Re: [Intel-gfx] [PATCH 17/28] drm/i915: use the new iterator in i915_gem_busy_ioctl v2

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

Am 05.10.21 um 14:40 schrieb Tvrtko Ursulin:
>
> On 05/10/2021 12:37, 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>
>> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>
> Reminder - r-b was retracted until at least more text is added to 
> commit message about pros and cons. But really some discussion had 
> inside the i915 team on the topic.

Sure, going to move those to a different branch.

But I really only see the following options:
1. Grab the lock.
2. Use the _unlocked variant with get/put.
3. Add another _rcu iterator just for this case.

I'm fine with either, but Daniel pretty much already rejected #3 and 
#2/#1 has more overhead then the original one.

Regards,
Christian.

>
> Regards,
>
> Tvrtko
>
>> ---
>>   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..dc72b36dae54 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 = 0;
>> +    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:
>>