linux-kernel - Re: sched_yield: delete sysctl_sched_compat

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Message-Id: <200711301429.15664.nickpiggin@yahoo.com.au>
Date:	Fri, 30 Nov 2007 14:29:15 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>, mingo@...e.hu,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: sched_yield: delete sysctl_sched_compat_yield

On Friday 30 November 2007 14:15, Zhang, Yanmin wrote:
> On Fri, 2007-11-30 at 13:46 +1100, Nick Piggin wrote:
> > On Wednesday 28 November 2007 09:57, Arjan van de Ven wrote:

> > > sounds like a bad idea; volanomark (well, technically the jvm behind
> > > it) is abusing sched_yield() by assuming it does something it really
> > > doesn't do, and as it happens some of the earlier 2.6 schedulers
> > > accidentally happened to behave in a way that was nice for this
> > > benchmark.
> >
> > OK, why is this still happening? Haven't we been asking JVMs to use
> > futexes or posix locking for years and years now? Are there any sane
> > jvms that _don't_ use yield?
>
> I think it's an issue of volanomark (a kind of java application) instead of
> JVM.

volanomark itself and not the jvm is calling sched_yield()? Do we have
any non-toy threaded java apps? (what's JAVA in the kernel-perf tests?)

> > > 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?
>
> There is no, so I suggest to set sched_compat_yield=1 by default.
> If sched_compat_yield=0, kernel almost does nothing but returns. When
> sched_compat_yield=1, it is closer to the meaning of sched_yield man page.

sched_yield() is really only defined for posix realtime scheduling
AFAIK, which talks about priority lists. 

SCHED_OTHER is defined to be a single priority, below the rest of the
realtime priorities. So at first you *might* say that the process
should then be made to run only after all other SCHED_OTHER processes,
however there is no such ordering requirement for SCHED_OTHER
scheduling. The SCHED_OTHER scheduler can run any task at any time.

That said, I think people would *expect* that call be much closer to
the compat behaviour than the current default. And that's definitely
what Linux has done in the past. So there really does need to be a
good reason to change it like this IMO.

> > As you say, for SCHED_OTHER tasks, yield
> > can do almost anything. We may as well do something that isn't a
> > regression...
>
> I just found SCHED_OTHER in man sched_setscheduler. Is it SCHED_NORMAL in
> the latest kernel?

Yes, SCHED_NORMAL is SCHED_OTHER. Don't know why it got renamed...
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