linux-kernel - Re: [PATCH RESEND] sched/nohz: Affine unpinned timers to housekeepers

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Date:	Mon, 24 Aug 2015 08:44:12 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Vatika Harlalka <vatikaharlalka@...il.com>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>
Subject: Re: [PATCH RESEND] sched/nohz: Affine unpinned timers to housekeepers


* Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> > here it's fully set - triggering the bug I'm worried about. So what am I 
> > missing, what prevents CONFIG_NO_HZ_FULL_ALL from crashing?
> 
> The boot CPU is excluded from tick_nohz_full_mask in tick_nohz_init(), which is 
> called from tick_init() which is called from start_kernel() shortly after 
> rcu_init():
> 
> 	cpu = smp_processor_id();
> 
> 	if (cpumask_test_cpu(cpu, tick_nohz_full_mask)) {
> 		pr_warning("NO_HZ: Clearing %d from nohz_full range for timekeeping\n", cpu);
> 		cpumask_clear_cpu(cpu, tick_nohz_full_mask);
> 	}
> 
> This happens after the call to tick_nohz_init_all() that does the 
> cpumask_setall() that you called out above.

Ah, indeed - I somehow missed that.

This brings up two other questions:

1)

the 'housekeeping CPU' is essentially the boot CPU. Yet we dedicate a full mask to 
it (housekeeping_mask - a variable mask to begin with) and recover the 
housekeeping CPU via:

+       return cpumask_any_and(housekeeping_mask, cpu_online_mask);

which can be pretty expensive, and which gets executed in two hotpaths:

kernel/time/hrtimer.c:  return &per_cpu(hrtimer_bases, get_nohz_timer_target());
kernel/time/timer.c:    return per_cpu_ptr(&tvec_bases, get_nohz_timer_target());

... why not just use a single housekeeping_cpu which would be way faster to pass 
down to the timer code?

2)

What happens if the boot CPU is offlined? (under CONFIG_BOOTPARAM_HOTPLUG_CPU0=y)

I don't see CPU hotplug callbacks fixing up the housekeeping_mask if the boot CPU 
is offlined.

Thanks,

	Ingo
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