linux-kernel - Re: [PATCH 07/13] sched: Enable IPI reception on timekeeper under nohz full system

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Message-ID: <20131218155036.GR19211@linux.vnet.ibm.com>
Date:	Wed, 18 Dec 2013 07:50:36 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.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 07/13] sched: Enable IPI reception on timekeeper under
 nohz full system

On Wed, Dec 18, 2013 at 03:49:17PM +0100, Frederic Weisbecker wrote:
> On Tue, Dec 17, 2013 at 03:52:48PM -0800, Paul E. McKenney wrote:
> > On Tue, Dec 17, 2013 at 11:51:26PM +0100, Frederic Weisbecker wrote:
> > > We need the default timekeeping CPU to be able to receive IPIs sent
> > > from full dynticks CPUs when they wake up from full system idle state.
> > > 
> > > Therefore we need an entrypoint from the scheduler IPI so that the
> > > need to poll on timekeeping duty is re-evaluated from irq_exit().
> > > 
> > > In order to achieve this, lets take the scheduler IPI everytime as long
> > > as there is at least one full dynticks CPU around. Full dynticks CPUs
> > > are interested too in taking scheduler IPIs to reevaluate their tick.
> > > 
> > > 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/sched/core.c | 6 +++---
> > >  1 file changed, 3 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index e85cda2..f46a7bc 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -1502,9 +1502,9 @@ void scheduler_ipi(void)
> > >  	if (tif_need_resched())
> > >  		set_preempt_need_resched();
> > > 
> > > -	if (llist_empty(&this_rq()->wake_list)
> > > -			&& !tick_nohz_full_cpu(smp_processor_id())
> > > -			&& !got_nohz_idle_kick())
> > > +	if (llist_empty(&this_rq()->wake_list) &&
> > > +	    !tick_nohz_full_enabled() &&
> > > +	    !got_nohz_idle_kick())
> > >  		return;
> > 
> > OK, this is what I was missing in my question about whether the
> > NO_HZ_FULL state was re-evaluated in the interrupt-return path.
> 
> I tend to write my patchset by splitting every single logical bricks and then only
> in the end I enable the feature.
> 
> But that makes a tradeoff between patchset granularity and global overview. And in the end,
> may be it's unbalanced toward overview.
> 
> Notwithstanding bisectability.
> 
> I remember I had similar reactions when I posted the initial full nohz patchset.
> 
> OTOH it's not good to have big all-in-one patches. And granular patchsets like this
> are better to focus discussions on each isolated topics.
> 
> There is a hard balance to find out here.

It is OK.  I have no idea what possessed me to review the patches in
reverse order.  I normally review in forward order.

							Thanx, Paul

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