Date: Wed, 2 Oct 2013 11:08:59 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: "Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Mel Gorman <mgorman@suse.de>, Rik van Riel <riel@redhat.com>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@kernel.org>, Andrea Arcangeli <aarcange@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>, Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Viresh Kumar <viresh.kumar@linaro.org>
Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()
Message-ID: <20131002090859.GE12926@twins.programming.kicks-ass.net>
References: <20130925175055.GA25914@redhat.com>
 <20130928144720.GL15690@laptop.programming.kicks-ass.net>
 <20130928163104.GA23352@redhat.com>
 <7632387.20FXkuCITr@vostro.rjw.lan>
 <524B0233.8070203@linux.vnet.ibm.com>
 <20131001173615.GW3657@laptop.programming.kicks-ass.net>
 <20131001174508.GA17411@redhat.com>
 <20131001175640.GQ15690@laptop.programming.kicks-ass.net>
 <20131001180750.GA18261@redhat.com>
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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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