[PATCH 3/5] dma-fence: Add a single fence fast path for fence merging

[Tvrtko Ursulin]

On 09/01/2025 15:47, Christian König wrote:
> And pushed to drm-misc-next.
> 
> Sorry I'm still catching up from the holidays,

Thank you! I afforded myself an extra ping with all this talk about 
unreliable/corrupt email.

Regards,

Tvrtko

> Am 09.01.25 um 11:53 schrieb Tvrtko Ursulin:
>>
>> Christian - it looks this patch could be merged now.
>>
>> Thanks,
>>
>> Tvrtko
>>
>> On 15/11/2024 10:21, Tvrtko Ursulin wrote:
>>> From: Tvrtko Ursulin <tvrtko.ursulin at igalia.com>
>>>
>>> Testing some workloads in two different scenarios, such as games running
>>> under Gamescope on a Steam Deck, or vkcube under a Plasma desktop, shows
>>> that in a significant portion of calls the dma_fence_unwrap_merge helper
>>> is called with just a single unsignalled fence.
>>>
>>> Therefore it is worthile to add a fast path for that case and so bypass
>>> the memory allocation and insertion sort attempts.
>>>
>>> Tested scenarios:
>>>
>>> 1) Hogwarts Legacy under Gamescope
>>>
>>> ~1500 calls per second to __dma_fence_unwrap_merge.
>>>
>>> Percentages per number of fences buckets, before and after checking for
>>> signalled status, sorting and flattening:
>>>
>>>     N       Before      After
>>>     0       0.85%
>>>     1      69.80%        ->   The new fast path.
>>>    2-9     29.36%        9%   (Ie. 91% of this bucket flattened to 1 
>>> fence)
>>>   10-19
>>>   20-40
>>>    50+
>>>
>>> 2) Cyberpunk 2077 under Gamescope
>>>
>>> ~2400 calls per second.
>>>
>>>     N       Before      After
>>>     0       0.71%
>>>     1      52.53%        ->    The new fast path.
>>>    2-9     44.38%      50.60%  (Ie. half resolved to a single fence)
>>>   10-19     2.34%
>>>   20-40     0.06%
>>>    50+
>>>
>>> 3) vkcube under Plasma
>>>
>>> 90 calls per second.
>>>
>>>     N       Before      After
>>>     0
>>>     1
>>>    2-9      100%         0%   (Ie. all resolved to a single fence)
>>>   10-19
>>>   20-40
>>>    50+
>>>
>>> In the case of vkcube all invocations in the 2-9 bucket were actually
>>> just two input fences.
>>>
>>> v2:
>>>   * Correct local variable name and hold on to unsignaled reference. 
>>> (Chistian)
>>>
>>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at igalia.com>
>>> Cc: Christian König <christian.koenig at amd.com>
>>> Cc: Friedrich Vock <friedrich.vock at gmx.de>
>>> ---
>>>   drivers/dma-buf/dma-fence-unwrap.c | 11 ++++++++++-
>>>   1 file changed, 10 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/dma-buf/dma-fence-unwrap.c 
>>> b/drivers/dma-buf/dma-fence-unwrap.c
>>> index 6345062731f1..2a059ac0ed27 100644
>>> --- a/drivers/dma-buf/dma-fence-unwrap.c
>>> +++ b/drivers/dma-buf/dma-fence-unwrap.c
>>> @@ -84,8 +84,8 @@ struct dma_fence *__dma_fence_unwrap_merge(unsigned 
>>> int num_fences,
>>>                          struct dma_fence **fences,
>>>                          struct dma_fence_unwrap *iter)
>>>   {
>>> +    struct dma_fence *tmp, *unsignaled = NULL, **array;
>>>       struct dma_fence_array *result;
>>> -    struct dma_fence *tmp, **array;
>>>       ktime_t timestamp;
>>>       int i, j, count;
>>>   @@ -94,6 +94,8 @@ struct dma_fence 
>>> *__dma_fence_unwrap_merge(unsigned int num_fences,
>>>       for (i = 0; i < num_fences; ++i) {
>>>           dma_fence_unwrap_for_each(tmp, &iter[i], fences[i]) {
>>>               if (!dma_fence_is_signaled(tmp)) {
>>> +                dma_fence_put(unsignaled);
>>> +                unsignaled = dma_fence_get(tmp);
>>>                   ++count;
>>>               } else {
>>>                   ktime_t t = dma_fence_timestamp(tmp);
>>> @@ -107,9 +109,16 @@ struct dma_fence 
>>> *__dma_fence_unwrap_merge(unsigned int num_fences,
>>>       /*
>>>        * If we couldn't find a pending fence just return a private 
>>> signaled
>>>        * fence with the timestamp of the last signaled one.
>>> +     *
>>> +     * Or if there was a single unsignaled fence left we can return it
>>> +     * directly and early since that is a major path on many workloads.
>>>        */
>>>       if (count == 0)
>>>           return dma_fence_allocate_private_stub(timestamp);
>>> +    else if (count == 1)
>>> +        return unsignaled;
>>> +
>>> +    dma_fence_put(unsignaled);
>>>         array = kmalloc_array(count, sizeof(*array), GFP_KERNEL);
>>>       if (!array)
>