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

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Date:	Thu, 07 Apr 2011 04:44:30 +0200
From:	Mike Galbraith <efault@....de>
To:	"Shi, Alex" <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 21:28 +0800, Shi, Alex wrote:
> >> > 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.
> 
> Good reminder! Do you have more detailed idea on this?

Other than 'don't do that'?  Nope.  sched_yield() semantics suck.

	-Mike

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