Re: [Intel-gfx] [PATCH 1/4] drm/i915/gt: Do not schedule normal requests immediately along virtual

From: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
To: Chris Wilson <chris@chris-wilson.co.uk>, intel-gfx@lists.freedesktop.org
Cc: stable@vger.kernel.org
Subject: Re: [Intel-gfx] [PATCH 1/4] drm/i915/gt: Do not schedule normal requests immediately along virtual
Date: Tue, 26 May 2020 09:29:47 +0100	[thread overview]
Message-ID: <35df1c6a-dc43-b123-c80c-8d1af00ea1b4@linux.intel.com> (raw)
In-Reply-To: <20200525202901.32244-1-chris@chris-wilson.co.uk>


On 25/05/2020 21:28, Chris Wilson wrote:
> When we push a virtual request onto the HW, we update the rq->engine to
> point to the physical engine. A request that is then submitted by the
> user that waits upon the virtual engine, but along the physical engine
> in use, will then see that it is due to be submitted to the same engine
> and take a shortcut (and be queued without waiting for the completion
> fence). However, the virtual request may be preempted (either by higher
> priority users, or by timeslicing) and removed from the physical engine
> to be migrated over to one of its siblings. The dependent normal request
> however is oblivious to the removal of the virtual request and remains
> queued to execute on HW, believing that once it reaches the head of its
> queue all of its predecessors will have completed executing!
> 
> Fixes: 6d06779e8672 ("drm/i915: Load balancing across a virtual engine")
> Testcase: igt/gem_exec_balancer/sliced
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Cc: <stable@vger.kernel.org> # v5.3+
> ---
>   drivers/gpu/drm/i915/i915_request.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> index c282719ad3ac..51588209bddd 100644
> --- a/drivers/gpu/drm/i915/i915_request.c
> +++ b/drivers/gpu/drm/i915/i915_request.c
> @@ -1074,7 +1074,7 @@ i915_request_await_request(struct i915_request *to, struct i915_request *from)
>   			return ret;
>   	}
>   
> -	if (to->engine == from->engine)
> +	if (is_power_of_2(to->execution_mask | READ_ONCE(from->execution_mask)))
>   		ret = i915_sw_fence_await_sw_fence_gfp(&to->submit,
>   						       &from->submit,
>   						       I915_FENCE_GFP);
> 

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

Regards,

Tvrtko