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SubjectRe: [PATCH v4] drm: Don't free jobs in wait_event_interruptible()
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On 26/09/2019 16:48, Grodzovsky, Andrey wrote:
>
> On 9/26/19 11:23 AM, Steven Price wrote:
>> On 26/09/2019 16:14, Grodzovsky, Andrey wrote:
>>> On 9/26/19 10:16 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. 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 v3:
>>>> * drm_sched_main() re-arms the timeout for the next job after calling
>>>> free_job()
>>>>
>>>> drivers/gpu/drm/scheduler/sched_main.c | 45 +++++++++++++++-----------
>>>> 1 file changed, 26 insertions(+), 19 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>>>> index 9a0ee74d82dc..148468447ba9 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);
>>>> }
>>>>
>>>> - /* 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 +696,21 @@ 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);
>>>> + /* queue timeout for next job */
>>>> + drm_sched_start_timeout(sched);
>>>> +
>>>> + cleanup_job = drm_sched_get_cleanup_job(sched);
>>>> + }
>>>
>>> Why drm_sched_start_timeout is called both here and inside
>>> drm_sched_get_cleanup_job ? And also why call it multiple times in the
>>> loop instead of only once after the loop is done  ?
>> Christian pointed out to be that the first thing
>> drm_sched_get_cleanup_job does is call cancel_delayed_work(), and if
>> that returns false then it bails out with a NULL return. So to actually
>> get another job (if one exists) the timeout has to be restarted.
>
>
> For this case where timeout work already in progress note that
> drm_sched_job_timedout restarts the timeout in it's end so it should be
> ok to restart the timeout unconditionally inside
> drm_sched_get_cleanup_job as it was done before.

I may have misinterpreted Christian[1], but I interpreted the below as
meaning that he'd prefer the caller (drm_sched_main()) to handle the
drm_sched_start_timeout() in this case:

On 26/09/2019 14:50, Koenig, Christian wrote:
>> Alternatively the caller could manually re-arm the timeout after
>> handling the job free.
>
> I don't see anything that could go wrong immediately, but that is
> probably the cleaner approach.

[1]
https://lore.kernel.org/lkml/d9b23ef8-f784-0bce-c34a-fa02002db1ea@amd.com/

I wasn't entirely sure if it was safe to leave the timeout running while
the job free was going on which would be the case if
drm_sched_get_cleanup_job() re-armed unconditionally.

>>
>> It's also necessary to restart the timeout in the case where the return
>> is NULL which is handled in the function itself.
>>
>> TBH I'm not sure whether this while loop is worth it - it may be better
>> to replace it with simply:
>>
>> if (cleanup_job) {
>> sched->ops->free_job(cleanup_job);
>> /* queue timeout for next job */
>> drm_sched_start_timeout(sched);
>> }
>>
>> The outer loop would then handle the next call to
>> drm_sched_get_cleanup_job() as necessary.
>
>
> What outer loop ?

There's a loop round the code in drm_sched_main():

while (!kthread_should_stop()) {
[...]
wait_event_interruptible(...)

while (cleanup_job) {...}

if (!entity)
continue;

[...]
}

So after handling the cleanup_job case, the outer loop will loop back
round to the wait_event_interruptible() call. One other concern is
whether it's possible for the thread to be signalled to stop before the
clean up has been completed. The while loop I added ensures that all
jobs are actually cleaned up before kthread_should_stop() is checked.

Steve

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