linux-kernel - Re: [PATCH 01/32] nohz: Move nohz load balancer selection into idle logic

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Date:	Tue, 30 Oct 2012 11:39:12 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	muming.wq@...il.com
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Clark Williams <clark.williams@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Avi Kivity <avi@...hat.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Kevin Hilman <khilman@...com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Sven-Thorsten Dietrich <thebigcorporation@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Alex Shi <alex.shi@...el.com>
Subject: Re: [PATCH 01/32] nohz: Move nohz load balancer selection into idle
 logic


On Tue, 2012-10-30 at 16:32 +0800, Charles Wang wrote:
> calc_load_exit_idle depends on updated jiffies, so you shouldn't move 
> this before tick_do_update_jiffies64.

OK, so it should be moved to the end of the if block. Note, that was a
change I made, as that function was added since Frederic did his code. I
looked into the code and it seemed that it should be moved for just idle
as well. But I agree with you that it should be after the jiffies
update.


> And why should we do nohz_balance_enter_idle in tick_nohz_idle_exit? 
> It's nohz_balance_exit_idle here.

OK, that's my fault as well. As Frederic's original patch just moved
select_nohz_load_balance(0). But the commit c1cc017c59 "sched/nohz:
Clean up select_nohz_load_balancer()" replaced it with
nohz_balance_enter_idle(cpu), and removed it on exit, there was nothing
to replace for the current code.

Knowing this patch was considered "buggy" instead of just not moving it
(and forgetting about it), I did the change to remind myself to talk
about it :-)

My question is now, is there any reason to keep that call there? Or can
we just remove it as well.

-- Steve


> 
> Regards,
> Charles
> 
> On 10/30/2012 04:27 AM, Steven Rostedt wrote:
> ［snipped］
>  > @@ -573,7 +573,6 @@ static void tick_nohz_restart_sched_tick(struct 
> tick_sched *ts, ktime_t now)
>  >  	tick_do_update_jiffies64(now);
>  >  	update_cpu_load_nohz();
>  >
>  > -	calc_load_exit_idle();
>  >  	touch_softlockup_watchdog();
>  >  	/*
>  >  	 * Cancel the scheduled timer and restore the tick
>  > @@ -628,6 +627,8 @@ void tick_nohz_idle_exit(void)
>  >  		tick_nohz_stop_idle(cpu, now);
>  >
>  >  	if (ts->tick_stopped) {
>  > +		nohz_balance_enter_idle(cpu);
>  > +		calc_load_exit_idle();
>  >  		tick_nohz_restart_sched_tick(ts, now);
>  >  		tick_nohz_account_idle_ticks(ts);
>  >  	}


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