linux-kernel - Re: [PATCH] hotplug: Optimize {get,put}_online

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Date:	Mon, 30 Sep 2013 22:11:47 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Oleg Nesterov <oleg@...hat.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

On Saturday, September 28, 2013 06:31:04 PM Oleg Nesterov wrote:
> On 09/28, Peter Zijlstra wrote:
> >
> > On Sat, Sep 28, 2013 at 02:48:59PM +0200, Oleg Nesterov wrote:
> >
> > > Please note that this wait_event() adds a problem... it doesn't allow
> > > to "offload" the final synchronize_sched(). Suppose a 4k cpu machine
> > > does disable_nonboot_cpus(), we do not want 2 * 4k * synchronize_sched's
> > > in this case. We can solve this, but this wait_event() complicates
> > > the problem.
> >
> > That seems like a particularly easy fix; something like so?
> 
> Yes, but...
> 
> > @@ -586,6 +603,11 @@ int disable_nonboot_cpus(void)
> >
> > +	cpu_hotplug_done();
> > +
> > +	for_each_cpu(cpu, frozen_cpus)
> > +		cpu_notify_nofail(CPU_POST_DEAD_FROZEN, (void*)(long)cpu);
> 
> This changes the protocol, I simply do not know if it is fine in general
> to do __cpu_down(another_cpu) without CPU_POST_DEAD(previous_cpu). Say,
> currently it is possible that CPU_DOWN_PREPARE takes some global lock
> released by CPU_DOWN_FAILED or CPU_POST_DEAD.
> 
> Hmm. Now that workqueues do not use CPU_POST_DEAD, it has only 2 users,
> mce_cpu_callback() and cpufreq_cpu_callback() and the 1st one even ignores
> this notification if FROZEN. So yes, probably this is fine, but needs an
> ack from cpufreq maintainers (cc'ed), for example to ensure that it is
> fine to call __cpufreq_remove_dev_prepare() twice without _finish().

To my eyes it will return -EBUSY when it tries to stop an already stopped
governor, which will cause the entire chain to fail I guess.

Srivatsa has touched that code most recently, so he should know better, though.

Thanks,
Rafael

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