[Intel-gfx] [PATCH 13/26] drm/i915: use the new iterator in i915_gem_busy_ioctl

[Christian König]

Am 22.09.21 um 13:46 schrieb Tvrtko Ursulin:
>
> On 22/09/2021 11:21, Tvrtko Ursulin wrote:
>>
>> On 22/09/2021 10:10, Christian König wrote:
>>> This makes the function much simpler since the complex
>>> retry logic is now handled else where.
>>>
>>> Signed-off-by: Christian König <christian.koenig at amd.com>
>>> ---
>>>   drivers/gpu/drm/i915/gem/i915_gem_busy.c | 35 
>>> ++++++++++--------------
>>>   1 file changed, 14 insertions(+), 21 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_busy.c 
>>> b/drivers/gpu/drm/i915/gem/i915_gem_busy.c
>>> index 6234e17259c1..313afb4a11c7 100644
>>> --- a/drivers/gpu/drm/i915/gem/i915_gem_busy.c
>>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_busy.c
>>> @@ -82,8 +82,8 @@ i915_gem_busy_ioctl(struct drm_device *dev, void 
>>> *data,
>>>   {
>>>       struct drm_i915_gem_busy *args = data;
>>>       struct drm_i915_gem_object *obj;
>>> -    struct dma_resv_list *list;
>>> -    unsigned int seq;
>>> +    struct dma_resv_iter cursor;
>>> +    struct dma_fence *fence;
>>>       int err;
>>>       err = -ENOENT;
>>> @@ -109,27 +109,20 @@ i915_gem_busy_ioctl(struct drm_device *dev, 
>>> void *data,
>>>        * to report the overall busyness. This is what the wait-ioctl 
>>> does.
>>>        *
>>>        */
>>> -retry:
>>> -    seq = raw_read_seqcount(&obj->base.resv->seq);
>>> -
>>> -    /* Translate the exclusive fence to the READ *and* WRITE engine */
>>> -    args->busy = 
>>> busy_check_writer(dma_resv_excl_fence(obj->base.resv));
>>> -
>>> -    /* Translate shared fences to READ set of engines */
>>> -    list = dma_resv_shared_list(obj->base.resv);
>>> -    if (list) {
>>> -        unsigned int shared_count = list->shared_count, i;
>>> -
>>> -        for (i = 0; i < shared_count; ++i) {
>>> -            struct dma_fence *fence =
>>> -                rcu_dereference(list->shared[i]);
>>> -
>>> +    args->busy = false;
>>
>> You can drop this line, especially since it is not a boolean. With that:
>>
>> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
>
> Having said this, one thing to add in the commit message is some 
> commentary that although simpler in code, the new implementation has a 
> lot more atomic instructions due all the extra fence get/put.
>
> Saying this because I remembered busy ioctl is quite an over-popular 
> one. Thinking about traces from some real userspaces I looked at in 
> the past.
>
> So I think ack from maintainers will be required here. Because I just 
> don't know if any performance impact will be visible or not. So view 
> my r-b as "code looks fine" but I am on the fence if it should 
> actually be merged. Probably leaning towards no actually - given how 
> the code is localised here and I dislike burdening old platforms with 
> more CPU time it could be cheaply left as is.

Well previously we would have allocated memory, which as far as I know 
has more overhead than a few extra atomic operations.

On the other hand I could as well stick with dma_resv_get_fences() here.

Regards,
Christian.

>
> Regards,
>
> Tvrtko
>
>
>>
>> Regards,
>>
>> Tvrtko
>>
>>> +    dma_resv_iter_begin(&cursor, obj->base.resv, true);
>>> +    dma_resv_for_each_fence_unlocked(&cursor, fence) {
>>> +        if (dma_resv_iter_is_restarted(&cursor))
>>> +            args->busy = 0;
>>> +
>>> +        if (dma_resv_iter_is_exclusive(&cursor))
>>> +            /* Translate the exclusive fence to the READ *and* 
>>> WRITE engine */
>>> +            args->busy |= busy_check_writer(fence);
>>> +        else
>>> +            /* Translate shared fences to READ set of engines */
>>>               args->busy |= busy_check_reader(fence);
>>> -        }
>>>       }
>>> -
>>> -    if (args->busy && read_seqcount_retry(&obj->base.resv->seq, seq))
>>> -        goto retry;
>>> +    dma_resv_iter_end(&cursor);
>>>       err = 0;
>>>   out:
>>>