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

[Chris Wilson]

On Wed, Jan 20, 2016 at 02:06:43PM +0000, Tvrtko Ursulin wrote:
> 
> On 20/01/16 13:55, Chris Wilson wrote:
> >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()).
> 
> So into engine->emit_request you are saying? That works just as well
> AFAICS, just making sure I understood correctly.

Oh, yeah you haven't got the unify add_request/emit_request patch...

So gen8_emit_request(), but considering the other patches in flight,
intel_logical_ring_advance_and_submit() will be the ultimate location.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre