From: Frederic Weisbecker <fweisbec@gmail.com>
To: Rik van Riel <riel@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
LKML <linux-kernel@vger.kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH 3/3] sched: Spare idle load balancing on nohz_full CPUs
Date: Tue, 20 Jun 2017 22:26:35 +0200 [thread overview]
Message-ID: <20170620202632.GA2918@lerouge> (raw)
In-Reply-To: <1497980547.20270.106.camel@redhat.com>
On Tue, Jun 20, 2017 at 01:42:27PM -0400, Rik van Riel wrote:
> On Mon, 2017-06-19 at 04:12 +0200, Frederic Weisbecker wrote:
> > Although idle load balancing obviously only concern idle CPUs, it can
> > be a disturbance on a busy nohz_full CPU. Indeed a CPU can only get
> > rid
> > of an idle load balancing duty once a tick fires while it runs a task
> > and this can take a while in a nohz_full CPU.
> >
> > We could fix that and escape the idle load balancing duty from the
> > very
> > idle exit path but that would bring unecessary overhead. Lets just
> > not
> > bother and leave that job to housekeeping CPUs (those outside
> > nohz_full
> > range). The nohz_full CPUs simply don't want any disturbance.
> >
> > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Ingo Molnar <mingo@kernel.org>
> > Cc: Rik van Riel <riel@redhat.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > ---
> > kernel/sched/fair.c | 4 ++++
> > 1 file changed, 4 insertions(+)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index d711093..cfca960 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -8659,6 +8659,10 @@ void nohz_balance_enter_idle(int cpu)
> > if (!cpu_active(cpu))
> > return;
> >
> > + /* Spare idle load balancing on CPUs that don't want to be
> > disturbed */
> > + if (!is_housekeeping_cpu(cpu))
> > + return;
> > +
> > if (test_bit(NOHZ_TICK_STOPPED, nohz_flags(cpu)))
> > return;
>
> I am not entirely convinced on this one.
>
> Doesn't the if (on_null_domain(cpu_rq(cpu)) test
> a few lines down take care of this already?
It shouldn't, since nohz_full= doesn't imply isolcpus= anymore.
Of course it does if the user manually adds them.
>
> Do we want nohz_full to always automatically
> imply that no idle balancing will happen, like
> on isolated CPUs?
You're making a good point in that I would prefer that nohz_full be
only about the tick and let some sort of separate isolation subsystem
deal with individual isolation features: nohz, workqueues, idle load
balancing, etc...
That's why I rather used is_housekeeping_cpu() and not !tick_nohz_full_cpu()
because for now housekeepers are ~tick_nohz_full_mask but later it should be
cpu_possible_mask by default or some given set of CPUs defined by the future
isolation subsystem.
Thanks.
next prev parent reply other threads:[~2017-06-20 20:26 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-19 2:11 [PATCH 0/3] sched: A few nohz_full improvements Frederic Weisbecker
2017-06-19 2:12 ` [PATCH 1/3] sched/loadavg: Generalize idle naming to nohz Frederic Weisbecker
2017-06-20 17:38 ` Rik van Riel
2017-06-22 11:10 ` [tip:sched/core] sched/loadavg: Generalize "_idle" naming to "_nohz" tip-bot for Frederic Weisbecker
2017-06-19 2:12 ` [PATCH 2/3] nohz: Move idle balancer registration to idle path Frederic Weisbecker
2017-06-20 17:39 ` Rik van Riel
2017-06-22 11:11 ` [tip:sched/core] nohz: Move idle balancer registration to the " tip-bot for Frederic Weisbecker
2017-06-19 2:12 ` [PATCH 3/3] sched: Spare idle load balancing on nohz_full CPUs Frederic Weisbecker
2017-06-20 17:42 ` Rik van Riel
2017-06-20 19:06 ` Mike Galbraith
2017-06-21 13:23 ` Frederic Weisbecker
2017-06-20 20:26 ` Frederic Weisbecker [this message]
2017-06-22 11:11 ` [tip:sched/core] sched/fair: " tip-bot for Frederic Weisbecker
2017-06-22 13:52 ` Frederic Weisbecker
2017-06-22 19:47 ` Ingo Molnar
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20170620202632.GA2918@lerouge \
--to=fweisbec@gmail.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@kernel.org \
--cc=peterz@infradead.org \
--cc=riel@redhat.com \
--cc=tglx@linutronix.de \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).