Re: [PATCH v9 12/32] timers: Fix nextevt calculation when no timers are pending

Sebastian Siewior
2023-12-04T16:03:56Z

On 2023-12-01 10:26:34 [+0100], Anna-Maria Behnsen wrote:
> When no timer is queued into an empty timer base, the next_expiry will not
> be updated. It was originally calculated as
>
> base->clk + NEXT_TIMER_MAX_DELTA
>
> When the timer base stays empty long enough (> NEXT_TIMER_MAX_DELTA), the
> next_expiry value of the empty base suggests that there is a timer pending
> soon. This might be more a kind of a theoretical problem, but the fix
> doesn't hurt.

So __run_timers() sets base::next_expiry to base->clk +
NEXT_TIMER_MAX_DELTA and then we have no more timers enqueued.

But wouldn't base->timers_pending remain false? Therefore it would use
"expires = KTIME_MAX" as return value (well cmp_next_hrtimer_event())?

Based on the code as of #11, it would only set timer_base::is_idle
wrongly false if it wraps around. Other than that, I don't see an issue.
What do I miss?

If you update it regardless here then it would make a difference to
run_local_timers() assuming we have still hrtimer which expire and this
next_expiry check might raise a softirq since it does not consider the
timers_pending value.

> Use only base->next_expiry value as nextevt when timers are
> pending. Otherwise nextevt will be jiffies + NEXT_TIMER_MAX_DELTA. As all
> information is in place, update base->next_expiry value of the empty timer
> base as well.

or consider timers_pending in run_local_timers()? An additional read vs
write?

> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -1944,10 +1943,20 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
> __forward_timer_base(base, basej);
>
> if (base->timers_pending) {
> + nextevt = base->next_expiry;
> +
> /* If we missed a tick already, force 0 delta */
> if (time_before(nextevt, basej))
> nextevt = basej;
> expires = basem + (u64)(nextevt - basej) * TICK_NSEC;
> + } else {
> + /*
> + * Move next_expiry for the empty base into the future to
> + * prevent a unnecessary raise of the timer softirq when the
an
> + * next_expiry value will be reached even if there is no timer
> + * pending.
> + */
> + base->next_expiry = nextevt;
> }
>
> /*

Sebastian