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

[Christian König]

On 5/8/25 19:25, James Zhu wrote:
> 
> 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();

Yeah, but drivers are not allowed to re-implement or even bypass that logic.

Regards,
Christian.

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