Re: [PATCH 2/2] drm/amdgpu: Use mod_delayed_work in JPEG/UVD/VCE/VCN ring_end_use hooks

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

Am 12.08.21 um 10:11 schrieb Michel Dänzer:
> On 2021-08-12 7:55 a.m., Koenig, Christian wrote:
>> Hi James,
>>
>> Evan seems to have understood how this all works together.
>>
>> See while any begin/end use critical section is active the work should not be active.
>>
>> When you handle only one ring you can just call cancel in begin use and schedule in end use. But when you have more than one ring you need a lock or counter to prevent concurrent work items to be started.
>>
>> Michelle's idea to use mod_delayed_work is a bad one because it assumes that the delayed work is still running.
> It merely assumes that the work may already have been scheduled before.
>
> Admittedly, I missed the cancel_delayed_work_sync calls for patch 2. While I think it can still have some effect when there's a single work item for multiple rings, as described by James, it's probably negligible, since presumably the time intervals between ring_begin_use and ring_end_use are normally much shorter than a second.
>
> So, while patch 2 is at worst a no-op (since mod_delayed_work is the same as schedule_delayed_work if the work hasn't been scheduled yet), I'm fine with dropping it.

Yeah, I think that would be much better.

>> Something similar applies to the first patch I think,
> There are no cancel work calls in that case, so the commit log is accurate TTBOMK.
>
> I noticed this because current mutter Git main wasn't able to sustain 60 fps on Navi 14 with a simple glxgears -fullscreen. mutter was dropping frames because its CPU work for a frame update occasionally took up to 3 ms, instead of the normal 2-300 microseconds. sysprof showed a lot of cycles spent in the functions which enable/disable GFXOFF in the HW.
>
>
>> so when this makes a difference it is actually a bug.
> There was certainly a bug though, which patch 1 fixes. :)

Agreed, just wanted to note that this is most likely not the right 
solution since Alex was already picking it up.

Going to reply separately on the new patch as well.

Regards,
Christian.