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

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Message-ID: <20131001150030.GA1801@redhat.com>
Date:	Tue, 1 Oct 2013 17:00:30 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.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>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

On 10/01, Paul E. McKenney wrote:
>
> On Tue, Oct 01, 2013 at 04:14:29PM +0200, Oleg Nesterov wrote:
> >
> > But please note another email, it seems to me we can simply kill
> > cpuhp_seq and all the barriers in cpuhp_readers_active_check().
>
> If you don't have cpuhp_seq, you need some other way to avoid
> counter overflow.

I don't think so. Overflows (espicially "unsigned") should be fine and
in fact we can't avoid them.

Say, a task does get() on CPU_0 and put() on CPU_1, after that we have

	CTR[0] == 1, CTR[1] = (unsigned)-1

iow, the counter was already overflowed (underflowed). But this is fine,
all we care about is  CTR[0] + CTR[1] == 0, and this is only true because
of another overflow.

But probably you meant another thing,

> Which might be provided by limited number of
> tasks, or, on 64-bit systems, 64-bit counters.

perhaps you meant that max_threads * max_depth can overflow the counter?
I don't think so... but OK, perhaps this counter should be u_long.

But how cpuhp_seq can help?

Oleg.

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