| Date | Thu, 1 Jun 2023 15:21:55 +0200 | From | Frederic Weisbecker <> | Subject | Re: [patch 18/20] posix-timers: Clarify posix_timer_fn() comments |
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On Tue, Apr 25, 2023 at 08:49:24PM +0200, Thomas Gleixner wrote: > 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.
BTW, what arranges for this to be called in softirq with interrupts enabled? The modes I see used here are HRTIMER_MODE_ABS or HRTIMER_MODE_REL and not their _SOFT counterparts.
> + * > + * Handles CLOCK_REALTIME, CLOCK_MONOTONIC, CLOCK_BOOTTIME and CLOCK_TAI > + * based timers. > */ > static enum hrtimer_restart posix_timer_fn(struct hrtimer *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
Or rather at least to the next jiffie, right? Because then in the next jiffie it gets re-evaluated in case a real signal handler might have been set in-between.
Or it could be:
+ * For now let timers with an interval less than a + * jiffie expire every jiffie (until a real sig handler + * is found set) 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. [...] > */ > -#ifdef CONFIG_HIGH_RES_TIMERS > - { > + if (IS_ENABLED(CONFIG_HIGHRES_TIMERS)) { > ktime_t kj = NSEC_PER_SEC / HZ;
Could be TICK_NSECS?
Thanks!
> > 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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