linux-kernel - Re: [GIT PULL] sched/core for v2.6.32

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Message-ID: <20090912054917.GB9420@elte.hu>
Date:	Sat, 12 Sep 2009 07:49:17 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Tony Luck <tony.luck@...el.com>
Cc:	Jesper Juhl <jj@...osbits.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>
Subject: Re: [GIT PULL] sched/core for v2.6.32

* Tony Luck <tony.luck@...el.com> wrote:

> On Fri, Sep 11, 2009 at 4:34 PM, Jesper Juhl <jj@...osbits.net> wrote:
> > That may be so; but most people I've ever talked to about multiple
> > processes, fork, vfork and the like, have mostly assumed child-runs-first.
> > That is just my personal experience.
> > So I get worried when that assumption is made false.
> 
> With multi-core cpus becoming (being?) the norm, almost all 
> systems are SMP now.  So child and parent can surely end up 
> running in parallel very often.  So applications that make 
> assumptions about child running first are going to be frequently 
> surprised.  Aren't they?

We had parent-runs-first briefly, in v2.6.23 - this got changed by 
v2.6.24 - but yes, it did trigger at least one app bug that i 
remember (dont remember which one it was though).

We are almost two years later now - maybe it works fine now.

In any case, as a precaution i made the sched_child_runs_first 
sysctl knob unconditional (previously it was under 
CONFIG_SCHED_DEBUG).

So if an old distro is upgraded with a new kernel (and user-space is 
not updated), it can be worked around by putting this into 
/etc/sysctl.conf:

    kernel.sched_child_runs_first = 1

You are right to suggest that due to SMP and due to the general 
non-determinism of preemption we _never_ made any 'promise' to run 
the child first.

It was a statistical property based on performance considerations - 
and now we flipped it around based on latency and for kbuild 
performance/throughput reasons: Serge Belyshev reported a 7% 
increase on a quad due to this change and i measured a 1.5% 
peak-kbuild performance increase.

So it's worth it for multiple reasons and even in the worst-case 
problems can be worked around easily and without rebooting the 
system.

	Ingo
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