From 84999b8bdb4969816c7bb7c14c3a55ed42aa4b94 Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Thu, 1 Jun 2023 21:07:37 +0200 Subject: posix-timers: Clarify posix_timer_fn() comments 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 Link: https://lore.kernel.org/r/874jnrdmrq.ffs@tglx --- kernel/time/posix-timers.c | 62 ++++++++++++++++++++++++---------------------- 1 file changed, 32 insertions(+), 30 deletions(-) (limited to 'kernel/time') diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c index d8d2169408f4..ae8799e97fcd 100644 --- a/kernel/time/posix-timers.c +++ b/kernel/time/posix-timers.c @@ -326,11 +326,11 @@ int posix_timer_event(struct k_itimer *timr, int si_private) } /* - * 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 interrupt (soft interrupt on RT kernels). + * + * Handles CLOCK_REALTIME, CLOCK_MONOTONIC, CLOCK_BOOTTIME and CLOCK_TAI + * based timers. */ static enum hrtimer_restart posix_timer_fn(struct hrtimer *timer) { @@ -348,9 +348,10 @@ static enum hrtimer_restart posix_timer_fn(struct hrtimer *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); @@ -359,34 +360,35 @@ static enum hrtimer_restart posix_timer_fn(struct hrtimer *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 and recheck for a + * valid signal handler. + * + * This avoids interrupt starvation in case of a + * very small interval, which would expire the + * timer immediately again. * - * 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. + * Moving now ahead of time by one jiffie tricks + * hrtimer_forward() to expire the timer later, + * while it still maintains the overrun accuracy + * for the price of a slight inconsistency in the + * timer_gettime() case. This is at least better + * than a timer storm. + * + * 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 - { - ktime_t kj = NSEC_PER_SEC / HZ; + if (IS_ENABLED(CONFIG_HIGHRES_TIMERS)) { + ktime_t kj = TICK_NSEC; 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; -- cgit