linux-kernel - Re: [RFC PATCH 0/6] stop_machine: kill stop_cpus_mutex and stop_cpus

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Message-ID: <20150626122330.GY19282@twins.programming.kicks-ass.net>
Date:	Fri, 26 Jun 2015 14:23:30 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	paulmck@...ux.vnet.ibm.com, tj@...nel.org, mingo@...hat.com,
	der.herr@...r.at, dave@...olabs.net, riel@...hat.com,
	viro@...IV.linux.org.uk, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/6] stop_machine: kill stop_cpus_mutex and
 stop_cpus_lock

On Fri, Jun 26, 2015 at 04:14:55AM +0200, Oleg Nesterov wrote:
> Not sure.
> 
> And note that this series kills stop_cpus_mutex, so that multiple
> stop_cpus()'s / stop_machine()'s can run in parallel if cpumask's
> do not overlap.
> 
> Note also the changelog in 6/6, we can simplify + optimize this code
> a bit more.
> 
> What do you think?

The problem I have with this is that it makes the better operation
(stop_two_cpus) slower while improving the worse operation (stop_cpus).

I would much prefer to keep stop_two_cpus() as proposed with taking two
cpu_stopper::lock instances and replacing the stop_cpu_mutex with a
percpu-rwsem.
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