Re: [Intel-gfx] [PATCH v2] drm/i915/gt: Transfer old virtual breadcrumbs to irq_worker

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

On 12/05/2020 14:22, Chris Wilson wrote:
> The second try at staging the transfer of the breadcrumb. In part one,
> we realised we could not simply move to the second engine as we were
> only holding the breadcrumb lock on the first. So in commit 6c81e21a4742
> ("drm/i915/gt: Stage the transfer of the virtual breadcrumb"), we
> removed it from the first engine and marked up this request to reattach
> the signaling on the new engine. However, this failed to take into
> account that we only attach the breadcrumb if the new request is added
> at the start of the queue, which if we are transferring, it is because
> we know there to be a request to be signaled (and hence we would not be
> attached).
> 
> In this attempt, we try to transfer the completed requests to the
> irq_worker on its rq->engine->breadcrumbs. This preserves the coupling
> between the rq and its breadcrumbs, so that
> i915_request_cancel_breadcrumb() does not attempt to manipulate the list
> under the wrong lock.
> 
> Fixes: 6c81e21a4742 ("drm/i915/gt: Stage the transfer of the virtual breadcrumb")
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> ---
> v2: rewrite from scratch with a new idea
> ---
>   drivers/gpu/drm/i915/gt/intel_breadcrumbs.c  | 33 ++++++++++++++++++++
>   drivers/gpu/drm/i915/gt/intel_engine.h       |  3 ++
>   drivers/gpu/drm/i915/gt/intel_engine_types.h |  2 ++
>   drivers/gpu/drm/i915/gt/intel_lrc.c          | 26 ++-------------
>   4 files changed, 41 insertions(+), 23 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> index cbedba857d43..e09dc162b508 100644
> --- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> +++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> @@ -155,6 +155,8 @@ static void signal_irq_work(struct irq_work *work)
>   	if (b->irq_armed && list_empty(&b->signalers))
>   		__intel_breadcrumbs_disarm_irq(b);
>   
> +	list_splice_init(&b->signaled_requests, &signal);
> +
>   	list_for_each_entry_safe(ce, cn, &b->signalers, signal_link) {
>   		GEM_BUG_ON(list_empty(&ce->signals));
>   
> @@ -255,6 +257,7 @@ void intel_engine_init_breadcrumbs(struct intel_engine_cs *engine)
>   
>   	spin_lock_init(&b->irq_lock);
>   	INIT_LIST_HEAD(&b->signalers);
> +	INIT_LIST_HEAD(&b->signaled_requests);
>   
>   	init_irq_work(&b->irq_work, signal_irq_work);
>   }
> @@ -274,6 +277,36 @@ void intel_engine_reset_breadcrumbs(struct intel_engine_cs *engine)
>   	spin_unlock_irqrestore(&b->irq_lock, flags);
>   }
>   
> +void intel_engine_transfer_breadcrumbs(struct intel_engine_cs *engine,
> +				       struct intel_context *ce)
> +{
> +	struct intel_breadcrumbs *b = &engine->breadcrumbs;
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&b->irq_lock, flags);
> +	if (!list_empty(&ce->signals)) {
> +		struct i915_request *rq, *next;
> +
> +		list_for_each_entry_safe(rq, next, &ce->signals, signal_link) {
> +			GEM_BUG_ON(rq->engine != engine);
> +			GEM_BUG_ON(!i915_request_completed(rq));

Do you remember why the breadcrumbs code uses local __request_completed 
helper?

 From here vvv

> +
> +			clear_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags);
> +			if (!__dma_fence_signal(&rq->fence))
> +				continue;
> +
> +			i915_request_get(rq);
> +			list_add_tail(&rq->signal_link, &b->signaled_requests);

^^^ to here looks like a block which could be shared with signal_irq_work.

> +		}
> +
> +		INIT_LIST_HEAD(&ce->signals);

Hm because list_add and not list_move you can't assert all have been 
unlinked.

> +		list_del_init(&ce->signal_link);
> +
> +		irq_work_queue(&b->irq_work);
> +	}
> +	spin_unlock_irqrestore(&b->irq_lock, flags);
> +}
> +
>   void intel_engine_fini_breadcrumbs(struct intel_engine_cs *engine)
>   {
>   }
> diff --git a/drivers/gpu/drm/i915/gt/intel_engine.h b/drivers/gpu/drm/i915/gt/intel_engine.h
> index cb789c8bf06b..45418f887953 100644
> --- a/drivers/gpu/drm/i915/gt/intel_engine.h
> +++ b/drivers/gpu/drm/i915/gt/intel_engine.h
> @@ -238,6 +238,9 @@ intel_engine_signal_breadcrumbs(struct intel_engine_cs *engine)
>   void intel_engine_reset_breadcrumbs(struct intel_engine_cs *engine);
>   void intel_engine_fini_breadcrumbs(struct intel_engine_cs *engine);
>   
> +void intel_engine_transfer_breadcrumbs(struct intel_engine_cs *engine,
> +				       struct intel_context *ce);
> +
>   void intel_engine_print_breadcrumbs(struct intel_engine_cs *engine,
>   				    struct drm_printer *p);
>   
> diff --git a/drivers/gpu/drm/i915/gt/intel_engine_types.h b/drivers/gpu/drm/i915/gt/intel_engine_types.h
> index c113b7805e65..e20b39eefd79 100644
> --- a/drivers/gpu/drm/i915/gt/intel_engine_types.h
> +++ b/drivers/gpu/drm/i915/gt/intel_engine_types.h
> @@ -377,6 +377,8 @@ struct intel_engine_cs {
>   		spinlock_t irq_lock;
>   		struct list_head signalers;
>   
> +		struct list_head signaled_requests;
> +
>   		struct irq_work irq_work; /* for use from inside irq_lock */
>   
>   		unsigned int irq_enabled;
> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> index 15716e4d6b76..ac32d494b07d 100644
> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> @@ -1821,30 +1821,10 @@ static bool virtual_matches(const struct virtual_engine *ve,
>   	return true;
>   }
>   
> -static void virtual_xfer_breadcrumbs(struct virtual_engine *ve,
> -				     struct i915_request *rq)
> +static void virtual_xfer_breadcrumbs(struct virtual_engine *ve)
>   {
> -	struct intel_engine_cs *old = ve->siblings[0];
> -
>   	/* All unattached (rq->engine == old) must already be completed */

This comments feels a bit out of place now.

> -
> -	spin_lock(&old->breadcrumbs.irq_lock);
> -	if (!list_empty(&ve->context.signal_link)) {
> -		list_del_init(&ve->context.signal_link);
> -
> -		/*
> -		 * We cannot acquire the new engine->breadcrumbs.irq_lock
> -		 * (as we are holding a breadcrumbs.irq_lock already),
> -		 * so attach this request to the signaler on submission.
> -		 * The queued irq_work will occur when we finally drop
> -		 * the engine->active.lock after dequeue.
> -		 */
> -		set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT, &rq->fence.flags);
> -
> -		/* Also transfer the pending irq_work for the old breadcrumb. */
> -		intel_engine_signal_breadcrumbs(rq->engine);
> -	}
> -	spin_unlock(&old->breadcrumbs.irq_lock);
> +	intel_engine_transfer_breadcrumbs(ve->siblings[0], &ve->context);

But isn't ve->siblings[0] the old engine at this point so new target 
engine would have to be explicitly passed in?

>   }
>   
>   #define for_each_waiter(p__, rq__) \
> @@ -2279,7 +2259,7 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
>   									engine);
>   
>   				if (!list_empty(&ve->context.signals))
> -					virtual_xfer_breadcrumbs(ve, rq);
> +					virtual_xfer_breadcrumbs(ve);
>   
>   				/*
>   				 * Move the bound engine to the top of the list
> 

Regards,

Tvrtko
_______________________________________________
Intel-gfx mailing list
Intel-gfx@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/intel-gfx