Re: [Intel-gfx] [PATCH v2 2/2] drm/i915/guc: Don't deadlock busyness stats vs reset

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 02/11/2022 19:21, John.C.Harrison@Intel.com wrote:
> From: John Harrison <John.C.Harrison@Intel.com>
> 
> The engine busyness stats has a worker function to do things like
> 64bit extend the 32bit hardware counters. The GuC's reset prepare
> function flushes out this worker function to ensure no corruption
> happens during the reset. Unforunately, the worker function has an
> infinite wait for active resets to finish before doing its work. Thus
> a deadlock would occur if the worker function had actually started
> just as the reset starts.
> 
> The function being used to lock the reset-in-progress mutex is called
> intel_gt_reset_trylock(). However, as noted it does not follow
> standard 'trylock' conventions and exit if already locked. So rename
> the current _trylock function to intel_gt_reset_lock_interruptible(),
> which is the behaviour it actually provides. In addition, add a new
> implementation of _trylock and call that from the busyness stats
> worker instead.
> 
> v2: Rename existing trylock to interruptible rather than trying to
> preserve the existing (confusing) naming scheme (review comments from
> Tvrtko).
> 
> Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
> ---
>   drivers/gpu/drm/i915/gem/i915_gem_mman.c       |  2 +-
>   drivers/gpu/drm/i915/gt/intel_reset.c          | 18 ++++++++++++++++--
>   drivers/gpu/drm/i915/gt/intel_reset.h          |  1 +
>   .../gpu/drm/i915/gt/uc/intel_guc_submission.c  |  4 +++-
>   4 files changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_mman.c b/drivers/gpu/drm/i915/gem/i915_gem_mman.c
> index e63329bc80659..c29efdef8313a 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_mman.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_mman.c
> @@ -330,7 +330,7 @@ static vm_fault_t vm_fault_gtt(struct vm_fault *vmf)
>   	if (ret)
>   		goto err_rpm;
>   
> -	ret = intel_gt_reset_trylock(ggtt->vm.gt, &srcu);
> +	ret = intel_gt_reset_lock_interruptible(ggtt->vm.gt, &srcu);
>   	if (ret)
>   		goto err_pages;
>   
> diff --git a/drivers/gpu/drm/i915/gt/intel_reset.c b/drivers/gpu/drm/i915/gt/intel_reset.c
> index 3159df6cdd492..24736ebee17c2 100644
> --- a/drivers/gpu/drm/i915/gt/intel_reset.c
> +++ b/drivers/gpu/drm/i915/gt/intel_reset.c
> @@ -1407,15 +1407,19 @@ void intel_gt_handle_error(struct intel_gt *gt,
>   	intel_runtime_pm_put(gt->uncore->rpm, wakeref);
>   }
>   
> -int intel_gt_reset_trylock(struct intel_gt *gt, int *srcu)
> +static int _intel_gt_reset_lock(struct intel_gt *gt, int *srcu, bool retry)
>   {
>   	might_lock(&gt->reset.backoff_srcu);
> -	might_sleep();
> +	if (retry)
> +		might_sleep();
>   
>   	rcu_read_lock();
>   	while (test_bit(I915_RESET_BACKOFF, &gt->reset.flags)) {
>   		rcu_read_unlock();
>   
> +		if (!retry)
> +			return -EBUSY;
> +
>   		if (wait_event_interruptible(gt->reset.queue,
>   					     !test_bit(I915_RESET_BACKOFF,
>   						       &gt->reset.flags)))
> @@ -1429,6 +1433,16 @@ int intel_gt_reset_trylock(struct intel_gt *gt, int *srcu)
>   	return 0;
>   }
>   
> +int intel_gt_reset_trylock(struct intel_gt *gt, int *srcu)
> +{
> +	return _intel_gt_reset_lock(gt, srcu, false);
> +}
> +
> +int intel_gt_reset_lock_interruptible(struct intel_gt *gt, int *srcu)
> +{
> +	return _intel_gt_reset_lock(gt, srcu, true);
> +}
> +
>   void intel_gt_reset_unlock(struct intel_gt *gt, int tag)
>   __releases(&gt->reset.backoff_srcu)
>   {
> diff --git a/drivers/gpu/drm/i915/gt/intel_reset.h b/drivers/gpu/drm/i915/gt/intel_reset.h
> index adc734e673870..25c975b6e8fc0 100644
> --- a/drivers/gpu/drm/i915/gt/intel_reset.h
> +++ b/drivers/gpu/drm/i915/gt/intel_reset.h
> @@ -39,6 +39,7 @@ int __intel_engine_reset_bh(struct intel_engine_cs *engine,
>   void __i915_request_reset(struct i915_request *rq, bool guilty);
>   
>   int __must_check intel_gt_reset_trylock(struct intel_gt *gt, int *srcu);
> +int __must_check intel_gt_reset_lock_interruptible(struct intel_gt *gt, int *srcu);
>   void intel_gt_reset_unlock(struct intel_gt *gt, int tag);
>   
>   void intel_gt_set_wedged(struct intel_gt *gt);
> diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> index 941613be3b9dd..92e514061d20b 100644
> --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> @@ -1401,7 +1401,9 @@ static void guc_timestamp_ping(struct work_struct *wrk)
>   
>   	/*
>   	 * Synchronize with gt reset to make sure the worker does not
> -	 * corrupt the engine/guc stats.
> +	 * corrupt the engine/guc stats. NB: can't actually block waiting
> +	 * for a reset to complete as the reset requires flushing out
> +	 * this worker thread if started. So waiting would deadlock.
>   	 */
>   	ret = intel_gt_reset_trylock(gt, &srcu);
>   	if (ret)

LGTM but I don't remember fully how ping worker and reset interact so 
I'll let Umesh r-b. Like is it okay to skip the ping or we'd need to 
re-schedule it ASAP due wrap issues? Maybe reset makes that pointless, I 
don't remember.

Regards,

Tvrtko