linux-kernel - Re: [PATCH 11/13] nohz: Wake up timekeeper on exit from sysidle state

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Message-ID: <20131217235233.GC18689@localhost.localdomain>
Date:	Wed, 18 Dec 2013 00:52:34 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	John Stultz <john.stultz@...aro.org>,
	Alex Shi <alex.shi@...aro.org>,
	Kevin Hilman <khilman@...aro.org>
Subject: Re: [PATCH 11/13] nohz: Wake up timekeeper on exit from sysidle state

On Tue, Dec 17, 2013 at 03:34:54PM -0800, Paul E. McKenney wrote:
> On Tue, Dec 17, 2013 at 11:51:30PM +0100, Frederic Weisbecker wrote:
> > When a full dynticks CPU wakes up from sysidle state, which means that
> > all full dynticks CPUs were previously sleeping, it's possible that
> > all the potential timekeeping CPUs are sleeping as well and nobody
> > maintains the associated duty.
> > 
> > But full dynticks CPUs don't run the tick by definition so we need
> > to wake up a timekeeper such that it can handle the timekeeping
> > duty on behalf of the freshly awoken full dyntick CPU.
> > 
> > To achieve this and ensure that this CPU won't deal with stale
> > jiffies values, lets wake up the default timekeeper using the right
> > API.
> > 
> > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Ingo Molnar <mingo@...nel.org>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > Cc: John Stultz <john.stultz@...aro.org>
> > Cc: Alex Shi <alex.shi@...aro.org>
> > Cc: Kevin Hilman <khilman@...aro.org>
> > ---
> >  kernel/rcu/tree_plugin.h | 2 +-
> >  kernel/time/tick-sched.c | 3 ++-
> >  2 files changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 1795265..b43e32d 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -2488,7 +2488,7 @@ void rcu_sysidle_force_exit(void)
> >  				      oldstate, RCU_SYSIDLE_NOT);
> >  		if (oldstate == newoldstate &&
> >  		    oldstate == RCU_SYSIDLE_FULL_NOTED) {
> > -			smp_send_reschedule(tick_timekeeping_default_cpu());
> > +			tick_nohz_full_kick_timekeeping();
> 
> OK, I guess I should look at the patches in order.  So yes, it is no
> longer safe to just kick tick_do_timer_cpu.  ;-)

Hehe, I do linear reviews as well ;)

Indeed it's no longer safe, especially since tick_do_timer_cpu can be TICK_DO_TIMER_NONE.
So this always kick CPU 0 instead (for now).
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