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

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Date:	Wed, 2 Oct 2013 11:08:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"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 Tue, Oct 01, 2013 at 08:07:50PM +0200, Oleg Nesterov wrote:
> > > But note that you do not strictly need this change. Just kill cpuhp_waitcount,
> > > then we can change cpu_hotplug_begin/end to use xxx_enter/exit we discuss in
> > > another thread, this should likely "join" all synchronize_sched's.
> >
> > That would still be 4k * sync_sched() == terribly long.
> 
> No? the next xxx_enter() avoids sync_sched() if rcu callback is still
> pending. Unless __cpufreq_remove_dev_finish() is "too slow" of course.

Hmm,. not in the version you posted; there xxx_enter() would only not do
the sync_sched if there's a concurrent 'writer', in which case it will
wait for it.

You only avoid the sync_sched in xxx_exit() and potentially join in the
sync_sched() of a next xxx_begin().

So with that scheme:

  for (i= ; i<4096; i++) {
    xxx_begin();
    xxx_exit();
  }

Will get 4096 sync_sched() calls from the xxx_begin() and all but the
last xxx_exit() will 'drop' the rcu callback.

And given the construct; I'm not entirely sure you can do away with the
sync_sched() in between. While its clear to me you can merge the two
into one; leaving it out entirely doesn't seem right.
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