From: Peter Zijlstra <peterz@infradead.org>
To: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: minyard@acm.org, linux-kernel@vger.kernel.org,
Corey Minyard <cminyard@mvista.com>
Subject: Re: [PATCH] sched: Initialize rq->age_stamp on processor start
Date: Tue, 18 Mar 2014 09:56:12 +0100 [thread overview]
Message-ID: <20140318085612.GP25546@laptop.programming.kicks-ass.net> (raw)
In-Reply-To: <1395115586.4883.36.camel@marge.simpson.net>
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@acm.org wrote:
> > From: Corey Minyard <cminyard@mvista.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@mvista.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.
next prev parent reply other threads:[~2014-03-18 8:56 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-03-18 0:05 [PATCH] sched: Initialize rq->age_stamp on processor start minyard
2014-03-18 4:06 ` Mike Galbraith
2014-03-18 8:56 ` Peter Zijlstra [this message]
2014-03-18 9:25 ` Mike Galbraith
2014-05-06 19:50 minyard
2014-05-08 16:13 ` Peter Zijlstra
2014-05-08 18:47 ` Corey Minyard
2014-05-08 18:47 ` minyard
2014-05-09 9:12 ` Peter Zijlstra
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