Re: [PATCH v4 5/6] drm/sched: Use parent fence instead of finished

Christian König
2022-09-29T14:58:13Z

Am 29.09.22 um 16:53 schrieb Steven Price:
> On 14/09/2022 17:43, Arvind Yadav wrote:
>> Using the parent fence instead of the finished fence
>> to get the job status. This change is to avoid GPU
>> scheduler timeout error which can cause GPU reset.
> I'm able to reproduce crashes on Panfrost and I believe this commit is
> the cause. Specifically it's possible for job->s_fence->parent to be NULL.
>
> The underlying issue seems to involve drm_sched_resubmit_jobs_ext() - if
> the run_jobs() callback returns an error it will set s_fence->parent to
> NULL after signalling s_fence->finished:
>
>> fence = sched->ops->run_job(s_job);
>> i++;
>>
>> if (IS_ERR_OR_NULL(fence)) {
>> if (IS_ERR(fence))
>> dma_fence_set_error(&s_fence->finished, PTR_ERR(fence));
>>
>> s_job->s_fence->parent = NULL;
> I don't understand the reasoning behind this change, but it doesn't seem
> right to be using the parent fence when we have code which can be
> setting that pointer to NULL.
>
> Since I don't understand the reasoning my only suggestion is to revert
> this patch (and potentially the dependent patch "dma-buf: Check status
> of enable-signaling bit on debug"?).
>
> Can anyone suggest a better fix?

Well, first of all please absolutely don't use
drm_sched_resubmit_jobs_ext()!

It was an extremely bad idea in amdgpu to approach GPU by re-submitting
jobs and it was an even worse idea to push this into the scheduler.

The design of dma_fence is that you submit that once and *only* once and
then get a result for this submission. If re-submission is desirable it
should be done in userspace or at least higher levels.

Apart from that, yes a NULL check is missing here but that should be
trivial to fix.

Thanks,
Christian.

>
> Thanks,
>
> Steve
>
>> Signed-off-by: Arvind Yadav <Arvind.Yadav@amd.com>
>> Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
>> ---
>>
>> changes in v1,v2 - Enable signaling for finished fence in sche_main()
>> is removed
>>
>> ---
>> drivers/gpu/drm/scheduler/sched_main.c | 4 ++--
>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
>> index e0ab14e0fb6b..2ac28ad11432 100644
>> --- a/drivers/gpu/drm/scheduler/sched_main.c
>> +++ b/drivers/gpu/drm/scheduler/sched_main.c
>> @@ -829,7 +829,7 @@ drm_sched_get_cleanup_job(struct drm_gpu_scheduler *sched)
>> job = list_first_entry_or_null(&sched->pending_list,
>> struct drm_sched_job, list);
>>
>> - if (job && dma_fence_is_signaled(&job->s_fence->finished)) {
>> + if (job && dma_fence_is_signaled(job->s_fence->parent)) {
>> /* remove job from pending_list */
>> list_del_init(&job->list);
>>
>> @@ -841,7 +841,7 @@ drm_sched_get_cleanup_job(struct drm_gpu_scheduler *sched)
>>
>> if (next) {
>> next->s_fence->scheduled.timestamp =
>> - job->s_fence->finished.timestamp;
>> + job->s_fence->parent->timestamp;
>> /* start TO timer for next job */
>> drm_sched_start_timeout(sched);
>> }