On Tue, Apr 07, 2015 at 04:20:41PM +0100, Chris Wilson wrote:
> This provides a nice boost to mesa in swap bound scenarios (as mesa
> throttles itself to the previous frame and given the scenario that will
> complete shortly). It will also provide a good boost to systems running
> with semaphores disabled and so frequently waiting on the GPU as it
> switches rings. In the most favourable of microbenchmarks, this can
> increase performance by around 15% - though in practice improvements
> will be marginal and rarely noticeable.
>
> v2: Account for user timeouts
> v3: Limit the spinning to a single jiffie (~1us) at most. On an
> otherwise idle system, there is no scheduler contention and so without a
> limit we would spin until the GPU is ready.
> v4: Drop forcewake - the lazy coherent access doesn't require it, and we
> have no reason to believe that the forcewake itself improves seqno
> coherency - it only adds delay.
>
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>
> Cc: Eero Tamminen <eero.t.tamminen@xxxxxxxxx>
> Cc: "Rantala, Valtteri" <valtteri.rantala@xxxxxxxxx>
Eero/Valtterri, do you have perf data for this one?
Thanks, Daniel
> ---
> drivers/gpu/drm/i915/i915_gem.c | 44 +++++++++++++++++++++++++++++++++++------
> 1 file changed, 38 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index c7d9ee2f708a..47650327204e 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -1181,6 +1181,29 @@ static bool missed_irq(struct drm_i915_private *dev_priv,
> return test_bit(ring->id, &dev_priv->gpu_error.missed_irq_rings);
> }
>
> +static int __i915_spin_request(struct drm_i915_gem_request *rq)
> +{
> + unsigned long timeout;
> +
> + if (i915_gem_request_get_ring(rq)->irq_refcount)
> + return -EBUSY;
> +
> + timeout = jiffies + 1;
> + while (!need_resched()) {
> + if (i915_gem_request_completed(rq, true))
> + return 0;
> +
> + if (time_after_eq(jiffies, timeout))
> + break;
> +
> + cpu_relax_lowlatency();
> + }
> + if (i915_gem_request_completed(rq, false))
> + return 0;
> +
> + return -EAGAIN;
> +}
> +
> /**
> * __i915_wait_request - wait until execution of request has finished
> * @req: duh!
> @@ -1225,12 +1248,20 @@ int __i915_wait_request(struct drm_i915_gem_request *req,
> if (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 for the next jiffie before touching IRQs */
> + ret = __i915_spin_request(req);
> + if (ret == 0)
> + goto out;
> +
> + if (!irq_test_in_progress && WARN_ON(!ring->irq_get(ring))) {
> + ret = -ENODEV;
> + goto out;
> + }
> +
> for (;;) {
> struct timer_list timer;
>
> @@ -1279,14 +1310,15 @@ int __i915_wait_request(struct drm_i915_gem_request *req,
> destroy_timer_on_stack(&timer);
> }
> }
> - now = ktime_get_raw_ns();
> - trace_i915_gem_request_wait_end(req);
> -
> if (!irq_test_in_progress)
> ring->irq_put(ring);
>
> finish_wait(&ring->irq_queue, &wait);
>
> +out:
> + now = ktime_get_raw_ns();
> + trace_i915_gem_request_wait_end(req);
> +
> if (timeout) {
> s64 tres = *timeout - (now - before);
>
> --
> 2.1.4
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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