[PATCH v5 1/2] timer: add a function to adjust timeouts to be upper bound

[Artem Savkov <asavkov@redhat.com>]

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