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Date: Tue, 27 Mar 2012 11:22:48 -0500 (CDT)
From: Christoph Lameter <cl@linux.com>
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To: Gilad Ben-Yossef <gilad@benyossef.com>
cc: Frederic Weisbecker <fweisbec@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, linaro-sched-sig@lists.linaro.org,
        Alessio Igor Bogani <abogani@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>, Avi Kivity <avi@redhat.com>,
        Chris Metcalf <cmetcalf@tilera.com>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Geoff Levand <geoff@infradead.org>, Ingo Molnar <mingo@kernel.org>,
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Subject: Re: [RFC][PATCH 00/32] Nohz cpusets v2 (adaptive tickless kernel)
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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.