Messages in this thread Patch in this message | | | From | Artem Savkov <> | Subject | [PATCH v5 1/2] timer: add a function to adjust timeouts to be upper bound | Date | Thu, 5 May 2022 15:18:10 +0200 |
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Current timer wheel implementation is optimized for performance and energy usage but lacks in precision. This, normally, is not a problem as most timers that use timer wheel are used for timeouts and thus rarely expire, instead they often get canceled or modified before expiration. Even when they don't, expiring a bit late is not an issue for timeout timers.
TCP keepalive timer is a special case, it's aim is to prevent timeouts, so triggering earlier rather than later is desired behavior. In a reported case the user had a 3600s keepalive timer for preventing firewall disconnects (on a 3650s interval). They observed keepalive timers coming in up to four minutes late, causing unexpected disconnects.
Add a new upper_bound_timeout() function that takes a relative timeout and adjusts it based on timer wheel granularity so that supplied value effectively becomes an upper bound for the timer.
This was previously discussed here: https://lore.kernel.org/all/20210302001054.4qgrvnkltvkgikzr@treble/T/#u
Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com> Signed-off-by: Artem Savkov <asavkov@redhat.com> --- include/linux/timer.h | 1 + kernel/time/timer.c | 14 ++++++++++++++ 2 files changed, 15 insertions(+)
diff --git a/include/linux/timer.h b/include/linux/timer.h index fda13c9d1256..b209d31d543f 100644 --- a/include/linux/timer.h +++ b/include/linux/timer.h @@ -168,6 +168,7 @@ static inline int timer_pending(const struct timer_list * timer) return !hlist_unhashed_lockless(&timer->entry); } +extern unsigned long upper_bound_timeout(unsigned long timeout); extern void add_timer_on(struct timer_list *timer, int cpu); extern int del_timer(struct timer_list * timer); extern int mod_timer(struct timer_list *timer, unsigned long expires); diff --git a/kernel/time/timer.c b/kernel/time/timer.c index 9dd2a39cb3b0..b087a481d06f 100644 --- a/kernel/time/timer.c +++ b/kernel/time/timer.c @@ -545,6 +545,20 @@ static int calc_wheel_index(unsigned long expires, unsigned long clk, return idx; } +/** + * upper_bound_timeout - return adjusted timeout + * @timeout: timeout value in jiffies + * + * This function return supplied timeout adjusted to timer wheel granularity + * effectively making supplied value an upper bound at which the timer will + * expire. + */ +unsigned long upper_bound_timeout(unsigned long timeout) +{ + return timeout - (timeout >> LVL_CLK_SHIFT); +} +EXPORT_SYMBOL(upper_bound_timeout); + static void trigger_dyntick_cpu(struct timer_base *base, struct timer_list *timer) { -- 2.34.1
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