[Intel-gfx] [PATCH 08/15] drm/i915: Slaughter the thundering i915_wait_request herd

[Chris Wilson]

On Sun, Nov 29, 2015 at 08:48:06AM +0000, Chris Wilson wrote:
> +	/* Optimistic spin for the next jiffie before touching IRQs */
> +	if (intel_breadcrumbs_add_waiter(req)) {
> +		ret = __i915_spin_request(req, state);
> +		if (ret == 0)
> +			goto out;

There are a couple of interesting side-effects here.  As we know start
up the irq in parallel and keep it running for longer, irq/i915 now
consumes a lot of CPU time (like 50-100%!) for synchronous batches, but
doesn't seem to interfere with latency (as spin_request is still nicely
running and catching the request completion). That should also still
work nicely on single cpu machines as the irq enabling should not
preempt us.  The second interesting side-effect is that the synchronous
loads that regressed with a 2us spin-request timeout are now happy again
at 2us. Also with the active-request and the can-spin check from
add_waiter, running a low fps game with a compositor is not burning the
CPU with any spinning.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre