Re: [patch] notifiers: fix blocking_notifier_call_chain() scalability

[Nick Piggin <nickpiggin@yahoo.com.au>]

Peter Zijlstra wrote:
> On Tue, 2007-01-23 at 10:45 +0100, Ingo Molnar wrote:

> > The fix is to enhance blocking_notifier_call_chain() to only take the 
> > lock if there appears to be work on the call-chain.
> >
> > With this patch applied i get nicely saturated system, and much higher 
> > munmap performance, on SMP systems.
> >
> > And as a bonus this also fixes a similar scalability bottleneck in the 
> > thread-exit codepath: profile_task_exit() ...
> >
> > Signed-off-by: Ingo Molnar <mingo@elte.hu>
>
>
> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
>
> > ---
> > kernel/sys.c |   15 +++++++++++----
> > 1 file changed, 11 insertions(+), 4 deletions(-)
> >
> > Index: linux/kernel/sys.c
> > ===================================================================
> > --- linux.orig/kernel/sys.c
> > +++ linux/kernel/sys.c
> > @@ -325,11 +325,18 @@ EXPORT_SYMBOL_GPL(blocking_notifier_chai
> > int blocking_notifier_call_chain(struct blocking_notifier_head *nh,
> > 		unsigned long val, void *v)
> > {
> > -	int ret;
> > +	int ret = NOTIFY_DONE;
> >
> > -	down_read(&nh->rwsem);
> > -	ret = notifier_call_chain(&nh->head, val, v);
> > -	up_read(&nh->rwsem);
> > +	/*
> > +	 * We check the head outside the lock, but if this access is
> > +	 * racy then it does not matter what the result of the test
> > +	 * is, we re-check the list after having taken the lock anyway:
> > +	 */

Great idea!

> > +	if (rcu_dereference(nh->head)) {

Except rcu_dereference() is not needed.

> > +		down_read(&nh->rwsem);
> > +		ret = notifier_call_chain(&nh->head, val, v);
> > +		up_read(&nh->rwsem);
> > +	}
> > 	return ret;
> > }

--
SUSE Labs, Novell Inc.
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