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

[Paul E. McKenney]

On Wed, Oct 02, 2013 at 04:00:20PM +0200, Oleg Nesterov wrote:
> On 10/02, Peter Zijlstra wrote:
> >
> > On Wed, Oct 02, 2013 at 02:13:56PM +0200, Oleg Nesterov wrote:
> > > In short: unless a gp elapses between _exit() and _enter(), the next
> > > _enter() does nothing and avoids synchronize_sched().
> >
> > That does however make the entire scheme entirely writer biased;
> 
> Well, this makes the scheme "a bit more" writer biased, but this is
> exactly what we want in this case.
> 
> We do not block the readers after xxx_exit() entirely, but we do want
> to keep them in SLOW state and avoid the costly SLOW -> FAST -> SLOW
> transitions.

Yes -- should help -a- -lot- for bulk write-side operations, such as
onlining all CPUs at boot time.  ;-)

							Thanx, Paul

> Lets even forget about disable_nonboot_cpus(), lets consider
> percpu_rwsem-like logic "in general".
> 
> Yes, it is heavily optimizied for readers. But if the writers come in
> a batch, or the same writer does down_write + up_write twice or more,
> I think state == FAST is pointless in between (if we can avoid it).
> This is the rare case (the writers should be rare), but if it happens
> it makes sense to optimize the writers too. And again, even
> 
> 	for (;;) {
> 		percpu_down_write();
> 		percpu_up_write();
> 	}
> 
> should not completely block the readers.
> 
> IOW. "turn sync_sched() into call_rcu_sched() in up_write()" is obviously
> a win. If the next down_write/xxx_enter "knows" that the readers are
> still in SLOW mode because gp was not completed yet, why should we
> add the artificial delay?
> 
> As for disable_nonboot_cpus(). You are going to move cpu_hotplug_begin()
> outside of the loop, this is the same thing.
> 
> Oleg.
> 

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