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

Thomas Gleixner
2023-06-01T18:43:27Z

On Thu, Jun 01 2023 at 15:21, Frederic Weisbecker wrote:
> 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.

Duh. My RT biased brain tricked me.

>> + *
>> + * 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...

Let me rephrase that.

>> + * 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?

Yep. Fixed it up.