Re: [PATCH] drm/i915: Optimistically spin for the request completion

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Mar 10, 2015 at 04:14:14PM +0000, Chris Wilson wrote:
> On Tue, Mar 10, 2015 at 04:06:19PM +0000, Chris Wilson wrote:
> > @@ -1235,12 +1257,20 @@ int __i915_wait_request(struct drm_i915_gem_request *req,
> >  	if (ring->id == RCS && INTEL_INFO(dev)->gen >= 6)
> >  		gen6_rps_boost(dev_priv, file_priv);
> >  
> > -	if (!irq_test_in_progress && WARN_ON(!ring->irq_get(ring)))
> > -		return -ENODEV;
> > -
> >  	/* Record current time in case interrupted by signal, or wedged */
> >  	trace_i915_gem_request_wait_begin(req);
> >  	before = ktime_get_raw_ns();
> > +
> > +	/* Optimistic spin before touching IRQs */
> Perhaps iff timeout == NULL, or pass it along and add a
> 
> if (timeout && timeout_after_eq(jiffies, timeout))
> 	break;
> 
> before the cpu_relax()?

I guess the answer for that is asking how many apps use short
opportunistic waits in the frame rendering loop, e.g. to wait for query
results and fall back to the ones from the previous frame if they're not
available?

Also do you have microbenchmark numbers for something midly ridiculous
like a loop of very short batches (enough ofc to cause a bit of delay) and
immediately stalling for them? It's definitely an awesome idea given that
every other lock and sync primitive does it too.
-Daniel
> 
> > +	ret = __i915_spin_request(req);
> > +	if (ret == 0)
> > +		goto out;
> 
> -- 
> Chris Wilson, Intel Open Source Technology Centre
> _______________________________________________
> Intel-gfx mailing list
> Intel-gfx@lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/intel-gfx

-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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