linux-kernel - Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2

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Message-ID: <20131010150444.51f9c929@gandalf.local.home>
Date:	Thu, 10 Oct 2013 15:04:44 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2

On Thu, 10 Oct 2013 11:49:15 -0700
Linus Torvalds <torvalds@...ux-foundation.org> wrote:
 
> Oh, and I'm sure there are several users that currently depend on
> being able to sleep over get_online_cpu's.  But I'm pretty sure it's
> "several", not "hundreds", and I think we could fix them up.

I'm wondering if we can have a for_each_cpu() that only disables
preemption in the loop. That is, each iteration will enable preemption,
but the loop itself will guarantee that the current cpu to process
still exists.

	rcu_read_lock();
	for_each_cpu(cpu, mask) {
		rcu_read_unlock();
		rcu_read_lock();
		if (!cpu_online(cpu))
			continue;
		[...]
	}
	rcu_read_unlock();

That way expensive loops wont stop the current CPU to process all
online CPUs.

Of course, it will miss a CPU that comes online. But it would still
need to handle the case of a CPU coming online after the final
put_online_cpus(), so I'm not sure that's a problem.

-- Steve
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