[Intel-gfx] [PATCH 3/3] drm/i915: Fix premature LRC unpin in GuC mode

[Chris Wilson]

On Wed, Jan 20, 2016 at 01:40:57PM +0000, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> 
> In GuC mode LRC pinning lifetime depends exclusively on the
> request liftime. Since that is terminated by the seqno update
> that opens up a race condition between GPU finishing writing
> out the context image and the driver unpinning the LRC.
> 
> To extend the LRC lifetime we will employ a similar approach
> to what legacy ringbuffer submission does.
> 
> We will start tracking the last submitted context per engine
> and keep it pinned until it is replaced by another one.
> 
> Note that the driver unload path is a bit fragile and could
> benefit greatly from efforts to unify the legacy and exec
> list submission code paths.
> 
> At the moment i915_gem_context_fini has special casing for the
> two which are potentialy not needed, and also depends on
> i915_gem_cleanup_ringbuffer running before itself.
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Issue: VIZ-4277
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Nick Hoath <nicholas.hoath at intel.com>
> ---
> I cannot test this with GuC but it passes BAT with execlists
> and some real world smoke tests.
> ---
>  drivers/gpu/drm/i915/i915_gem_context.c | 4 +++-
>  drivers/gpu/drm/i915/intel_lrc.c        | 7 +++++++
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
> index c25083c78ba7..0b419e165836 100644
> --- a/drivers/gpu/drm/i915/i915_gem_context.c
> +++ b/drivers/gpu/drm/i915/i915_gem_context.c
> @@ -438,7 +438,9 @@ void i915_gem_context_fini(struct drm_device *dev)
>  	for (i = 0; i < I915_NUM_RINGS; i++) {
>  		struct intel_engine_cs *ring = &dev_priv->ring[i];
>  
> -		if (ring->last_context)
> +		if (ring->last_context && i915.enable_execlists)
> +			intel_lr_context_unpin(ring->last_context, ring);
> +		else if (ring->last_context)
>  			i915_gem_context_unreference(ring->last_context);
>  
>  		ring->default_context = NULL;
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index 5c3f57fed916..b8a7e126d6d2 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -918,6 +918,7 @@ int intel_execlists_submission(struct i915_execbuffer_params *params,
>  	struct intel_engine_cs  *ring = params->ring;
>  	struct drm_i915_private *dev_priv = dev->dev_private;
>  	struct intel_ringbuffer *ringbuf = params->ctx->engine[ring->id].ringbuf;
> +	struct intel_context    *ctx = params->request->ctx;
>  	u64 exec_start;
>  	int instp_mode;
>  	u32 instp_mask;
> @@ -982,6 +983,12 @@ int intel_execlists_submission(struct i915_execbuffer_params *params,
>  
>  	trace_i915_gem_ring_dispatch(params->request, params->dispatch_flags);
>  
> +	if (ring->last_context && ring->last_context != ctx) {
> +		intel_lr_context_unpin(ring->last_context, ring);
> +		intel_lr_context_pin(ctx, ring);
> +		ring->last_context = ctx;
> +	}

I think this is the wrong location and should be part of submitting the
context inside the engine (because intel_execlists_submission should not
as it is entirely duplicating the common GEM batch submision code and
the unique part is engine->add_request()).

Note that it should be:

if (engine->last_context != request->ctx) {
	if (engine->last_context)
		intel_lr_context_unpin(engine->last_context, engine);
	engine->last_context = request->ctx;
	intel_lr_context_pin(engine->last_context, engine);
}
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre