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

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Message-ID: <20131001153451.GB3515@redhat.com>
Date:	Tue, 1 Oct 2013 17:34:52 +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 04:48:20PM +0200, Peter Zijlstra wrote:
> > On Tue, Oct 01, 2013 at 07:45:37AM -0700, Paul E. McKenney wrote:
> > > If you don't have cpuhp_seq, you need some other way to avoid
> > > counter overflow.  Which might be provided by limited number of
> > > tasks, or, on 64-bit systems, 64-bit counters.
> >
> > How so? PID space is basically limited to 30 bits, so how could we
> > overflow a 32bit reference counter?
>
> Nesting.

Still it seems that UINT_MAX / PID_MAX_LIMIT has enough room.

But again, OK lets make it ulong. The question is, how cpuhp_seq can
help and why we can't kill it.

Oleg.

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