linux-kernel - Re: [tip:sched/core] sched: Keep kthreads at default priority

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Message-Id: <1252757951.6084.5.camel@marge.simson.net>
Date:	Sat, 12 Sep 2009 14:19:11 +0200
From:	Mike Galbraith <efault@....de>
To:	Martin Steigerwald <Martin@...htvoll.de>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>, mingo@...hat.com,
	hpa@...or.com, tglx@...utronix.de, mingo@...e.hu,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/core] sched: Keep kthreads at default priority

On Sat, 2009-09-12 at 13:48 +0200, Martin Steigerwald wrote:
> Am Mittwoch 09 September 2009 schrieb Mike Galbraith:
> > On Wed, 2009-09-09 at 19:06 +0200, Peter Zijlstra wrote:
> > > On Wed, 2009-09-09 at 09:55 -0700, Dmitry Torokhov wrote:
> > > > On Wed, Sep 09, 2009 at 03:37:34PM +0000, tip-bot for Mike Galbraith 
> wrote:
> > > > > diff --git a/kernel/kthread.c b/kernel/kthread.c
> > > > > index eb8751a..5fe7099 100644
> > > > > --- a/kernel/kthread.c
> > > > > +++ b/kernel/kthread.c
> > > > > @@ -16,8 +16,6 @@
> > > > >  #include <linux/mutex.h>
> > > > >  #include <trace/events/sched.h>
> > > > >
> > > > > -#define KTHREAD_NICE_LEVEL (-5)
> > > > > -
> > > >
> > > > Why don't we just redefine it to 0? We may find out later that we'd
> > > > still prefer to have kernel threads have boost.
> > >
> > > Seems sensible, also the traditional reasoning behind this nice level
> > > is that kernel threads do work on behalf of multiple tasks. Its a
> > > kind of prio ceiling thing.
> > 
> > True.  None of our current threads are heavy enough to matter much.
> 
> Does it make sense to have this as a tunable? Where does it matter? Server 
> workloads?

I don't think it should be a knob.  It only makes a difference to
kthreads that are heavy CPU users.  If one pops up as a performance
problem, IMHO, it should be tweaked separately.  Running at default
weight saves a bit of unnecessary math for the common case.

	-Mike

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