linux-kernel - Re: [RFC][PATCH 00/32] Nohz cpusets v2 (adaptive tickless kernel)

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Message-ID: <alpine.DEB.2.00.1203271116370.1170@router.home>
Date:	Tue, 27 Mar 2012 11:22:48 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Gilad Ben-Yossef <gilad@...yossef.com>
cc:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linaro-sched-sig@...ts.linaro.org,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Avi Kivity <avi@...hat.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Geoff Levand <geoff@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Max Krasnyansky <maxk@...lcomm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Stephen Hemminger <shemminger@...tta.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Sven-Thorsten Dietrich <thebigcorporation@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Zen Lin <zen@...nhuawei.org>
Subject: Re: [RFC][PATCH 00/32] Nohz cpusets v2 (adaptive tickless kernel)

On Tue, 27 Mar 2012, Gilad Ben-Yossef wrote:

> +static void vmstat_update(struct work_struct *w)
> +{
> +	int cpu, this_cpu = smp_processor_id();
> +	int sleepy_cpu_counter = 0;
> +	static spinlock_t lock;
> +
> +	if(spin_trylock(&lock)) {

Trylock would cause cache bouncing between vmstat runs on various
processors. The reason that vmstat_update exists is to avoid these cache
bounces. Please no exclusive cacheline acquisiton by all cpus by default.

The best method would be to assign a sacrifical lamb cpu and check if we
are running on that cpu. That way taking a lock can be avoided.

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