Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

[Paul E. McKenney]

On Tue, Oct 01, 2013 at 04:14:29PM +0200, Oleg Nesterov wrote:
> On 09/30, Paul E. McKenney wrote:
> >
> > On Fri, Sep 27, 2013 at 10:41:16PM +0200, Peter Zijlstra wrote:
> > > On Fri, Sep 27, 2013 at 08:15:32PM +0200, Oleg Nesterov wrote:
> > > > On 09/26, Peter Zijlstra wrote:
> >
> > [ . . . ]
> >
> > > > > +static bool cpuhp_readers_active_check(void)
> > > > >  {
> > > > > +	unsigned int seq = per_cpu_sum(cpuhp_seq);
> > > > > +
> > > > > +	smp_mb(); /* B matches A */
> > > > > +
> > > > > +	/*
> > > > > +	 * In other words, if we see __get_online_cpus() cpuhp_seq increment,
> > > > > +	 * we are guaranteed to also see its __cpuhp_refcount increment.
> > > > > +	 */
> > > > >
> > > > > +	if (per_cpu_sum(__cpuhp_refcount) != 0)
> > > > > +		return false;
> > > > >
> > > > > +	smp_mb(); /* D matches C */
> > > >
> > > > It seems that both barries could be smp_rmb() ? I am not sure the comments
> > > > from srcu_readers_active_idx_check() can explain mb(), note that
> > > > __srcu_read_lock() always succeeds unlike get_cpus_online().
> > >
> > > I see what you mean; cpuhp_readers_active_check() is all purely reads;
> > > there are no writes to order.
> > >
> > > Paul; is there any argument for the MB here as opposed to RMB; and if
> > > not should we change both these and SRCU?
> >
> > Given that these memory barriers execute only on the semi-slow path,
> > why add the complexity of moving from smp_mb() to either smp_rmb()
> > or smp_wmb()?  Straight smp_mb() is easier to reason about and more
> > robust against future changes.
> 
> But otoh this looks misleading, and the comments add more confusion.
> 
> 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.  Which might be provided by limited number of
tasks, or, on 64-bit systems, 64-bit counters.

							Thanx, Paul

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