[PATCH] drm/scheduler: fix race condition in load balancer

[Christian König]

Hi Nirmoy,

Am 15.01.20 um 12:04 schrieb Nirmoy:
> Hi Christian,
>
> On 1/14/20 5:01 PM, Christian König wrote:
>>
>>> Before this patch:
>>>
>>> sched_name     num of many times it got scheduled
>>> =========      ==================================
>>> sdma0          314
>>> sdma1          32
>>> comp_1.0.0     56
>>> comp_1.1.0     0
>>> comp_1.1.1     0
>>> comp_1.2.0     0
>>> comp_1.2.1     0
>>> comp_1.3.0     0
>>> comp_1.3.1     0
>>>
>>> After this patch:
>>>
>>> sched_name     num of many times it got scheduled
>>> =========      ==================================
>>>   sdma1          243
>>>   sdma0          164
>>>   comp_1.0.1     14
>>>   comp_1.1.0     11
>>>   comp_1.1.1     10
>>>   comp_1.2.0     15
>>>   comp_1.2.1     14
>>>   comp_1.3.0     10
>>>   comp_1.3.1     10
>>
>> Well that is still rather nice to have, why does that happen?
>
> I think I know why it happens. At init all entity's rq gets assigned 
> to sched_list[0]. I put some prints to check what we compare in 
> drm_sched_entity_get_free_sched.
>
> It turns out most of the time it compares zero values(num_jobs(0) < 
> min_jobs(0)) so most of the time 1st rq(sdma0, comp_1.0.0) was picked 
> by drm_sched_entity_get_free_sched.

Well that is expected because the unit tests always does 
submission,wait,submission,wait,submission,wait.... So the number of 
jobs in the scheduler becomes zero in between.

> This patch was not correct , had an extra atomic_inc(num_jobs) in 
> drm_sched_job_init. This probably added bit of randomness I think, 
> which helped in better job distribution.

Mhm, that might not be a bad idea after all. We could rename num_jobs 
into something like like score and do a +1 in drm_sched_rq_add_entity() 
and a -1 in drm_sched_rq_remove_entity().

That should have pretty much the effect we want to have.

> I've updated my previous RFC patch which uses time consumed by each 
> sched for load balance with a twist of ignoring previously scheduled 
> sched/rq. Let me know what do you think.

I didn't had time yet to wrap my head around that in detail, but at 
least of hand Luben is right that the locking looks really awkward.

And I would rather like to avoid a larger change like this for a nice to 
have for testing feature.

Regards,
Christian.

>
>
> Regards,
>
> Nirmoy
>
>>
>> Christian.
>>
>>