From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
Josh Triplett <josh@joshtriplett.org>,
Steven Rostedt <rostedt@goodmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: Re: [PATCH] rcu: Only pin GP kthread when full dynticks is actually used
Date: Fri, 13 Jun 2014 09:20:26 -0700 [thread overview]
Message-ID: <20140613162026.GR4581@linux.vnet.ibm.com> (raw)
In-Reply-To: <20140613160318.GM6635@localhost.localdomain>
On Fri, Jun 13, 2014 at 06:03:20PM +0200, Frederic Weisbecker wrote:
> On Fri, Jun 13, 2014 at 08:55:48AM -0700, Paul E. McKenney wrote:
> > On Fri, Jun 13, 2014 at 02:55:45PM +0200, Frederic Weisbecker wrote:
> > > On Thu, Jun 12, 2014 at 07:05:54PM -0700, Paul E. McKenney wrote:
> > > > On Thu, Jun 12, 2014 at 06:24:32PM -0700, Paul E. McKenney wrote:
> > > > > On Fri, Jun 13, 2014 at 02:16:59AM +0200, Frederic Weisbecker wrote:
> > > > > > CONFIG_NO_HZ_FULL may be enabled widely on distros nowadays but actual
> > > > > > users should be a tiny minority, if actually any.
> > > > > >
> > > > > > Also there is a risk that affining the GP kthread to a single CPU could
> > > > > > end up noticeably reducing RCU performances and increasing energy
> > > > > > consumption.
> > > > > >
> > > > > > So lets affine the GP kthread only when nohz full is actually used
> > > > > > (ie: when the nohz_full= parameter is filled or CONFIG_NO_HZ_FULL_ALL=y)
> > > > > >
> > > > > > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> > > > > > Cc: Josh Triplett <josh@joshtriplett.org>
> > > > > > Cc: Steven Rostedt <rostedt@goodmis.org>
> > > > > > Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > > > > > ---
> > > > > > kernel/rcu/tree_plugin.h | 10 +++++++---
> > > > > > 1 file changed, 7 insertions(+), 3 deletions(-)
> > > > > >
> > > > > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > > > > > index cbc2c45..726f52c 100644
> > > > > > --- a/kernel/rcu/tree_plugin.h
> > > > > > +++ b/kernel/rcu/tree_plugin.h
> > > > > > @@ -2843,12 +2843,16 @@ static bool rcu_nohz_full_cpu(struct rcu_state *rsp)
> > > > > > */
> > > > > > static void rcu_bind_gp_kthread(void)
> > > > > > {
> > > > > > -#ifdef CONFIG_NO_HZ_FULL
> > > > > > - int cpu = ACCESS_ONCE(tick_do_timer_cpu);
> > > > > > + int cpu;
> > > > > > +
> > > > > > + if (!tick_nohz_full_enabled())
> > > > > > + return;
> > > > > > +
> > > > > > + cpu = ACCESS_ONCE(tick_do_timer_cpu);
> > > > > >
> > > > > > if (cpu < 0 || cpu >= nr_cpu_ids)
> > > > > > return;
> > > > > > +
> > > > > > if (raw_smp_processor_id() != cpu)
> > > > > > set_cpus_allowed_ptr(current, cpumask_of(cpu));
> > > > > > -#endif /* #ifdef CONFIG_NO_HZ_FULL */
> > > > > > }
> > > > >
> > > > > Hello, Frederic,
> > > > >
> > > > > I have the following queued. Shall I port yours on top of mine, or is
> > > > > there an issue with mine?
> > > >
> > > > OK, I suppose I should show you what it looks like ported on top of mine.
> > >
> > > No need to keep my patch as long as yours goes in. It fixes all the issue.
> > >
> > > >
> > > > I didn't understand the removal of "#ifdef CONFIG_NO_HZ_FULL", so I
> > > > left that.
> > >
> > > Yeah that's because the:
> > >
> > > if (!tick_nohz_full_enabled())
> > > return;
> > >
> > > returns unconditionally if CONFIG_NO_HZ_FULL=n. So the whole function
> > > becomes dead code that should be detected and removed by gcc. So same
> > > result as with the ifdef.
> >
> > Well, that part of your patch is worthwhile, then! And I do therefore
> > need to restructure to invoke tick_nohz_full_enabled() at the very
> > beginning of the function.
>
> Well if no CPU is full dynticks, the sole effect of this is that the kthread
> will be re-affined globally... So no big issue.
>
> Although that can make you spare a call to set_cpus_allowed_ptr(), which can
> be worthwile after all :)
Good point, I must keep the #ifdef, but can also add the call.
Thanx, Paul
> > > > I also didn't understand how dumping the GP kthreads onto
> > > > a single CPU could affect energy efficiency all that much, so I omitted
> > > > that from the commit log. If I am missing something, please enlighten me.
> > >
> > > Because if the kthread is pinned to CPU 0 and a grace period is in progress,
> > > the kthread is going to disturb the CPU 0 regardless of its possibly deep sleep
> > > state. OTOH if the kthread is widely affine, it can be scheduled to idle CPUs
> > > that are less cold and thus wake them from less deep power state.
> > >
> > > Well that all assuming that the scheduler takes care of these deep idle state.
> > > But I believe it does iirc...
> >
> > My guess is that enough stuff gets dumped on CPU 0 to make this a non-issue,
> > but point taken nonetheless.
>
> Ok.
>
next prev parent reply other threads:[~2014-06-13 16:20 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-06-13 0:16 [PATCH] rcu: Only pin GP kthread when full dynticks is actually used Frederic Weisbecker
2014-06-13 1:24 ` Paul E. McKenney
2014-06-13 1:35 ` Paul E. McKenney
2014-06-13 12:47 ` Frederic Weisbecker
2014-06-13 15:52 ` Paul E. McKenney
2014-06-13 16:00 ` Frederic Weisbecker
2014-06-13 16:16 ` Paul E. McKenney
2014-06-13 16:21 ` Frederic Weisbecker
2014-06-13 16:44 ` Josh Triplett
2014-06-13 20:48 ` Paul E. McKenney
2014-06-13 21:10 ` Josh Triplett
2014-06-13 22:49 ` Paul E. McKenney
2014-06-13 23:10 ` Frederic Weisbecker
2014-06-13 23:27 ` Paul E. McKenney
2014-06-13 23:39 ` Frederic Weisbecker
2014-06-14 5:06 ` Paul E. McKenney
2014-06-14 11:26 ` Paul E. McKenney
2014-06-14 13:10 ` Frederic Weisbecker
2014-06-14 14:29 ` Paul E. McKenney
2014-06-14 13:05 ` Frederic Weisbecker
2014-06-13 20:49 ` Paul E. McKenney
2014-06-13 23:13 ` Frederic Weisbecker
2014-06-13 23:22 ` Paul E. McKenney
2014-06-13 2:05 ` Paul E. McKenney
2014-06-13 12:55 ` Frederic Weisbecker
2014-06-13 15:55 ` Paul E. McKenney
2014-06-13 16:03 ` Frederic Weisbecker
2014-06-13 16:20 ` Paul E. McKenney [this message]
2014-06-13 16:10 ` Paul E. McKenney
2014-06-13 12:42 ` Frederic Weisbecker
2014-06-13 15:58 ` Paul E. McKenney
2014-06-13 16:09 ` Frederic Weisbecker
2014-06-13 16:23 ` Paul E. McKenney
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