Subject: Re: [tip:sched/core] sched: Keep kthreads at default priority
From: Mike Galbraith <efault@gmx.de>
To: Martin Steigerwald <Martin@lichtvoll.de>
Cc: linux-kernel@vger.kernel.org, Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Dmitry Torokhov <dmitry.torokhov@gmail.com>, mingo@redhat.com,
       hpa@zytor.com, tglx@linutronix.de, mingo@elte.hu,
       linux-tip-commits@vger.kernel.org
In-Reply-To: <200909121348.38528.Martin@lichtvoll.de>
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	 (sfid-20090909_202324_097104_7D52F860)  <200909121348.38528.Martin@lichtvoll.de>
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Date: Sat, 12 Sep 2009 14:19:11 +0200
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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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