linux-kernel - Re: __i915_spin

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Date:	Fri, 13 Nov 2015 22:12:12 +0000
From:	Chris Wilson <chris@...is-wilson.co.uk>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Mike Galbraith <umgwanakikbuti@...il.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	DRI Development <dri-devel@...ts.freedesktop.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: __i915_spin_request() sucks

On Fri, Nov 13, 2015 at 09:22:52AM -0700, Jens Axboe wrote:
> On 11/13/2015 09:13 AM, Mike Galbraith wrote:
> >On Fri, 2015-11-13 at 08:36 -0700, Jens Axboe wrote:
> >>Previous patch was obvious pre-coffee crap, this should work for using
> >>ktime to spin max 1usec.
> >
> >1us seems a tad low.  I doubt the little wooden gears and pulleys of my
> >core2 Toshiba Satellite lappy can get one loop ground out in a usec :)
> 
> Maybe it is, it's based off the original intent of the function,
> though. See the original commit referenced.

I've been looking at numbers from one laptop and I can set the timeout
at 2us before we see a steep decline in what is more or less synchronous
request handling (which affects a variety of rendering workloads).

Looking around, other busy loops seem to use local_clock() (i.e. rdstcll
with a fair wind). Is that worth using here?
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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