On 8/13/2021 8:10 PM, Michel Dänzer wrote:
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@xxxxxxxxxx>
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@xxxxxxxxxx>
---
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).
I meant the original implementation of
amdgpu_device_delay_enable_gfx_off().
If you indeed want to use _sync, there is a small problem with this
implementation also which is roughly equivalent to the original problem
you faced.
amdgpu_gfx_off_ctrl(disable) locks mutex
calls cancel_delayed_work_sync
amdgpu_device_delay_enable_gfx_off already started running
mutex_trylock fails and schedules another one
amdgpu_gfx_off_ctrl(enable)
schedules_delayed_work() - Delay is not extended, it's the same as when
it's rearmed from work item.
Probably, overthinking about the solution. Looking back, mod_ version is
simpler :). May be just delay it further everytime there is a call with
enable instead of doing it only for req_cnt==0?
Thanks,
Lijo
[...], 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. :)
