[RFC 0/4] DRM scheduler fixes, or not, or incorrect kind

[Tvrtko Ursulin]

On 09/09/2024 09:47, Philipp Stanner wrote:
> Hi,
> 
> On Fri, 2024-09-06 at 19:06 +0100, Tvrtko Ursulin wrote:
>> From: Tvrtko Ursulin <tvrtko.ursulin at igalia.com>
>>
>> In a recent conversation with Christian there was a thought that
>> drm_sched_entity_modify_sched() should start using the entity-
>>> rq_lock to be
>> safe against job submission and simultaneous priority changes.
> 
> There are also FIXMEs in gpu_scheduler.h that might be related.

Yes there appears to be a good number of dodgy/unclear areas there.

>> The kerneldoc accompanying that function however is a bit unclear to
>> me. For
>> instance is amdgpu simply doing it wrongly by not serializing the two
>> in the
>> driver? Or is the comment referring to some other race condition than
>> which is
>> of concern in this series?
>>
>> To cut the long story short, first three patches try to fix this race
>> in three
>> places I *think* can manifest in different ways.
>>
>> Last patch is a trivial optimisation I spotted can be easily done.
> 
> I took a look and at least to me it doesn't appear to be that trivial,
> mostly because it takes two locks.

The code does take two locks, but the patch itself does not change any 
of that. It just splits the locked helper out so re-locking is avoided.

> Would you mind branching that out as a separate patch so that the
> series would 100% address bugs?

I wanted to start the series with fixes so backporting will work. If I 
put the optimisation first then fixes will not trivially apply to older 
kernels. Assuming they are correct of course.

Regards,

Tvrtko

> 
> P.	
> 
>>
>> Cc: Christian König <christian.koenig at amd.com>
>> Cc: Alex Deucher <alexander.deucher at amd.com>
>> Cc: Luben Tuikov <ltuikov89 at gmail.com>
>> Cc: Matthew Brost <matthew.brost at intel.com>
>>
>> Tvrtko Ursulin (4):
>>    drm/sched: Add locking to drm_sched_entity_modify_sched
>>    drm/sched: Always wake up correct scheduler in
>>      drm_sched_entity_push_job
>>    drm/sched: Always increment correct scheduler score
>>    drm/sched: Optimise drm_sched_entity_push_job
>>
>>   drivers/gpu/drm/scheduler/sched_entity.c | 17 ++++++++++++-----
>>   drivers/gpu/drm/scheduler/sched_main.c   | 21 ++++++++++++++-------
>>   include/drm/gpu_scheduler.h              |  1 +
>>   3 files changed, 27 insertions(+), 12 deletions(-)
>>
>