Re: [PATCH] drm/panfrost: fix runtime pm imbalance on error

[Steven Price]

On 21/05/2020 08:00, dinghao.liu@xxxxxxxxxx wrote:
> Hi Steve,
>
> There are two bailing out points in panfrost_job_hw_submit(): one is
> the error path beginning from pm_runtime_get_sync(), the other one is
> the error path beginning from WARN_ON() in the if statement. The pm
> imbalance fixed in this patch is between these two paths. I think the
> caller of panfrost_job_hw_submit() cannot distinguish this imbalance
> outside this function.

My point is the caller expects panfrost_job_hw_submit() to increase the 
PM reference count. Since panfrost_job_hw_submit() cannot return an 
error (it's void return) we cannot signal to the caller that the 
reference hasn't been taken.

> panfrost_job_timedout() calls pm_runtime_put_noidle() for every job it
> finds, but all jobs are added to the pfdev->jobs just before calling
> panfrost_job_hw_submit(). Therefore I think the imbalance still exists.

My point's exactly that - the "jobs are added to pfdev->jobs just before 
calling panfrost_job_hw_submit()". Since we don't have a way for 
panfrost_job_hw_submit() to fail it must unconditionally take any 
references that will then be freed later on.

> But I'm not very sure if we should add pm_runtime_put on the error path
> after pm_runtime_get_sync(), or remove pm_runtime_put one the error path
> after WARN_ON().

The pm_runtime_put after the WARN_ON() is a bug. Sorry this is probably 
what confused you - clearly the WARN_ON() situation is never meant to 
happen in the first place, so hopefully this isn't actually possible.

Feel free to send a patch removing it! ;)

> As for the problem about panfrost_devfreq_record_busy(), this may be a
> new bug and requires independent patch to fix it.

Indeed, I'll post a proper patch for that later - I just spotted it 
while looking at the code.

Thanks,

Steve

> Regards,
> Dinghao
>
>
> > On 20/05/2020 12:05, Dinghao Liu wrote:
> > > pm_runtime_get_sync() increments the runtime PM usage counter even
> > > the call returns an error code. Thus a pairing decrement is needed
> > > on the error handling path to keep the counter balanced.
> > >
> > > Signed-off-by: Dinghao Liu <dinghao.liu@xxxxxxxxxx>
> >
> > Actually I think we have the opposite problem. To be honest we don't
> > handle this situation very well. By the time panfrost_job_hw_submit() is
> > called the job has already been added to the pfdev->jobs array, so it's
> > considered submitted even if it never actually lands on the hardware. So
> > in the case of this function bailing out early we will then (eventually)
> > hit a timeout and trigger a GPU reset.
> >
> > panfrost_job_timedout() iterates through the pfdev->jobs array and calls
> > pm_runtime_put_noidle() for each job it finds. So there's no inbalance
> > here that I can see.
> >
> > Have you actually observed the situation where pm_runtime_get_sync()
> > returns a failure?
> >
> > HOWEVER, it appears that by bailing out early the call to
> > panfrost_devfreq_record_busy() is never made, which as far as I can see
> > means that there may be an extra call to panfrost_devfreq_record_idle()
> > when the jobs have timed out. Which could underflow the counter.
> >
> > But equally looking at panfrost_job_timedout(), we only call
> > panfrost_devfreq_record_idle() *once* even though multiple jobs might be
> > processed.
> >
> > There's a completely untested patch below which in theory should fix that...
> >
> > Steve
> >
> > ----8<---
> > diff --git a/drivers/gpu/drm/panfrost/panfrost_job.c
> > b/drivers/gpu/drm/panfrost/panfrost_job.c
> > index 7914b1570841..f9519afca29d 100644
> > --- a/drivers/gpu/drm/panfrost/panfrost_job.c
> > +++ b/drivers/gpu/drm/panfrost/panfrost_job.c
> > @@ -145,6 +145,8 @@ static void panfrost_job_hw_submit(struct
> > panfrost_job *job, int js)
> >    	u64 jc_head = job->jc;
> >    	int ret;
> >
> > +	panfrost_devfreq_record_busy(pfdev);
> > +
> >    	ret = pm_runtime_get_sync(pfdev->dev);
> >    	if (ret < 0)
> >    		return;
> > @@ -155,7 +157,6 @@ static void panfrost_job_hw_submit(struct
> > panfrost_job *job, int js)
> >    	}
> >
> >    	cfg = panfrost_mmu_as_get(pfdev, &job->file_priv->mmu);
> > -	panfrost_devfreq_record_busy(pfdev);
> >
> >    	job_write(pfdev, JS_HEAD_NEXT_LO(js), jc_head & 0xFFFFFFFF);
> >    	job_write(pfdev, JS_HEAD_NEXT_HI(js), jc_head >> 32);
> > @@ -410,12 +411,12 @@ static void panfrost_job_timedout(struct
> > drm_sched_job *sched_job)
> >    	for (i = 0; i < NUM_JOB_SLOTS; i++) {
> >    		if (pfdev->jobs[i]) {
> >    			pm_runtime_put_noidle(pfdev->dev);
> > +			panfrost_devfreq_record_idle(pfdev);
> >    			pfdev->jobs[i] = NULL;
> >    		}
> >    	}
> >    	spin_unlock_irqrestore(&pfdev->js->job_lock, flags);
> >
> > -	panfrost_devfreq_record_idle(pfdev);
> >    	panfrost_device_reset(pfdev);
> >
> >    	for (i = 0; i < NUM_JOB_SLOTS; i++)