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List:       linaro-sched-sig
Subject:    [PATCH 35/41] sched: Update rq clock before idle balancing
From:       fweisbec () gmail ! com (Frederic Weisbecker)
Date:       2012-05-02 10:55:35
Message-ID: 20120502105532.GB22061 () somewhere
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On Wed, May 02, 2012 at 11:36:07AM +0800, Michael Wang wrote:
> On 05/01/2012 07:55 AM, Frederic Weisbecker wrote:
> 
> > idle_balance() is called from schedule() right before we schedule the
> > idle task. It needs to record the idle timestamp at that time and for
> > this the rq clock must be accurate. If the CPU is running tickless
> > we need to update the rq clock manually.
> > 
> > Signed-off-by: Frederic Weisbecker <fweisbec at gmail.com>
> > Cc: Alessio Igor Bogani <abogani at kernel.org>
> > Cc: Andrew Morton <akpm at linux-foundation.org>
> > Cc: Avi Kivity <avi at redhat.com>
> > Cc: Chris Metcalf <cmetcalf at tilera.com>
> > Cc: Christoph Lameter <cl at linux.com>
> > Cc: Daniel Lezcano <daniel.lezcano at linaro.org>
> > Cc: Geoff Levand <geoff at infradead.org>
> > Cc: Gilad Ben Yossef <gilad at benyossef.com>
> > Cc: Hakan Akkan <hakanakkan at gmail.com>
> > Cc: Ingo Molnar <mingo at kernel.org>
> > Cc: Kevin Hilman <khilman at ti.com>
> > Cc: Max Krasnyansky <maxk at qualcomm.com>
> > Cc: Paul E. McKenney <paulmck at linux.vnet.ibm.com>
> > Cc: Peter Zijlstra <peterz at infradead.org>
> > Cc: Stephen Hemminger <shemminger at vyatta.com>
> > Cc: Steven Rostedt <rostedt at goodmis.org>
> > Cc: Sven-Thorsten Dietrich <thebigcorporation at gmail.com>
> > Cc: Thomas Gleixner <tglx at linutronix.de>
> > ---
> >  kernel/sched/fair.c |    1 +
> >  1 files changed, 1 insertions(+), 0 deletions(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index eff80e0..cd871e7 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -4638,6 +4638,7 @@ void idle_balance(int this_cpu, struct rq *this_rq)
> >  	int pulled_task = 0;
> >  	unsigned long next_balance = jiffies + HZ;
> > 
> > +	update_nohz_rq_clock(this_rq);
> 
> 
> I'm not sure but why we have to care nohz? if we really need an accurate
> clock, we should do the update anyway, don't we?

This concerns adaptive tickless CPUs only. So I wanted to keep the overhead
low for CPUs that are not in adaptive tickless mode. update_nohz_rq_clock()
takes care of that. It only updates the rq clock if the CPU is adaptive tickless.

> 
> Some thing also confused me is the description:
> "If the CPU is running tickless we need to update the rq clock manually."
> 
> I think the cpu will enter tickless mode only when the idle thread
> already switched in, then invoke
> tick_nohz_idle_enter->tick_nohz_stop_sched_tick, isn't it?

An adaptive tickless CPU tries to shutdown the tick even when the CPU
is not idle. By the time we are about to sleep and schedule the idle
task, we may be already tickless for a while.

> 
> And if we invoke idle_balance for a cpu, that means it hasn't enter
> idle(current task is not idle task), so how can such a cpu in tickless mode?
> 
> Regards,
> Michael Wang
> 
> >  	this_rq->idle_stamp = this_rq->clock;
> > 
> >  	if (this_rq->avg_idle < sysctl_sched_migration_cost)
> 
> --
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