linux-kernel - Re: [PATCH] sched: Initialize rq->age

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Date:	Tue, 18 Mar 2014 09:56:12 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	minyard@....org, linux-kernel@...r.kernel.org,
	Corey Minyard <cminyard@...sta.com>
Subject: Re: [PATCH] sched: Initialize rq->age_stamp on processor start

On Tue, Mar 18, 2014 at 05:06:26AM +0100, Mike Galbraith wrote:
> CC maintainer improves patch aerodynamics.

hehe, for sure. I have very little time to look at lkml these days and
there's a near 100% chance I'll miss anything that doesn't hit the
inbox.

> On Mon, 2014-03-17 at 19:05 -0500, minyard@....org wrote: 
> > From: Corey Minyard <cminyard@...sta.com>
> > 
> > If the sched_clock time starts at a large value, the kernel will spin
> > in sched_avg_update for a long time while rq->age_stamp catches up
> > with rq->clock.
> > 
> > The comment in kernel/sched/clock.c says that there is no strict promise
> > that it starts at zero.  So initialize rq->age_stamp when a cpu starts up
> > to avoid this.
> > 
> > I was seeing long delays on a simulator that didn't start the clock at
> > zero.  This might also be an issue on reboots on processors that don't
> > re-initialize the timer to zero on reset, and when using kexec.
> > 
> > Signed-off-by: Corey Minyard <cminyard@...sta.com>
> > ---
> >  kernel/sched/core.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> > 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index b46131e..5be3d4a 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -5037,11 +5037,20 @@ static struct notifier_block migration_notifier = {
> >  	.priority = CPU_PRI_MIGRATION,
> >  };
> >  
> > +static void __cpuinit set_cpu_rq_start_time(void)
> > +{
> > +	int cpu = smp_processor_id();
> > +	struct rq *rq = cpu_rq(cpu);
> > +	rq->age_stamp = sched_clock_cpu(cpu);
> > +}
> 
> rq->age_stamp must lag rq->clock.  See scale_rt_power(), and what
> happens when it munches magic timewarp mushrooms.
> 
> > +
> >  static int sched_cpu_active(struct notifier_block *nfb,
> >  				      unsigned long action, void *hcpu)
> >  {
> >  	switch (action & ~CPU_TASKS_FROZEN) {
> >  	case CPU_STARTING:
> > +		set_cpu_rq_start_time();
> > +		/* fall through */
> >  	case CPU_DOWN_FAILED:
> >  		set_cpu_active((long)hcpu, true);
> >  		return NOTIFY_OK;
> > @@ -6922,6 +6931,7 @@ void __init sched_init(void)
> >  	init_sched_fair_class();
> >  
> >  	scheduler_running = 1;
> > +	set_cpu_rq_start_time();

I would put it one line up; that scheduler_running=1 is the last thing
we should do.
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