Re: [PATCH 13/26] drm/i915: use the new iterator in i915_gem_busy_ioctl

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

Am 22.09.21 um 14:20 schrieb Tvrtko Ursulin:
>
> On 22/09/2021 13:15, Christian König wrote:
>> 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@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@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.
>
> It doesn't, it only uses dma_resv_excl_fence and dma_resv_shared_list.

Yeah, ok then that's not really an option any more.

I think Daniel and I are totally on the same page that we won't allow 
this RCU dance in the drivers any more.

Regards,
Christian.

>
> Regards,
>
> Tvrtko
>
>> 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:
>>>>>
>>