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

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Date:	Mon, 24 Aug 2015 03:28:51 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	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

On Sun, Aug 23, 2015 at 09:01:01AM -0700, Paul E. McKenney wrote:
> 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.
> 
> Or is a recent patch that I missed changing this?

Exactly, this happens right after tick_nohz_full_mask is filled and
makes sure that at least the boot CPU runs as a housekeeper. We
also make sure that it can't become offline later.

Thanks.
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