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

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Message-ID: <20131002140020.GA25256@redhat.com>
Date:	Wed, 2 Oct 2013 16:00:20 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 10/02, Peter Zijlstra wrote:
>
> On Wed, Oct 02, 2013 at 02:13:56PM +0200, Oleg Nesterov wrote:
> > In short: unless a gp elapses between _exit() and _enter(), the next
> > _enter() does nothing and avoids synchronize_sched().
>
> That does however make the entire scheme entirely writer biased;

Well, this makes the scheme "a bit more" writer biased, but this is
exactly what we want in this case.

We do not block the readers after xxx_exit() entirely, but we do want
to keep them in SLOW state and avoid the costly SLOW -> FAST -> SLOW
transitions.

Lets even forget about disable_nonboot_cpus(), lets consider
percpu_rwsem-like logic "in general".

Yes, it is heavily optimizied for readers. But if the writers come in
a batch, or the same writer does down_write + up_write twice or more,
I think state == FAST is pointless in between (if we can avoid it).
This is the rare case (the writers should be rare), but if it happens
it makes sense to optimize the writers too. And again, even

	for (;;) {
		percpu_down_write();
		percpu_up_write();
	}

should not completely block the readers.

IOW. "turn sync_sched() into call_rcu_sched() in up_write()" is obviously
a win. If the next down_write/xxx_enter "knows" that the readers are
still in SLOW mode because gp was not completed yet, why should we
add the artificial delay?

As for disable_nonboot_cpus(). You are going to move cpu_hotplug_begin()
outside of the loop, this is the same thing.

Oleg.

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