From 91b0c352ace9afec1fb51590c7b8bd60e0eb9fbd Mon Sep 17 00:00:00 2001 From: Chris Wilson Date: Fri, 11 Dec 2015 11:32:57 +0000 Subject: [PATCH] drm/i915: Break busywaiting for requests on pending signals The busywait in __i915_spin_request() does not respect pending signals and so may consume the entire timeslice for the task instead of returning to userspace to handle the signal. In the worst case this could cause a delay in signal processing of 20ms, which would be a noticeable jitter in cursor tracking. If a higher resolution signal was being used, for example to provide fairness of a server timeslices between clients, we could expect to detect some unfairness between clients (i.e. some windows not updating as fast as others). This issue was noticed when inspecting a report of poor interactivity resulting from excessively high __i915_spin_request usage. Fixes regression from commit 2def4ad99befa25775dd2f714fdd4d92faec6e34 [v4.2] Author: Chris Wilson Date: Tue Apr 7 16:20:41 2015 +0100 drm/i915: Optimistically spin for the request completion v2: Try to assess the impact of the bug Signed-off-by: Chris Wilson Reviewed-by: Tvrtko Ursulin Cc: Jens Axboe Cc; "Rogozhkin, Dmitry V" Cc: Daniel Vetter Cc: Tvrtko Ursulin Cc: Eero Tamminen Cc: "Rantala, Valtteri" Cc: stable@vger.kernel.org Signed-off-by: Daniel Vetter Link: http://patchwork.freedesktop.org/patch/msgid/1449833608-22125-2-git-send-email-chris@chris-wilson.co.uk --- drivers/gpu/drm/i915/i915_gem.c | 13 ++++++++----- 1 file changed, 8 insertions(+), 5 deletions(-) diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c index adc339a5ab09..44f747c78128 100644 --- a/drivers/gpu/drm/i915/i915_gem.c +++ b/drivers/gpu/drm/i915/i915_gem.c @@ -1146,7 +1146,7 @@ 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 *req) +static int __i915_spin_request(struct drm_i915_gem_request *req, int state) { unsigned long timeout; @@ -1158,6 +1158,9 @@ static int __i915_spin_request(struct drm_i915_gem_request *req) if (i915_gem_request_completed(req, true)) return 0; + if (signal_pending_state(state, current)) + break; + if (time_after_eq(jiffies, timeout)) break; @@ -1197,6 +1200,7 @@ int __i915_wait_request(struct drm_i915_gem_request *req, struct drm_i915_private *dev_priv = dev->dev_private; const bool irq_test_in_progress = ACCESS_ONCE(dev_priv->gpu_error.test_irq_rings) & intel_ring_flag(ring); + int state = interruptible ? TASK_INTERRUPTIBLE : TASK_UNINTERRUPTIBLE; DEFINE_WAIT(wait); unsigned long timeout_expire; s64 before, now; @@ -1229,7 +1233,7 @@ int __i915_wait_request(struct drm_i915_gem_request *req, before = ktime_get_raw_ns(); /* Optimistic spin for the next jiffie before touching IRQs */ - ret = __i915_spin_request(req); + ret = __i915_spin_request(req, state); if (ret == 0) goto out; @@ -1241,8 +1245,7 @@ int __i915_wait_request(struct drm_i915_gem_request *req, for (;;) { struct timer_list timer; - prepare_to_wait(&ring->irq_queue, &wait, - interruptible ? TASK_INTERRUPTIBLE : TASK_UNINTERRUPTIBLE); + prepare_to_wait(&ring->irq_queue, &wait, state); /* We need to check whether any gpu reset happened in between * the caller grabbing the seqno and now ... */ @@ -1260,7 +1263,7 @@ int __i915_wait_request(struct drm_i915_gem_request *req, break; } - if (interruptible && signal_pending(current)) { + if (signal_pending_state(state, current)) { ret = -ERESTARTSYS; break; } -- 2.20.1