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SubjectRe: [GIT PULL] sched/core for v2.6.32

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

> On Fri, Sep 11, 2009 at 4:34 PM, Jesper Juhl <jj@chaosbits.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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