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

[Christian König]

And pushed to drm-misc-next.

Sorry I'm still catching up from the holidays,
Christian.

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)