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

[James Zhu]

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!

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