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Message-Id: <1220819281.8687.100.camel@twins.programming.kicks-ass.net>
Date: Sun, 07 Sep 2008 22:28:01 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Mike Travis <travis@....com>, Ingo Molnar <mingo@...e.hu>,
davej@...emonkey.org.uk, David Miller <davem@...emloft.net>,
Eric Dumazet <dada1@...mosbay.com>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
Jack Steiner <steiner@....com>,
Jeremy Fitzhardinge <jeremy@...p.org>,
Jes Sorensen <jes@....com>, "H. Peter Anvin" <hpa@...or.com>,
Thomas Gleixner <tglx@...utronix.de>,
linux-kernel@...r.kernel.org, Gautham R Shenoy <ego@...ibm.com>
Subject: Re: [RFC 07/13] sched: Reduce stack size requirements in
kernel/sched.c
On Sun, 2008-09-07 at 04:00 -0700, Andrew Morton wrote:
> make cpu_hotplug.refcount an atomic_t.
That might actually be a worthwhile idea, but it will not make
get_online_cpus() atomic. The whole point of get_online_cpus() is to
serialize against actual hotplug operations, so it will have to sleep at
some point.
Now, turning cpu_hotplug.refcount into an atomic_t might be worthwhile
because it will reduce the amount of atomic operations in its fastpath
from 2 to 1.
You'd have to make recount==1 the stable situation and use
atomic_inc_unless() and atomic_dec_and_test() in get_online_cpus() and
put_online_cpus() resp. that way !refcount can signify a hotplug
operation and we'd fall back into the slow paths.
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