linux-kernel - Re: [PATCH] drm: Don't free jobs in wait_event

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Message-ID: <cc0b260c-059d-7f55-288e-c48f30eb84e3@amd.com>
Date:   Wed, 25 Sep 2019 15:56:46 +0000
From:   "Grodzovsky, Andrey" <Andrey.Grodzovsky@....com>
To:     Steven Price <steven.price@....com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        "Koenig, Christian" <Christian.Koenig@....com>
CC:     "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Deucher, Alexander" <Alexander.Deucher@....com>,
        Nayan Deshmukh <nayan26deshmukh@...il.com>,
        Sharat Masetty <smasetty@...eaurora.org>
Subject: Re: [PATCH] drm: Don't free jobs in wait_event_interruptible()

On 9/25/19 11:14 AM, Steven Price wrote:

> drm_sched_cleanup_jobs() attempts to free finished jobs, however because
> it is called as the condition of wait_event_interruptible() it must not
> sleep. Unfortunately some free callbacks (notably for Panfrost) do sleep.
>
> Instead let's rename drm_sched_cleanup_jobs() to
> drm_sched_get_cleanup_job() and simply return a job for processing if
> there is one. The caller can then call the free_job() callback outside
> the wait_event_interruptible() where sleeping is possible before
> re-checking and returning to sleep if necessary.
>
> Signed-off-by: Steven Price <steven.price@....com>
> ---
>   drivers/gpu/drm/scheduler/sched_main.c | 44 ++++++++++++++------------
>   1 file changed, 24 insertions(+), 20 deletions(-)
>
> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> index 9a0ee74d82dc..0ed4aaa4e6d1 100644
> --- a/drivers/gpu/drm/scheduler/sched_main.c
> +++ b/drivers/gpu/drm/scheduler/sched_main.c
> @@ -622,43 +622,41 @@ static void drm_sched_process_job(struct dma_fence *f, struct dma_fence_cb *cb)
>   }
>   
>   /**
> - * drm_sched_cleanup_jobs - destroy finished jobs
> + * drm_sched_get_cleanup_job - fetch the next finished job to be destroyed
>    *
>    * @sched: scheduler instance
>    *
> - * Remove all finished jobs from the mirror list and destroy them.
> + * Returns the next finished job from the mirror list (if there is one)
> + * ready for it to be destroyed.
>    */
> -static void drm_sched_cleanup_jobs(struct drm_gpu_scheduler *sched)
> +static struct drm_sched_job *
> +drm_sched_get_cleanup_job(struct drm_gpu_scheduler *sched)
>   {
> +	struct drm_sched_job *job = NULL;
>   	unsigned long flags;
>   
>   	/* Don't destroy jobs while the timeout worker is running */
>   	if (sched->timeout != MAX_SCHEDULE_TIMEOUT &&
>   	    !cancel_delayed_work(&sched->work_tdr))
> -		return;
> -
> -
> -	while (!list_empty(&sched->ring_mirror_list)) {
> -		struct drm_sched_job *job;
> +		return NULL;
>   
> -		job = list_first_entry(&sched->ring_mirror_list,
> +	job = list_first_entry_or_null(&sched->ring_mirror_list,
>   				       struct drm_sched_job, node);
> -		if (!dma_fence_is_signaled(&job->s_fence->finished))
> -			break;
>   
> -		spin_lock_irqsave(&sched->job_list_lock, flags);
> +	spin_lock_irqsave(&sched->job_list_lock, flags);
> +
> +	if (job && dma_fence_is_signaled(&job->s_fence->finished)) {
>   		/* remove job from ring_mirror_list */
>   		list_del_init(&job->node);
> -		spin_unlock_irqrestore(&sched->job_list_lock, flags);
> -
> -		sched->ops->free_job(job);
> +	} else {
> +		job = NULL;
> +		/* queue timeout for next job */
> +		drm_sched_start_timeout(sched);
>   	}
>   
> -	/* queue timeout for next job */
> -	spin_lock_irqsave(&sched->job_list_lock, flags);
> -	drm_sched_start_timeout(sched);
>   	spin_unlock_irqrestore(&sched->job_list_lock, flags);
>   
> +	return job;
>   }
>   
>   /**
> @@ -698,12 +696,18 @@ static int drm_sched_main(void *param)
>   		struct drm_sched_fence *s_fence;
>   		struct drm_sched_job *sched_job;
>   		struct dma_fence *fence;
> +		struct drm_sched_job *cleanup_job = NULL;
>   
>   		wait_event_interruptible(sched->wake_up_worker,
> -					 (drm_sched_cleanup_jobs(sched),
> +					 (cleanup_job = drm_sched_get_cleanup_job(sched)) ||
>   					 (!drm_sched_blocked(sched) &&
>   					  (entity = drm_sched_select_entity(sched))) ||
> -					 kthread_should_stop()));
> +					 kthread_should_stop());


Can't we just call drm_sched_cleanup_jobs right here, remove all the 
conditions in wait_event_interruptible (make it always true) and after 
drm_sched_cleanup_jobs is called test for all those conditions and 
return to sleep if they evaluate to false ? drm_sched_cleanup_jobs is 
called unconditionally inside wait_event_interruptible anyway... This is 
more of a question to Christian.

Andrey


> +
> +		while (cleanup_job) {
> +			sched->ops->free_job(cleanup_job);
> +			cleanup_job = drm_sched_get_cleanup_job(sched);
> +		}
>   
>   		if (!entity)
>   			continue;