linux-kernel - Re: [PATCH] sched: recover sched_yield task running time increase

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Date:	Wed, 06 Apr 2011 09:01:29 +0200
From:	Mike Galbraith <efault@....de>
To:	"Alex,Shi" <alex.shi@...el.com>
Cc:	Rik van Riel <riel@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"a.p.zijlstra@...llo.nl" <a.p.zijlstra@...llo.nl>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"Chen, Tim C" <tim.c.chen@...el.com>,
	"Li, Shaohua" <shaohua.li@...el.com>
Subject: Re: [PATCH] sched: recover sched_yield task running time increase

On Wed, 2011-04-06 at 14:15 +0800, Alex,Shi wrote:
> On Wed, 2011-04-06 at 13:07 +0800, Rik van Riel wrote:
> > On 04/05/2011 06:33 PM, Alex Shi wrote:
> > > commit ac53db596cc08ecb8040c removed the sched_yield task running
> > > time increase, so the yielded task get more opportunity to be launch
> > > again. That may not the caller want to be. And this also causes
> > > volano benchmark drop 50~80 percent performance on core2/NHM/WSM
> > > machines. This patch recover the sched_yield task vruntime up.
> > >
> > > Signed-off-by: alex.shi@...el.com
> > 
> > NACK
> > 
> > This was switched off by default and under
> > the sysctl sched_compat_yield for a reason.
> > 
> > Reintroducing it under that sysctl option
> > may be acceptable, but by default it would
> > be doing the wrong thing for other workloads.
> 
> I can implement this as sysctl option. But when I checked again the man
> page of sched_yield. I have some concerns on this. 
> 
> ----
>        int sched_yield(void);
> 
> DESCRIPTION
>        A  process  can  relinquish  the processor voluntarily without blocking by calling sched_yield().
>        The process will then be moved to the end of the queue for its static priority and a new  process
>        gets to run.
> ----
> 
> If a application calls sched_yield system call, most of time it is not
> want to be launched again right now. so the man page said "the caller
> process will then be moved to the _end_ of the queue..."

Moving a yielding nice 0 task behind a SCHED_IDLE (or nice 19) task
could be incredibly painful.

	-Mike

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