Re: [patch 18/20] posix-timers: Clarify posix_timer_fn() comments

Frederic Weisbecker
2023-06-01T13:22:25Z

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;
>