Re: [PATCH] drm/amdgpu: Cancel delayed work when GFXOFF is disabled

["Michel Dänzer" <michel@daenzer.net>]

On 2021-08-13 4:14 p.m., Lazar, Lijo wrote:
> On 8/13/2021 7:04 PM, Michel Dänzer wrote:
>> On 2021-08-13 1:50 p.m., Lazar, Lijo wrote:
>>> On 8/13/2021 3:59 PM, Michel Dänzer wrote:
>>>> From: Michel Dänzer <mdaenzer@redhat.com>
>>>>
>>>> schedule_delayed_work does not push back the work if it was already
>>>> scheduled before, so amdgpu_device_delay_enable_gfx_off ran ~100 ms
>>>> after the first time GFXOFF was disabled and re-enabled, even if GFXOFF
>>>> was disabled and re-enabled again during those 100 ms.
>>>>
>>>> This resulted in frame drops / stutter with the upcoming mutter 41
>>>> release on Navi 14, due to constantly enabling GFXOFF in the HW and
>>>> disabling it again (for getting the GPU clock counter).
>>>>
>>>> To fix this, call cancel_delayed_work_sync when GFXOFF transitions from
>>>> enabled to disabled. This makes sure the delayed work will be scheduled
>>>> as intended in the reverse case.
>>>>
>>>> In order to avoid a deadlock, amdgpu_device_delay_enable_gfx_off needs
>>>> to use mutex_trylock instead of mutex_lock.
>>>>
>>>> v2:
>>>> * Use cancel_delayed_work_sync & mutex_trylock instead of
>>>>     mod_delayed_work.
>>>>
>>>> Signed-off-by: Michel Dänzer <mdaenzer@redhat.com>
>>>> ---
>>>>    drivers/gpu/drm/amd/amdgpu/amdgpu_device.c | 11 ++++++++++-
>>>>    drivers/gpu/drm/amd/amdgpu/amdgpu_gfx.c    | 13 +++++++------
>>>>    drivers/gpu/drm/amd/amdgpu/amdgpu_gfx.h    |  3 +++
>>>>    3 files changed, 20 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
>>>> index f3fd5ec710b6..8b025f70706c 100644
>>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
>>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
>>>> @@ -2777,7 +2777,16 @@ static void amdgpu_device_delay_enable_gfx_off(struct work_struct *work)
>>>>        struct amdgpu_device *adev =
>>>>            container_of(work, struct amdgpu_device, gfx.gfx_off_delay_work.work);
>>>>    -    mutex_lock(&adev->gfx.gfx_off_mutex);
>>>> +    /* mutex_lock could deadlock with cancel_delayed_work_sync in amdgpu_gfx_off_ctrl. */
>>>> +    if (!mutex_trylock(&adev->gfx.gfx_off_mutex)) {
>>>> +        /* If there's a bug which causes amdgpu_gfx_off_ctrl to be called with enable=true
>>>> +         * when adev->gfx.gfx_off_req_count is already 0, we might race with that.
>>>> +         * Re-schedule to make sure gfx off will be re-enabled in the HW eventually.
>>>> +         */
>>>> +        schedule_delayed_work(&adev->gfx.gfx_off_delay_work, AMDGPU_GFX_OFF_DELAY_ENABLE);
>>>> +        return;
>>>
>>> This is not needed and is just creating another thread to contend for mutex.
>>
>> Still not sure what you mean by that. What other thread?
> 
> Sorry, I meant it schedules another workitem and delays GFXOFF enablement further. For ex: if it was another function like gfx_off_status holding the lock at the time of check.
> 
>>
>>> The checks below take care of enabling gfxoff correctly. If it's already in gfx_off state, it doesn't do anything. So I don't see why this change is needed.
>>
>> mutex_trylock is needed to prevent the deadlock discussed before and below.
>>
>> schedule_delayed_work is needed due to this scenario hinted at by the comment:
>>
>> 1. amdgpu_gfx_off_ctrl locks mutex, calls schedule_delayed_work
>> 2. amdgpu_device_delay_enable_gfx_off runs, calls mutex_trylock, which fails
>>
>> GFXOFF would never get re-enabled in HW in this case (until amdgpu_gfx_off_ctrl calls schedule_delayed_work again).
>>
>> (cancel_delayed_work_sync guarantees there's no pending delayed work when it returns, even if amdgpu_device_delay_enable_gfx_off calls schedule_delayed_work)
>>
> 
> I think we need to explain based on the original code before. There is an asssumption here that the only other contention of this mutex is with the gfx_off_ctrl function.

Not really.


> As far as I understand if the work has already started running when schedule_delayed_work is called, it will insert another in the work queue after delay. Based on that understanding I didn't find a problem with the original code.

Original code as in without this patch or the mod_delayed_work patch? If so, the problem is not when the work has already started running. It's that when it hasn't started running yet, schedule_delayed_work doesn't change the timeout for the already scheduled work, so it ends up enabling GFXOFF earlier than intended (and thus at all in scenarios when it's not supposed to).


> [...], there could be cases where it could have gone to gfxoff right after gfx_off_status releases the lock, but it doesn't delaying it further. That would be the case if some other function is also introduced which takes this mutex.

I really don't think we need to worry about amdgpu_get_gfx_off_status, since it's only called from debugfs (and should be very short). If something hits that debugfs file and it causes higher energy consumption, that's a "doctor, it hurts if I do this" kind of problem.

We can worry about future users of the mutex when they show up.


>>>> @@ -569,9 +566,13 @@ void amdgpu_gfx_off_ctrl(struct amdgpu_device *adev, bool enable)
>>>>            adev->gfx.gfx_off_req_count--;
>>>>          if (enable && !adev->gfx.gfx_off_state && !adev->gfx.gfx_off_req_count) {
>>>> -        schedule_delayed_work(&adev->gfx.gfx_off_delay_work, GFX_OFF_DELAY_ENABLE);
>>>> -    } else if (!enable && adev->gfx.gfx_off_state) {
>>>> -        if (!amdgpu_dpm_set_powergating_by_smu(adev, AMD_IP_BLOCK_TYPE_GFX, false)) {
>>>> +        schedule_delayed_work(&adev->gfx.gfx_off_delay_work, AMDGPU_GFX_OFF_DELAY_ENABLE);
>>>> +    } else if (!enable) {
>>>> +        if (adev->gfx.gfx_off_req_count == 1 && !adev->gfx.gfx_off_state)
>>>> +            cancel_delayed_work_sync(&adev->gfx.gfx_off_delay_work);
>>>
>>> This has the deadlock problem as discussed in the other thread.
>>
>> It does not. If amdgpu_device_delay_enable_gfx_off runs while amdgpu_gfx_off_ctrl holds the mutex,
>> mutex_trylock fails and the former bails.
> 
> Ok, but now it creates a case of re-arming the work item from the work.
> 
> TBH I didn't understand the problem on having to use _sync itself and not cancel_delayed_work().
> 
> The edge case you mentioned for a cancel_delayed_work looks like a rare case
> 
> amdgpu_gfx_off_ctrl(disable) gets the lock
> amdgpu_device_delay_enable_gfx_off - waits for the lock
> amdgpu_gfx_off_ctrl(enable) gets the lock again  (this has to be matching call for the previous disable)
> 
> This scenario looks highly improbable as in general we expect some other work that needs to be done done between disable/enable.

At least for the case that started me on this journey (reading the GFX clock counter), that should be very short, just a couple of register reads.

I agree it's highly improbable, I'm trying to make it impossible. :)


-- 
Earthling Michel Dänzer               |               https://redhat.com
Libre software enthusiast             |             Mesa and X developer