[PATCH] drm/amdkfd: improve performance with XNACK enable

[James Zhu]

On 2025-05-08 11:20, James Zhu wrote:
>
> On 2025-05-08 10:50, Christian König wrote:
>> On 5/8/25 16:46, James Zhu wrote:
>>> When XNACK on, hang or low performance is observed with some test 
>>> cases.
>>> The restoring page process has unexpected stuck during 
>>> evicting/restoring
>>> if some bo's flag has KFD_IOCTL_SVM_FLAG_GPU_ALWAYS_MAPPED setting:
>>> 1. when xnack on, retry pagefault will invoke restoring pages process
>>> 2. A. if there is enough VRAM space, simply migrating pages from ram 
>>> to vram
>>>     B. if there is no enough VRAM space left, searching resource LRU 
>>> list, and
>>>        scheduling a new eviction work queue to evict LRU bo from 
>>> vram to ram
>>>        first, then resume restoring pages process, or waiting for 
>>> eviction
>>>        timeout and try to schedule evicting next LRU bo
>>> 3. for case 2B, if bo has KFD_IOCTL_SVM_FLAG_GPU_ALWAYS_MAPPED setting,
>>>     queue eviction will be triggered.So restoring work queue will be 
>>> scheduled.
>>> 4. step 1, restoring pages process will hold one mm->mmap_lock's 
>>> read until
>>>     restoring pages is completed
>>>     step 2B, evictiion work queue process will hold one 
>>> mm->mmap_lock's read
>>>     until evicting bo is completed
>>>     step 3, restoring work queue process is trying to acquire one 
>>> mm->mmap_lock's
>>>     write after the above two mm->mmap_lock's read are released, and 
>>> in the
>>>     meantime which will block all following mm->mmap_lock's read 
>>> request.
>>> 5. in step 2, if the first eviction bo's size is big enough for step 1
>>>     restoring pages request, everything is fine. if not, which means 
>>> that the
>>>     mm->mmap_lock's read step 1 won't be release right the way. In 
>>> step 3, first
>>>     eviction bo's restoring work queue will compete for 
>>> mm->mmap_lock's write,
>>>     the second and following LRU bo's evictiion work queue will be 
>>> blocked by
>>>     tring to acquire mm->mmap_lock's read until timeout. All 
>>> restoring pages
>>>     process will be stuck here.
>>> Using down_write_trylock to replace mmap_write_lock will help not 
>>> block the
>>> second and following evictiion work queue process.
>>>
>>> Signed-off-by: James Zhu <James.Zhu at amd.com>
>>> ---
>>>   drivers/gpu/drm/amd/amdkfd/kfd_svm.c | 6 +++++-
>>>   1 file changed, 5 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/gpu/drm/amd/amdkfd/kfd_svm.c 
>>> b/drivers/gpu/drm/amd/amdkfd/kfd_svm.c
>>> index 72be6e152e88..5f6ed70559b7 100644
>>> --- a/drivers/gpu/drm/amd/amdkfd/kfd_svm.c
>>> +++ b/drivers/gpu/drm/amd/amdkfd/kfd_svm.c
>>> @@ -1794,7 +1794,11 @@ svm_range_list_lock_and_flush_work(struct 
>>> svm_range_list *svms,
>>>   {
>>>   retry_flush_work:
>>>       flush_work(&svms->deferred_list_work);
>>> -    mmap_write_lock(mm);
>>> +        while (true) {
>>> +                if (down_write_trylock(&(mm->mmap_lock)))
>>> +                        break;
>>> +                schedule();
>>> +        }
>> Oh, stuff like that is usually an absolutely clear NAK from upstream.
>>
>> As far as I know that is not something we can do so easily.
>>
>> Would it be possible to wait for progress instead of calling 
>> schedule() here?
>
> [JZ]  At 1st beginning, I am thinking adding sync with restoring pages 
> done.
>
> but the original restoring work design philosophy is blindly scheduled 
> after certain delay.
>
> the changes with sync may take more time and risk. I would like Felix 
> and Philip give comments
>
> if there is efficient and safe way to fix it. Thanks!
[JZ] BTW, in worse case, mmap_write_lock will fall into 
rwsem_down_write_slowpath(), schedule_preempt_disabled() and schedule();

>
>>
>> Regards,
>> Christian.
>>
>>>         if (list_empty(&svms->deferred_range_list))
>>>           return;