[PATCH 2/2] drm/amdgpu: make an improvement on amdgpu_hmm_range_get_pages

[Christian König]

Am 12.12.23 um 00:43 schrieb Felix Kuehling:
>
> On 2023-12-11 05:38, Christian König wrote:
>> Am 09.12.23 um 00:01 schrieb James Zhu:
>>> Needn't do schedule for each hmm_range_fault, and use cond_resched
>>> to replace schedule.
>>
>> cond_resched() is usually NAKed upstream since it is a NO-OP in most 
>> situations.
>
> That's weird, because https://docs.kernel.org/RCU/stallwarn.html 
> specifically recommends it to resolve RCU stall warnings. I previously 
> told James to use that instead of schedule().

I haven't followed the full discussion and rational about that either, 
could be that the documentation is outdated.

The latest RFC to remove cond_resched() and explaining the background 
can be found here: https://lwn.net/Articles/950581/

Regards,
Christian.

>
> Regards,
>   Felix
>
>
>>
>> IIRC there was even a patch set to completely remove it.
>>
>> Christian.
>>
>>>
>>> Signed-off-by: James Zhu <James.Zhu at amd.com>
>>> ---
>>>   drivers/gpu/drm/amd/amdgpu/amdgpu_hmm.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_hmm.c 
>>> b/drivers/gpu/drm/amd/amdgpu/amdgpu_hmm.c
>>> index b24eb5821fd1..c77c4eceea46 100644
>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_hmm.c
>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_hmm.c
>>> @@ -199,6 +199,7 @@ int amdgpu_hmm_range_get_pages(struct 
>>> mmu_interval_notifier *notifier,
>>>           hmm_range->notifier_seq = mmu_interval_read_begin(notifier);
>>>           r = hmm_range_fault(hmm_range);
>>>           if (unlikely(r)) {
>>> +            cond_resched();
>>>               /*
>>>                * FIXME: This timeout should encompass the retry from
>>>                * mmu_interval_read_retry() as well.
>>> @@ -212,7 +213,6 @@ int amdgpu_hmm_range_get_pages(struct 
>>> mmu_interval_notifier *notifier,
>>>               break;
>>>           hmm_range->hmm_pfns += MAX_WALK_BYTE >> PAGE_SHIFT;
>>>           hmm_range->start = hmm_range->end;
>>> -        schedule();
>>>       } while (hmm_range->end < end);
>>>         hmm_range->start = start;
>>