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 2def4ad9 [v4.2] Author: Chris Wilson <chris@chris-wilson.co.uk> 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 <chris@chris-wilson.co.uk> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com> Cc: Jens Axboe <axboe@kernel.dk> Cc; "Rogozhkin, Dmitry V" <dmitry.v.rogozhkin@intel.com> Cc: Daniel Vetter <daniel.vetter@ffwll.ch> Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com> Cc: Eero Tamminen <eero.t.tamminen@intel.com> Cc: "Rantala, Valtteri" <valtteri.rantala@intel.com> Cc: stable@vger.kernel.org Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch> Link: http://patchwork.freedesktop.org/patch/msgid/1449833608-22125-2-git-send-email-chris@chris-wilson.co.uk
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