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

["Lazar, Lijo" <lijo.lazar@amd.com>]

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. 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.

The other problem is amdgpu_get_gfx_off_status() also uses the same 
mutex. So it won't be knowing which thread it would be contending 
against and blindly creates more work items.

> +	}
> +
>   	if (!adev->gfx.gfx_off_state && !adev->gfx.gfx_off_req_count) {
>   		if (!amdgpu_dpm_set_powergating_by_smu(adev, AMD_IP_BLOCK_TYPE_GFX, true))
>   			adev->gfx.gfx_off_state = true;
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_gfx.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_gfx.c
> index a0be0772c8b3..da4c46db3093 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_gfx.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_gfx.c
> @@ -28,9 +28,6 @@
>   #include "amdgpu_rlc.h"
>   #include "amdgpu_ras.h"
>   
> -/* delay 0.1 second to enable gfx off feature */
> -#define GFX_OFF_DELAY_ENABLE         msecs_to_jiffies(100)
> -
>   /*
>    * GPU GFX IP block helpers function.
>    */
> @@ -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.

Thanks,
Lijo

> +		if (adev->gfx.gfx_off_state &&
> +		    !amdgpu_dpm_set_powergating_by_smu(adev, AMD_IP_BLOCK_TYPE_GFX, false)) {
>   			adev->gfx.gfx_off_state = false;
>   
>   			if (adev->gfx.funcs->init_spm_golden) {
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_gfx.h b/drivers/gpu/drm/amd/amdgpu/amdgpu_gfx.h
> index d43fe2ed8116..dcdb505bb7f4 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_gfx.h
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_gfx.h
> @@ -32,6 +32,9 @@
>   #include "amdgpu_rlc.h"
>   #include "soc15.h"
>   
> +/* delay 0.1 second to enable gfx off feature */
> +#define AMDGPU_GFX_OFF_DELAY_ENABLE msecs_to_jiffies(100)
> +
>   /* GFX current status */
>   #define AMDGPU_GFX_NORMAL_MODE			0x00000000L
>   #define AMDGPU_GFX_SAFE_MODE			0x00000001L
>