linux-kernel - Re: [patch 18/20] posix-timers: Clarify posix_timer

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Date:   Thu, 1 Jun 2023 15:21:55 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        syzbot+5c54bd3eb218bb595aa9@...kaller.appspotmail.com,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [patch 18/20] posix-timers: Clarify posix_timer_fn() comments

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@...utronix.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;
>