linux-kernel - Re: Make yield_task

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Message-Id: <1203583439.6243.119.camel@lappy>
Date:	Thu, 21 Feb 2008 09:43:59 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	balbir@...ux.vnet.ibm.com
Cc:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peter@...gramming.kicks-ass.net>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Make yield_task_fair more efficient


On Thu, 2008-02-21 at 13:09 +0530, Balbir Singh wrote:

> sched_yield() is supported API

For SCHED_FIFO/SCHED_RR.

>  and also look at http://lkml.org/lkml/2007/9/19/351.

Read on (http://lkml.org/lkml/2007/9/19/371) and find:
        
        The sched_yield() behaviour is actually very well-defined for RT
        tasks (now, whether it's a good interface to use or not is still
        open to debate, but at least it's a _defined_ interface ;), and
        I think we should at least try to approximate that behaviour for
        normal tasks, even if they aren't RT.

sched_yield() just isn't a very useful API except in RT and even there
one can question it.

> I am trying to make sched_yield() efficient
> when compat_sched_yield is turned on (which is most likely), since people will
> want that behaviour (Hint, please read the man-page for sched_yield).

I did, so what? read the POSIX spec - its just plain undefined for
SCHED_OTHER. We already do something 'sensible' for those few broken
applications relying on unspecified behaviour.

> There are
> already several applications using sched_yield(), so they all suffer.

Who is using it, where is their source so we can show its faster to not
use it?

Really, hiding behind closed sores doesn't make it good.


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