Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752033AbbKONeN (ORCPT ); Sun, 15 Nov 2015 08:34:13 -0500 Received: from mail.fireflyinternet.com ([87.106.93.118]:56268 "EHLO fireflyinternet.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751322AbbKONeJ (ORCPT ); Sun, 15 Nov 2015 08:34:09 -0500 X-Default-Received-SPF: pass (skip=forwardok (res=PASS)) x-ip-name=78.156.65.138; From: Chris Wilson To: Jens Axboe , intel-gfx@lists.freedesktop.org, linux-kernel@vger.kernel.org Cc: dri-devel@lists.freedesktop.org, Chris Wilson , Daniel Vetter , Tvrtko Ursulin , Eero Tamminen , "Rantala, Valtteri" , stable@kernel.vger.org Subject: [PATCH 2/2] drm/i915: Limit the busy wait on requests to 2us not 10ms! Date: Sun, 15 Nov 2015 13:32:44 +0000 Message-Id: <1447594364-4206-2-git-send-email-chris@chris-wilson.co.uk> X-Mailer: git-send-email 2.6.2 In-Reply-To: <1447594364-4206-1-git-send-email-chris@chris-wilson.co.uk> References: <1447594364-4206-1-git-send-email-chris@chris-wilson.co.uk> X-Originating-IP: 78.156.65.138 X-Country: code=GB country="United Kingdom" ip=78.156.65.138 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3392 Lines: 98 When waiting for high frequency requests, the finite amount of time required to set up the irq and wait upon it limits the response rate. By busywaiting on the request completion for a short while we can service the high frequency waits as quick as possible. However, if it is a slow request, we want to sleep as quickly as possible. The tradeoff between waiting and sleeping is roughly the time it takes to sleep on a request, on the order of a microsecond. Based on measurements from big core, I have set the limit for busywaiting as 2 microseconds. The code currently uses the jiffie clock, but that is far too coarse (on the order of 10 milliseconds) and results in poor interactivity as the CPU ends up being hogged by slow requests. To get microsecond resolution we need to use a high resolution timer. The cheapest of which is polling local_clock(), but that is only valid on the same CPU. If we switch CPUs because the task was preempted, we can also use that as an indicator that the system is too busy to waste cycles on spinning and we should sleep instead. __i915_spin_request was introduced in commit 2def4ad99befa25775dd2f714fdd4d92faec6e34 [v4.2] Author: Chris Wilson Date: Tue Apr 7 16:20:41 2015 +0100 drm/i915: Optimistically spin for the request completion Reported-by: Jens Axboe Link: https://lkml.org/lkml/2015/11/12/621 Cc: Jens Axboe Cc; "Rogozhkin, Dmitry V" Cc: Daniel Vetter Cc: Tvrtko Ursulin Cc: Eero Tamminen Cc: "Rantala, Valtteri" Cc: stable@kernel.vger.org --- drivers/gpu/drm/i915/i915_gem.c | 28 +++++++++++++++++++++++++--- 1 file changed, 25 insertions(+), 3 deletions(-) diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c index 740530c571d1..2a88158bd1f7 100644 --- a/drivers/gpu/drm/i915/i915_gem.c +++ b/drivers/gpu/drm/i915/i915_gem.c @@ -1146,14 +1146,36 @@ static bool missed_irq(struct drm_i915_private *dev_priv, return test_bit(ring->id, &dev_priv->gpu_error.missed_irq_rings); } +static u64 local_clock_us(unsigned *cpu) +{ + u64 t; + + *cpu = get_cpu(); + t = local_clock() >> 10; + put_cpu(); + + return t; +} + +static bool busywait_stop(u64 timeout, unsigned cpu) +{ + unsigned this_cpu; + + if (time_after64(local_clock_us(&this_cpu), timeout)) + return true; + + return this_cpu != cpu; +} + static int __i915_spin_request(struct drm_i915_gem_request *req, int state) { - unsigned long timeout; + u64 timeout; + unsigned cpu; if (i915_gem_request_get_ring(req)->irq_refcount) return -EBUSY; - timeout = jiffies + 1; + timeout = local_clock_us(&cpu) + 2; while (!need_resched()) { if (i915_gem_request_completed(req, true)) return 0; @@ -1161,7 +1183,7 @@ static int __i915_spin_request(struct drm_i915_gem_request *req, int state) if (signal_pending_state(state, current)) break; - if (time_after_eq(jiffies, timeout)) + if (busywait_stop(timeout, cpu)) break; cpu_relax_lowlatency(); -- 2.6.2 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/