linux-kernel - Re: [PATCH RESEND v4] sched/fair: Add advisory flag for borrowing a timeslice

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Message-ID: <20150113112516.GH25256@twins.programming.kicks-ass.net>
Date:	Tue, 13 Jan 2015 12:25:16 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Khalid Aziz <khalid.aziz@...cle.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>, corbet@....net,
	mingo@...hat.com, hpa@...or.com, akpm@...ux-foundation.org,
	rientjes@...gle.com, ak@...ux.intel.com, mgorman@...e.de,
	raistlin@...ux.it, kirill.shutemov@...ux.intel.com,
	atomlin@...hat.com, avagin@...nvz.org, gorcunov@...nvz.org,
	serge.hallyn@...onical.com, athorlton@....com, oleg@...hat.com,
	vdavydov@...allels.com, daeseok.youn@...il.com,
	keescook@...omium.org, yangds.fnst@...fujitsu.com,
	sbauer@....utah.edu, vishnu.ps@...sung.com, axboe@...com,
	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH RESEND v4] sched/fair: Add advisory flag for borrowing a
 timeslice

On Tue, Dec 23, 2014 at 01:46:58PM -0500, Rik van Riel wrote:
> An uncontended futex is taken without ever going into kernel
> space. Adaptive spinning allows short duration futexes to be
> taken without going into kernel space.

The going into kernel is a red herring afaict, a no-op syscall costs ~180
cycles or something like that. So sure you can do a wee spin (~90 cycles
to try and amortize that), but I really doubt that this is the problem.
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