@@ -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);
@@ -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)
{
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(+)