| From | Thomas Gleixner <> | Subject | [patch 18/20] posix-timers: Clarify posix_timer_fn() comments | Date | Tue, 25 Apr 2023 20:49:24 +0200 (CEST) |
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Make the issues vs. SIG_IGN understandable and remove the 15 years old promise that a proper solution is already on the horizon.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de> --- kernel/time/posix-timers.c | 56 +++++++++++++++++++++------------------------ 1 file changed, 27 insertions(+), 29 deletions(-)
--- a/kernel/time/posix-timers.c +++ b/kernel/time/posix-timers.c @@ -325,11 +325,11 @@ int posix_timer_event(struct k_itimer *t } /* - * This function gets called when a POSIX.1b interval timer expires. It - * is used as a callback from the kernel internal timer. The - * run_timer_list code ALWAYS calls with interrupts on. - - * This code is for CLOCK_REALTIME* and CLOCK_MONOTONIC* timers. + * This function gets called when a POSIX.1b interval timer expires from + * the HRTIMER soft interrupt with interrupts enabled. + * + * Handles CLOCK_REALTIME, CLOCK_MONOTONIC, CLOCK_BOOTTIME and CLOCK_TAI + * based timers. */ static enum hrtimer_restart posix_timer_fn(struct hrtimer *timer) { @@ -347,9 +347,10 @@ static enum hrtimer_restart posix_timer_ if (posix_timer_event(timr, si_private)) { /* - * signal was not sent because of sig_ignor - * we will not get a call back to restart it AND - * it should be restarted. + * The signal was not queued due to SIG_IGN. As a + * consequence the timer is not going to be rearmed from + * the signal delivery path. But as a real signal handler + * can be installed later the timer must be rearmed here. */ if (timr->it_interval != 0) { ktime_t now = hrtimer_cb_get_time(timer); @@ -358,34 +359,31 @@ static enum hrtimer_restart posix_timer_ * FIXME: What we really want, is to stop this * timer completely and restart it in case the * SIG_IGN is removed. This is a non trivial - * change which involves sighand locking - * (sigh !), which we don't want to do late in - * the release cycle. + * change to the signal handling code. + * + * For now let timers with an interval less than a + * jiffie expire every jiffie to avoid softirq + * starvation in case of SIG_IGN and a very small + * interval, which would put the timer right back + * on the softirq pending list. Moving now ahead of + * time tricks hrtimer_forward() to expire the + * timer later, while it still maintains the + * overrun accuracy for the price of a slightly + * inconsistency in the timer_gettime() case. This + * is at least better than a starved softirq. * - * For now we just let timers with an interval - * less than a jiffie expire every jiffie to - * avoid softirq starvation in case of SIG_IGN - * and a very small interval, which would put - * the timer right back on the softirq pending - * list. By moving now ahead of time we trick - * hrtimer_forward() to expire the timer - * later, while we still maintain the overrun - * accuracy, but have some inconsistency in - * the timer_gettime() case. This is at least - * better than a starved softirq. A more - * complex fix which solves also another related - * inconsistency is already in the pipeline. + * Only required when high resolution timers are + * enabled as the periodic tick based timers are + * automatically aligned to the next tick. */ -#ifdef CONFIG_HIGH_RES_TIMERS - { + if (IS_ENABLED(CONFIG_HIGHRES_TIMERS)) { ktime_t kj = NSEC_PER_SEC / HZ; if (timr->it_interval < kj) now = ktime_add(now, kj); } -#endif - timr->it_overrun += hrtimer_forward(timer, now, - timr->it_interval); + + timr->it_overrun += hrtimer_forward(timer, now, timr->it_interval); ret = HRTIMER_RESTART; ++timr->it_requeue_pending; timr->it_active = 1;
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