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

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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++)

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