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SubjectRe: [PATCH v3] drm: Don't free jobs in wait_event_interruptible()
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On 26/09/2019 14:37, Koenig, Christian wrote:
> Am 26.09.19 um 14:31 schrieb Steven Price:
>> 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. Unfortuantly some free callbacks (notibly 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@arm.com>
>> ---
>>
>> Changes from v2:
>> * Actually move list_first_entry_or_null() within the lock
>>
>> drivers/gpu/drm/scheduler/sched_main.c | 42 ++++++++++++++------------
>> 1 file changed, 23 insertions(+), 19 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>> index 9a0ee74d82dc..e4bd792f2b29 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;
>> -
>> + return NULL;
>>
>> - while (!list_empty(&sched->ring_mirror_list)) {
>> - struct drm_sched_job *job;
>> + spin_lock_irqsave(&sched->job_list_lock, flags);
>>
>> - 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);
>> + 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);
>
> Sorry I've only seen this right now: This won't work.
>
> See you always need to start the timeout, or cancel_delayed_work() will
> abort the whole thing after returning the first job.

Good spot!

My logic was that drm_sched_main() would call again and that would
re-arm the timeout after the job has been completed.

But looking at this more closely I see that isn't actually going to work
reliably - if the work has been cancelled the first time then
cancel_delayed_work() will return false causing an early out and the
timeout isn't re-armed.

Is it as simple as moving drm_sched_start_timeout() out of the else case
and unconditionally re-arming the timeout? I'm worried about races with
the job that is being returned, although it has already been removed
from ring_mirror_list so perhaps it's safe to do?

Alternatively the caller could manually re-arm the timeout after
handling the job free.

Steve

> Christian.
>
>> }
>>
>> - /* 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());
>> +
>> + while (cleanup_job) {
>> + sched->ops->free_job(cleanup_job);
>> + cleanup_job = drm_sched_get_cleanup_job(sched);
>> + }
>>
>> if (!entity)
>> continue;
>
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> dri-devel@lists.freedesktop.org
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