linux-kernel - Re: sched_yield: delete sysctl_sched_compat

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Date:	Fri, 30 Nov 2007 14:02:36 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>, mingo@...e.hu,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: sched_yield: delete sysctl_sched_compat_yield

On Friday 30 November 2007 13:51, Arjan van de Ven wrote:
> On Fri, 30 Nov 2007 13:46:22 +1100
>
> Nick Piggin <nickpiggin@...oo.com.au> wrote:
> > > Todays kernel has a different behavior somewhat (and before people
> > > scream "regression"; sched_yield() behavior isn't really specified
> > > and doesn't make any sense at all, whatever you get is what you
> > > get.... it's pretty much an insane defacto behavior that is
> > > incredibly tied to which decisions the scheduler makes how, and no
> > > app can depend on that
> >
> > It is a performance regression. Is there any reason *not* to use the
> > "compat" yield by default? As you say, for SCHED_OTHER tasks, yield
> > can do almost anything. We may as well do something that isn't a
> > regression..
>
> it just makes OTHER tests/benchmarks regress.... this is one of those
> things where you just can't win.

OK, which ones? Because java is slightly important...


> > > in any way. In fact, I've proposed to make sched_yield() just do an
> > > msleep(1)... that'd be closer to what sched_yield is supposed to do
> > > standard wise than any of the current behaviors .... ;_
> >
> > What makes you say that? IIRC of all the things that sched_yeild can
> > do, it is not allowed to block. So this is about the only thing that
> > will break the standard...
>
> sched_yield OF COURSE can block.. it's a schedule call after all!

In unix, blocking ~= removed from runqueue, no?

OF COURSE it is allowed to cooperatively schedule another task, but
I don't see why you think it should so obviously be allowed to block
/ sleep.

It breaks the basically only invariant of sched_yeild in that the
task will no longer run when there is nothing else running.
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