[Intel-gfx] [PATCH] drm/i915/gt: Drain the breadcrumbs just once

[Matthew Brost]

On Thu, Dec 17, 2020 at 09:15:24AM +0000, Chris Wilson wrote:
> Matthew Brost pointed out that the while-loop on a shared breadcrumb was
> inherently fraught with danger as it competed with the other users of
> the breadcrumbs. However, in order to completely drain the re-arming irq
> worker, the while-loop is a necessity, despite my optimism that we could
> force cancellation with a couple of irq_work invocations.
> 
> Given that we can't merely drop the while-loop, use an activity counter on
> the breadcrumbs to detect when we are parking the breadcrumbs for the
> last time.
> 
> Based on a patch by Matthew Brost.
> 

This was more or less exactly what I was going to suggest.
Reviewed-by: Matthew Brost <matthew.brost at intel.com>

> Reported-by: Matthew Brost <matthew.brost at intel.com>
> Suggested-by: Matthew Brost <matthew.brost at intel.com>
> Fixes: 9d5612ca165a ("drm/i915/gt: Defer enabling the breadcrumb interrupt to after submission")
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Matthew Brost <matthew.brost at intel.com>
> ---
>  drivers/gpu/drm/i915/gt/intel_breadcrumbs.c       | 10 ++++++----
>  drivers/gpu/drm/i915/gt/intel_breadcrumbs.h       | 13 ++++++++++++-
>  drivers/gpu/drm/i915/gt/intel_breadcrumbs_types.h |  6 ++++--
>  drivers/gpu/drm/i915/gt/intel_engine_pm.c         |  1 +
>  4 files changed, 23 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> index 00918300f53f..3c62fd6daa76 100644
> --- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> +++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> @@ -332,17 +332,19 @@ void intel_breadcrumbs_reset(struct intel_breadcrumbs *b)
>  	spin_unlock_irqrestore(&b->irq_lock, flags);
>  }
>  
> -void intel_breadcrumbs_park(struct intel_breadcrumbs *b)
> +void __intel_breadcrumbs_park(struct intel_breadcrumbs *b)
>  {
> -	/* Kick the work once more to drain the signalers */
> +	if (!READ_ONCE(b->irq_armed))
> +		return;
> +
> +	/* Kick the work once more to drain the signalers, and disarm the irq */
>  	irq_work_sync(&b->irq_work);
> -	while (unlikely(READ_ONCE(b->irq_armed))) {
> +	while (READ_ONCE(b->irq_armed) && !atomic_read(&b->active)) {
>  		local_irq_disable();
>  		signal_irq_work(&b->irq_work);
>  		local_irq_enable();
>  		cond_resched();
>  	}
> -	GEM_BUG_ON(!list_empty(&b->signalers));
>  }
>  
>  void intel_breadcrumbs_free(struct intel_breadcrumbs *b)
> diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.h b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.h
> index ed3d1deabfbd..75cc9cff3ae3 100644
> --- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.h
> +++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.h
> @@ -19,7 +19,18 @@ intel_breadcrumbs_create(struct intel_engine_cs *irq_engine);
>  void intel_breadcrumbs_free(struct intel_breadcrumbs *b);
>  
>  void intel_breadcrumbs_reset(struct intel_breadcrumbs *b);
> -void intel_breadcrumbs_park(struct intel_breadcrumbs *b);
> +void __intel_breadcrumbs_park(struct intel_breadcrumbs *b);
> +
> +static inline void intel_breadcrumbs_unpark(struct intel_breadcrumbs *b)
> +{
> +	atomic_inc(&b->active);
> +}
> +
> +static inline void intel_breadcrumbs_park(struct intel_breadcrumbs *b)
> +{
> +	if (atomic_dec_and_test(&b->active))
> +		__intel_breadcrumbs_park(b);
> +}
>  
>  static inline void
>  intel_engine_signal_breadcrumbs(struct intel_engine_cs *engine)
> diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs_types.h b/drivers/gpu/drm/i915/gt/intel_breadcrumbs_types.h
> index a74bb3062bd8..d85a6f74fb87 100644
> --- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs_types.h
> +++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs_types.h
> @@ -29,8 +29,7 @@
>   * the overhead of waking that client is much preferred.
>   */
>  struct intel_breadcrumbs {
> -	/* Not all breadcrumbs are attached to physical HW */
> -	struct intel_engine_cs *irq_engine;
> +	atomic_t active;
>  
>  	spinlock_t signalers_lock; /* protects the list of signalers */
>  	struct list_head signalers;
> @@ -40,6 +39,9 @@ struct intel_breadcrumbs {
>  	struct irq_work irq_work; /* for use from inside irq_lock */
>  	unsigned int irq_enabled;
>  	bool irq_armed;
> +
> +	/* Not all breadcrumbs are attached to physical HW */
> +	struct intel_engine_cs *irq_engine;
>  };
>  
>  #endif /* __INTEL_BREADCRUMBS_TYPES__ */
> diff --git a/drivers/gpu/drm/i915/gt/intel_engine_pm.c b/drivers/gpu/drm/i915/gt/intel_engine_pm.c
> index 499b09cb4acf..d74e748f677a 100644
> --- a/drivers/gpu/drm/i915/gt/intel_engine_pm.c
> +++ b/drivers/gpu/drm/i915/gt/intel_engine_pm.c
> @@ -65,6 +65,7 @@ static int __engine_unpark(struct intel_wakeref *wf)
>  	if (engine->unpark)
>  		engine->unpark(engine);
>  
> +	intel_breadcrumbs_unpark(engine->breadcrumbs);
>  	intel_engine_unpark_heartbeat(engine);
>  	return 0;
>  }
> -- 
> 2.20.1
>