but without actually running the
> > batchbuffer". The downside is that the submit ioctl isn't perfectly
> > transaction anymore, but I don't think that matters for tha tail
> > stuff, which is generally just some out-fence installing. That
> > generally never fails.
>
> So I hacked up:
>
> ------
> diff --git a/drivers/gpu/drm/scheduler/sched_fence.c
> b/drivers/gpu/drm/scheduler/sched_fence.c
> index 3aa6351d2101..88e66dbc9515 100644
> --- a/drivers/gpu/drm/scheduler/sched_fence.c
> +++ b/drivers/gpu/drm/scheduler/sched_fence.c
> @@ -176,6 +176,7 @@ struct drm_sched_fence
> *drm_sched_fence_create(struct drm_sched_entity *entity,
>         fence->sched = entity->rq->sched;
>         spin_lock_init(&fence->lock);
>
> +       seq = atomic_inc_return(&entity->fence_seq);
>         seq = atomic_inc_return(&entity->fence_seq);
>         dma_fence_init(&fence->scheduled, &drm_sched_fence_ops_scheduled,
>                        &fence->lock, entity->fence_context, seq);
> diff --git a/drivers/gpu/drm/scheduler/sched_main.c
> b/drivers/gpu/drm/scheduler/sched_main.c
> index fcc601962e92..583e85adbbe0 100644
> --- a/drivers/gpu/drm/scheduler/sched_main.c
> +++ b/drivers/gpu/drm/scheduler/sched_main.c
> @@ -593,6 +593,7 @@ int drm_sched_job_init(struct drm_sched_job *job,
>         if (!job->s_fence)
>                 return -ENOMEM;
>         job->id = atomic64_inc_return(&sched->job_id_count);
> +       job->id = atomic64_inc_return(&sched->job_id_count);
>
>         INIT_LIST_HEAD(&job->list);
>
> ------
>
> (I guess the job->id part shouldn't really be needed, that looks like
> it is only used by amdgpu)
>
> This didn't cause any problems that I could see.  So I don't *think* a
> failure after drm_sched_job_init() is really problematic, as long as
> things are serialized between drm_sched_job_init() and
> drm_sched_entity_push_job().
>
> I also noticed that in the atomic commit path, the out-fences are
> initialized before atomic-check.. so there should be plenty of
> precedent for skipping fence seqno's.

Oh I think I remember now. The reason why the split into init/arm is
so that you can keep your critical section only around job_arm() and
push_job(). My very first version just pulled the jobs_init() of that
for most drivers to where I needed it, and that would result in a bit
chaos because the fences would signal out of order potentially. But
yeah I guess bailing out is fine with the scheduler.

Do you want me to tune down the commit message a bit, it's not a must
to submit the job, but just makes a bit more sense than bailing out
with a fence seqno reserved?
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch