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

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Date:	Wed, 25 Sep 2013 19:50:55 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	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>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

On 09/25, Peter Zijlstra wrote:
>
> On Wed, Sep 25, 2013 at 05:55:15PM +0200, Oleg Nesterov wrote:
>
> > > +static inline void get_online_cpus(void)
> > > +{
> > > +	might_sleep();
> > > +
> > > +	/* Support reader-in-reader recursion */
> > > +	if (current->cpuhp_ref++) {
> > > +		barrier();
> > > +		return;
> > > +	}
> > > +
> > > +	preempt_disable();
> > > +	if (likely(!__cpuhp_writer))
> > > +		__this_cpu_inc(__cpuhp_refcount);
> >
> > mb() to ensure the reader can't miss, say, a STORE done inside
> > the cpu_hotplug_begin/end section.
> >
> > put_online_cpus() needs mb() as well.
>
> OK, I'm not getting this; why isn't the sync_sched sufficient to get out
> of this fast path without barriers?

Aah, sorry, I didn't notice this version has another synchronize_sched()
in cpu_hotplug_done().

Then I need to recheck again...

No. Too tired too ;) damn LSB test failures...

> > > +	if (atomic_dec_and_test(&cpuhp_waitcount))
> > > +		wake_up_all(&cpuhp_writer);
> >
> > Same problem as in previous version. __get_online_cpus() succeeds
> > without incrementing __cpuhp_refcount. "goto start" can't help
> > afaics.
>
> I added a goto into the cond-block, not before the cond; but see the
> version below.

"into the cond-block" doesn't look right too, at first glance. This
always succeeds, but by this time another writer can already hold
the lock.

Oleg.

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