linux-kernel - Re: [RFC PATCH] check_preempt_tick should not compare vruntime with wall time

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Message-Id: <1284430252.7386.4.camel@marge.simson.net>
Date:	Tue, 14 Sep 2010 04:10:52 +0200
From:	Mike Galbraith <efault@....de>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tony Lindgren <tony@...mide.com>
Subject: Re: [RFC PATCH] check_preempt_tick should not compare vruntime
 with wall time

On Mon, 2010-09-13 at 12:16 -0400, Mathieu Desnoyers wrote:
> * Peter Zijlstra (peterz@...radead.org) wrote:
> > On Mon, 2010-09-13 at 09:56 -0400, Mathieu Desnoyers wrote:
> [...]
> > > >  static void
> > > >  check_preempt_tick(struct cfs_rq *cfs_rq, struct sched_entity *curr)
> > > >  {
> > > > -     unsigned long ideal_runtime, delta_exec;
> > > > +     unsigned long slice = sched_slice(cfs_rq, curr);
> > > 
> > > So you still compute the sched_slice(), based on sched_period(), based on
> > > sysctl_sched_min_granularity *= nr_running when there are more than nr_latency
> > > running threads.
> > 
> > What's wrong with that? I keep asking you, you keep not giving an
> > answer. Stop focussing on nr_latency, its an by produce not a
> > fundamental entity.
> > 
> >  period := max(latency, min_gran * nr_running)
> > 
> > See, no nr_latency -- the one and only purpose of nr_latency is avoiding
> > that multiplication when possible.
> 
> OK, the long IRC discussions we just had convinced me that the current scheme
> takes things into account by adapting the granularity dynamically, but also got
> me to notice that check_preempt seems to compare vruntime with wall time, which
> is utterly incorrect. So maybe all my patch was doing was to expose this bug:

It's not wall time, it's just a distance.  But I'm not attached to it by
any means, if something else works better, do that :)

	-Mike

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