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

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Date:	Wed, 25 Sep 2013 17:16:42 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	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>
Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

On 09/24, Peter Zijlstra wrote:
>
> On Tue, Sep 24, 2013 at 08:00:05PM +0200, Oleg Nesterov wrote:
> >
> > Yes, we need to ensure gcc doesn't reorder this code so that
> > do_something() comes before get_online_cpus(). But it can't? At least
> > it should check current->cpuhp_ref != 0 first? And if it is non-zero
> > we do not really care, we are already in the critical section and
> > this ->cpuhp_ref has only meaning in put_online_cpus().
> >
> > Confused...
>
>
> So the reason I put it in was because of the inline; it could possibly
> make it do:

[...snip...]

> In which case the recursive fast path doesn't have a barrier() between
> taking the ref and starting do_something().

Yes, but my point was, this can only happen in recursive fast path.
And in this case (I think) we do not care, we are already in the critical
section.

current->cpuhp_ref doesn't matter at all until we call put_online_cpus().

Suppose that gcc knows for sure that current->cpuhp_ref != 0. Then I
think, for example,

	get_online_cpus();
	do_something();
	put_online_cpus();

converted to

	do_something();
	current->cpuhp_ref++;
	current->cpuhp_ref--;

is fine. do_something() should not depend on ->cpuhp_ref.

OK, please forget. I guess I will never understand this ;)

Oleg.

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