On Mon, 9 Jun 2014, Stanislav Fomichev wrote:
> In hrtimer_get_next_event we unconditionally lock/unlock spinlock, even if
> it's
> not required (hrtimer_hres_active() != 0). This patch adds fast path
> when highres is active so we don't execute unnecessary operations.
>
> We can safely do lockless check because:
> - hrtimer_get_next_event is always called with interrupts disabled;
> - we may switch to hres only from softirq handler with disabled interrupts.
>
> Because we only care about hres_active which may be changed only from
> local CPU, we can use interrupt context for synchronization.
>
> run_timer_softirq
> hrtimer_run_pending
> hrtimer_switch_to_hres
> local_irq_save <-
> base->hres_active = 0
>
> tick_nohz_idle_enter
> local_irq_disable <-
> __tick_nohz_idle_enter
> tick_nohz_stop_sched_tick
> get_next_timer_interrupt
> cmp_next_hrtimer_event
> hrtimer_get_next_event
> check base->hres_active
>
> irq_exit <- irq context
> tick_irq_exit
> tick_nohz_irq_exit
> tick_nohz_full_stop_tick
> tick_nohz_stop_sched_tick
> ... <see above>
> __tick_nohz_idle_enter
> ... <see above>
Sigh. Is anybody actually reading and trying to understand what I
write in reviews?
> Also the lockless check wants a comment why it is correct.
Is it that hard? A comment is NOT a uberlenghty explanation in the
changelog. A comment starts with /* and ends with */ and is in the
code.
/*
* Called with interrupts disabled and therefor
* protected against a switch to high resolution mode.
*/
That's a comment, right?
Thanks,
tglx
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