On Fri, Jan 12, 2018 at 02:22:58PM -0500, Luiz Capitulino wrote:
> On Thu, 4 Jan 2018 05:25:36 +0100
> Frederic Weisbecker <frede...@kernel.org> wrote:
>
> > When a CPU runs in full dynticks mode, a 1Hz tick remains in order to
> > keep the scheduler stats alive. However this residual tick is a burden
> > for bare metal tasks that can't stand any interruption at all, or want
> > to minimize them.
> >
> > Adding the boot parameter "isolcpus=nohz_offload" will now outsource
> > these scheduler ticks to the global workqueue so that a housekeeping CPU
> > handles that tick remotely.
> >
> > Note it's still up to the user to affine the global workqueues to the
> > housekeeping CPUs through /sys/devices/virtual/workqueue/cpumask or
> > domains isolation.
> >
> > Signed-off-by: Frederic Weisbecker <frede...@kernel.org>
> > Cc: Chris Metcalf <cmetc...@mellanox.com>
> > Cc: Christoph Lameter <c...@linux.com>
> > Cc: Luiz Capitulino <lcapitul...@redhat.com>
> > Cc: Mike Galbraith <efa...@gmx.de>
> > Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com>
> > Cc: Peter Zijlstra <pet...@infradead.org>
> > Cc: Rik van Riel <r...@redhat.com>
> > Cc: Thomas Gleixner <t...@linutronix.de>
> > Cc: Wanpeng Li <kernel...@gmail.com>
> > Cc: Ingo Molnar <mi...@kernel.org>
> > ---
> > kernel/sched/core.c | 88
> > ++++++++++++++++++++++++++++++++++++++++++++++--
> > kernel/sched/isolation.c | 4 +++
> > kernel/sched/sched.h | 2 ++
> > 3 files changed, 91 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index d72d0e9..b964890 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -3052,9 +3052,14 @@ void scheduler_tick(void)
> > */
> > u64 scheduler_tick_max_deferment(void)
> > {
> > - struct rq *rq = this_rq();
> > - unsigned long next, now = READ_ONCE(jiffies);
> > + struct rq *rq;
> > + unsigned long next, now;
> >
> > + if (!housekeeping_cpu(smp_processor_id(), HK_FLAG_TICK_SCHED))
> > + return ktime_to_ns(KTIME_MAX);
> > +
> > + rq = this_rq();
> > + now = READ_ONCE(jiffies);
> > next = rq->last_sched_tick + HZ;
> >
> > if (time_before_eq(next, now))
> > @@ -3062,7 +3067,82 @@ u64 scheduler_tick_max_deferment(void)
> >
> > return jiffies_to_nsecs(next - now);
> > }
> > -#endif
> > +
> > +struct tick_work {
> > + int cpu;
> > + struct delayed_work work;
> > +};
> > +
> > +static struct tick_work __percpu *tick_work_cpu;
> > +
> > +static void sched_tick_remote(struct work_struct *work)
> > +{
> > + struct delayed_work *dwork = to_delayed_work(work);
> > + struct tick_work *twork = container_of(dwork, struct tick_work, work);
> > + int cpu = twork->cpu;
> > + struct rq *rq = cpu_rq(cpu);
> > + struct rq_flags rf;
> > +
> > + /*
> > + * Handle the tick only if it appears the remote CPU is running
> > + * in full dynticks mode. The check is racy by nature, but
> > + * missing a tick or having one too much is no big deal.
> > + */
> > + if (!idle_cpu(cpu) && tick_nohz_tick_stopped_cpu(cpu)) {
> > + rq_lock_irq(rq, &rf);
> > + update_rq_clock(rq);
> > + rq->curr->sched_class->task_tick(rq, rq->curr, 0);
> > + rq_unlock_irq(rq, &rf);
> > + }
>
> OK, so this executes task_tick() remotely. What about account_process_tick()?
> Don't we need it as well?
Nope, tasks in nohz_full mode have their special accounting that doesn't
rely on the tick.
>
> In particular, when I run a hog application on a nohz_full core configured
> with tick offload, I can see in top that the CPU usage goes from 100%
> to idle for a few seconds every couple of seconds. Could this be related?
>
> Also, in my testing I'm sometimes seeing the tick. Sometimes at 10 or
> 20 seconds interval. Is this expected? I'll dig deeper next week.
That's expected, see the changelog: the offload is not affine by default.
You need to either also isolate the domains:
isolcpus=nohz_offload,domain
or tweak the workqueue cpumask through:
/sys/devices/virtual/workqueue/cpumask
Thanks.
