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

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

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.

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:
>>