linux-kernel - Re: [PATCH 2/2] nohz: make nohz

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Date:	Mon, 06 Apr 2015 15:28:28 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>,
	Chris Metcalf <cmetcalf@...hip.com>
CC:	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] nohz: make nohz_full imply isolcpus

On 04/03/2015 11:43 PM, Mike Galbraith wrote:

> Speaking of microsecond savers, the (ick) deferment experiment below
> cut 60 core jitter in half.  Shooting the clocksource watchdog fixes
> alternating ~15us/~5us tick on my desktop box.
>
> With workqueue twiddles and whatnot floating around, the thing is
> starting to look viable.

Doesn't look too bad to me, though the changes below
could probably use some comments when turned into a
final patch :)

> ---
>   kernel/sched/core.c       |    5 +++--
>   kernel/time/clocksource.c |    5 +++++
>   2 files changed, 8 insertions(+), 2 deletions(-)
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2604,12 +2604,13 @@ u64 scheduler_tick_max_deferment(void)
>          struct rq *rq = this_rq();
>          unsigned long next, now = ACCESS_ONCE(jiffies);
>
> -       next = rq->last_sched_tick + HZ;
> +       next = (rq->last_sched_tick + HZ) | (rq->clock & 0x3f);
>
>          if (time_before_eq(next, now))
>                  return 0;
>
> -       return jiffies_to_nsecs(next - now);
> +       /* Add noise to avoid CPUs colliding at tick boundaries */
> +       return jiffies_to_nsecs(next - now) | (rq->clock & 0xfffff);
>   }
>   #endif
>
> --- a/kernel/time/clocksource.c
> +++ b/kernel/time/clocksource.c
> @@ -267,8 +267,13 @@ static void clocksource_watchdog(unsigne
>           * to each other.
>           */
>          next_cpu = cpumask_next(raw_smp_processor_id(), cpu_online_mask);
> +skip_nohz_full:
>          if (next_cpu >= nr_cpu_ids)
>                  next_cpu = cpumask_first(cpu_online_mask);
> +       if (next_cpu && tick_nohz_full_cpu(next_cpu)) {
> +               next_cpu = cpumask_next(next_cpu, cpu_online_mask);
> +               goto skip_nohz_full;
> +       }
>          watchdog_timer.expires += WATCHDOG_INTERVAL;
>          add_timer_on(&watchdog_timer, next_cpu);
>   out:
> --
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