Date: Wed, 2 Oct 2013 18:31:52 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
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: <20131002163152.GA16233@redhat.com>
References: <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> <20131002090859.GE12926@twins.programming.kicks-ass.net> <20131002121356.GA21581@redhat.com> <20131002133137.GG28601@twins.programming.kicks-ass.net> <20131002140020.GA25256@redhat.com> <20131002151734.GT3081@twins.programming.kicks-ass.net>
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On 10/02, Peter Zijlstra wrote:
>
> On Wed, Oct 02, 2013 at 04:00:20PM +0200, Oleg Nesterov wrote:
> > And again, even
> >
> > 	for (;;) {
> > 		percpu_down_write();
> > 		percpu_up_write();
> > 	}
> >
> > should not completely block the readers.
>
> Sure there's a tiny window, but don't forget that a reader will have to
> wait for the gp_state cacheline to transfer to shared state and the
> per-cpu refcount cachelines to be brought back into exclusive mode and
> the above can be aggressive enough that by that time we'll observe
> state == blocked again.

Sure, but don't forget that other callers of cpu_down() do a lot more
work before/after they actually call cpu_hotplug_begin/end().

> So I'll stick to waitcount -- as you can see in the patches I've just
> posted.

I still do not believe we need this waitcount "in practice" ;)

But even if I am right this is minor and we can reconsider this later,
so please forget.

Oleg.

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