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SubjectRe: [patch 18/39] tick: sched: Force tick interrupt and get rid of softirq magic
On Tue, Apr 14, 2015 at 09:08:54PM -0000, Thomas Gleixner wrote:
> We already got rid of the hrtimer reprogramming loops and hoops as
> hrtimer now enforces an interrupt if the enqueued time is in the past.
>
> Do the same for the nohz non highres mode. That gets rid of the need
> to raise the softirq which only serves the purpose of getting the
> machine out of the inner idle loop.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Cc: John Stultz <john.stultz@linaro.org>
> Cc: Preeti U Murthy <preeti@linux.vnet.ibm.com>
> Cc: Marcelo Tosatti <mtosatti@redhat.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
>
> ---
> kernel/time/tick-sched.c | 83 ++++++++++++++++-------------------------------
> 1 file changed, 29 insertions(+), 54 deletions(-)
>
> Index: tip/kernel/time/tick-sched.c
> ===================================================================
> --- tip.orig/kernel/time/tick-sched.c
> +++ tip/kernel/time/tick-sched.c
> @@ -565,6 +565,20 @@ u64 get_cpu_iowait_time_us(int cpu, u64
> }
> EXPORT_SYMBOL_GPL(get_cpu_iowait_time_us);
>
> +static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
> +{
> + hrtimer_cancel(&ts->sched_timer);
> + hrtimer_set_expires(&ts->sched_timer, ts->last_tick);
> +
> + /* Forward the time to expire in the future */
> + hrtimer_forward(&ts->sched_timer, now, tick_period);
> +
> + if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
> + hrtimer_start_expires(&ts->sched_timer, HRTIMER_MODE_ABS_PINNED);
> + else
> + tick_program_event(hrtimer_get_expires(&ts->sched_timer), 1);
> +}
> +
> static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
> ktime_t now, int cpu)
> {
> @@ -691,22 +705,18 @@ static ktime_t tick_nohz_stop_sched_tick
> if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
> hrtimer_cancel(&ts->sched_timer);
> goto out;
> - }
> + }
>
> - if (ts->nohz_mode == NOHZ_MODE_HIGHRES) {
> - hrtimer_start(&ts->sched_timer, expires,
> - HRTIMER_MODE_ABS_PINNED);
> - goto out;
> - } else if (!tick_program_event(expires, 0))
> - goto out;
> - /*
> - * We are past the event already. So we crossed a
> - * jiffie boundary. Update jiffies and raise the
> - * softirq.
> - */
> - tick_do_update_jiffies64(ktime_get());
> + if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
> + hrtimer_start(&ts->sched_timer, expires,
> + HRTIMER_MODE_ABS_PINNED);
> + else
> + tick_program_event(expires, 1);
> + } else {
> + /* Tick is stopped, but required now. Enforce it */
> + tick_nohz_restart(ts, now);
> }
> - raise_softirq_irqoff(TIMER_SOFTIRQ);
> +

But the reprogramming happens only under "if ((long)delta_jiffies >= 1)".
Probably this condition should go away as well.

In the end, the possible side effect, at least on low-res, is that timers
which are already expired will be handled on the next tick instead of now.
But probably it doesn't matter much to have a one-tick delay.


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