Date: Mon, 16 Nov 2015 11:22:38 +0000
From: Chris Wilson <chris@chris-wilson.co.uk>
To: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Jens Axboe <axboe@kernel.dk>, intel-gfx@lists.freedesktop.org,
        linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Eero Tamminen <eero.t.tamminen@intel.com>,
        "Rantala, Valtteri" <valtteri.rantala@intel.com>,
        stable@kernel.vger.org
Subject: Re: [PATCH 1/2] drm/i915: Break busywaiting for requests on pending
 signals
Message-ID: <20151116112238.GR569@nuc-i3427.alporthouse.com>
Mail-Followup-To: Chris Wilson <chris@chris-wilson.co.uk>,
	Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>,
	Jens Axboe <axboe@kernel.dk>, intel-gfx@lists.freedesktop.org,
	linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org,
	Daniel Vetter <daniel.vetter@ffwll.ch>,
	Eero Tamminen <eero.t.tamminen@intel.com>,
	"Rantala, Valtteri" <valtteri.rantala@intel.com>,
	stable@kernel.vger.org
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 <5649A7C2.90206@linux.intel.com>
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On Mon, Nov 16, 2015 at 09:54:10AM +0000, Tvrtko Ursulin wrote:
> 
> Hi,
> 
> On 15/11/15 13:32, Chris Wilson wrote:
> >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.
> 
> Obviously correct to break the spin, but if spending a jiffie to
> react to signals was the only problem then it is not too severe.
> 
> Add something to the commit message about how it was found/reported
> and about the severity of impact, etc?

Perhaps:

At 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. This issue was noticed when inspecting a
report of poor interactivity resulting from excessively high
__i915_spin_request usage.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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