linux-kernel - Re: [PATCH tip/core/rcu 07/23] rcu: Provide OOM handler to motivate lazy RCU callbacks

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Message-ID: <20120906203254.GU2448@linux.vnet.ibm.com>
Date:	Thu, 6 Sep 2012 13:32:54 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu, patches@...aro.org,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH tip/core/rcu 07/23] rcu: Provide OOM handler to motivate
 lazy RCU callbacks

On Thu, Sep 06, 2012 at 07:46:16PM +0200, Peter Zijlstra wrote:
> On Thu, 2012-09-06 at 10:41 -0700, Paul E. McKenney wrote:
> > On Thu, Sep 06, 2012 at 09:52:53AM -0400, Steven Rostedt wrote:
> > > On Thu, 2012-09-06 at 15:46 +0200, Peter Zijlstra wrote:
> > > > On Thu, 2012-08-30 at 11:18 -0700, Paul E. McKenney wrote:
> > > > > +       get_online_cpus();
> > > > > +       for_each_online_cpu(cpu)
> > > > > +               for_each_rcu_flavor(rsp)
> > > > > +                       smp_call_function_single(cpu, rcu_oom_notify_cpu,
> > > > > +                                                rsp, 1);
> > > > > +       put_online_cpus(); 
> > > > 
> > > > I guess blasting IPIs around is better than OOM but still.. do you
> > > > really need to wait for each cpu individually, or would a construct
> > > > using on_each_cpu() be possible, or better yet, on_each_cpu_cond()?
> > 
> > I rejected on_each_cpu_cond() because it disables preemption across
> > a scan of all CPUs.  Probably need to fix that at some point...
> 
> It would be rather straight fwd to make a variant that does
> get_online_cpus() though.. but even then there's smp_call_function()
> that does a broadcast, avoiding the need to spray individual IPIs and
> wait for each CPU individually.

And in this case I can live with inexactness with respect to CPUs actually
being hotplugged, so smp_call_function() does sound good.

							Thanx, Paul

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