Messages in this thread | | | Date | Mon, 29 Jan 2018 16:38:39 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 4/6] sched/isolation: Residual 1Hz scheduler tick offload |
| |
On Fri, Jan 19, 2018 at 01:02:18AM +0100, Frederic Weisbecker 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. > > The usual boot parameters "nohz_full=" or "isolcpus=nohz" will now > outsource these scheduler ticks to the global workqueue so that a > housekeeping CPU handles those remotely. > > Note that in the case of using isolcpus, 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 > "isolcpus=nohz,domain".
I would very much like a few words on why sched_class::task_tick() is safe to call remote -- from a quick look I think it actually is, but it would be good to have some words here.
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index d72d0e9..c79500c 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -3062,7 +3062,82 @@ 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); > + } > + > + queue_delayed_work(system_unbound_wq, dwork, HZ);
Do we want something that tracks the actual interrer arrival time of this work, such that we can detect and warn if the book-keeping thing is failing to keep up?
> +} > + > +static void sched_tick_start(int cpu) > +{ > + struct tick_work *twork; > + > + if (housekeeping_cpu(cpu, HK_FLAG_TICK)) > + return;
This all looks very static :-(, you can't reconfigure this nohz_full crud after boot?
> + WARN_ON_ONCE(!tick_work_cpu); > + > + twork = per_cpu_ptr(tick_work_cpu, cpu); > + twork->cpu = cpu; > + INIT_DELAYED_WORK(&twork->work, sched_tick_remote); > + queue_delayed_work(system_unbound_wq, &twork->work, HZ); > +}
Similarly, I think we want a few words about how unbound workqueues are expected to behave vs NUMA.
AFAICT unbound workqueues by default prefer to run on a cpu in the same node, but if no cpu is available, it doesn't go looking for the nearest node that does have a cpu, it just punts to whatever random cpu.
| |