linux-kernel - Re: [PATCH 1/9] x86/intel_cqm: Modify hot cpu notification handling

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Message-ID: <20150729164416.GA25159@twins.programming.kicks-ass.net>
Date:	Wed, 29 Jul 2015 18:44:16 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vikas Shivappa <vikas.shivappa@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org, vikas.shivappa@...el.com,
	x86@...nel.org, hpa@...or.com, tglx@...utronix.de,
	mingo@...nel.org, tj@...nel.org, matt.fleming@...el.com,
	will.auld@...el.com, glenn.p.williamson@...el.com,
	kanaka.d.juvva@...el.com
Subject: Re: [PATCH 1/9] x86/intel_cqm: Modify hot cpu notification handling

On Wed, Jul 01, 2015 at 03:21:02PM -0700, Vikas Shivappa wrote:
> +/*
> + * Temporary cpumask used during hot cpu notificaiton handling. The usage
> + * is serialized by hot cpu locks.
> + */
> +static cpumask_t tmp_cpumask;

So the problem with this is that its 512 bytes on your general distro
config. And this patch set includes at least 3 of them

So you've just shot 1k5 bytes of .data for no reason.

I know tglx whacked you over the head for this, but is this really worth
it? I mean, nobody sane should care about hotplug performance, so who
cares if we iterate a bunch of cpus on the abysmal slow path called
hotplug.
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