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Date: Thu, 7 Apr 2022 09:52:41 +0200 From: Artem Savkov <asavkov@...hat.com> To: Thomas Gleixner <tglx@...utronix.de>, Josh Poimboeuf <jpoimboe@...hat.com>, Anna-Maria Behnsen <anna-maria@...utronix.de>, netdev@...r.kernel.org Cc: davem@...emloft.net, yoshfuji@...ux-ipv6.org, dsahern@...nel.org, linux-kernel@...r.kernel.org, Artem Savkov <asavkov@...hat.com> Subject: [PATCH v4 1/2] timer: add a function to adjust timeouts to be upper bound 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. This commit adds 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@...hat.com> Signed-off-by: Artem Savkov <asavkov@...hat.com> --- include/linux/timer.h | 1 + kernel/time/timer.c | 68 ++++++++++++++++++++++++++++++++++--------- 2 files changed, 56 insertions(+), 13 deletions(-) diff --git a/include/linux/timer.h b/include/linux/timer.h index fda13c9d1256c..b209d31d543f0 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 85f1021ad4595..a645b62e257e2 100644 --- a/kernel/time/timer.c +++ b/kernel/time/timer.c @@ -507,28 +507,38 @@ static inline unsigned calc_index(unsigned long expires, unsigned lvl, return LVL_OFFS(lvl) + (expires & LVL_MASK); } -static int calc_wheel_index(unsigned long expires, unsigned long clk, - unsigned long *bucket_expiry) +static inline int get_wheel_lvl(unsigned long delta) { - unsigned long delta = expires - clk; - unsigned int idx; - if (delta < LVL_START(1)) { - idx = calc_index(expires, 0, bucket_expiry); + return 0; } else if (delta < LVL_START(2)) { - idx = calc_index(expires, 1, bucket_expiry); + return 1; } else if (delta < LVL_START(3)) { - idx = calc_index(expires, 2, bucket_expiry); + return 2; } else if (delta < LVL_START(4)) { - idx = calc_index(expires, 3, bucket_expiry); + return 3; } else if (delta < LVL_START(5)) { - idx = calc_index(expires, 4, bucket_expiry); + return 4; } else if (delta < LVL_START(6)) { - idx = calc_index(expires, 5, bucket_expiry); + return 5; } else if (delta < LVL_START(7)) { - idx = calc_index(expires, 6, bucket_expiry); + return 6; } else if (LVL_DEPTH > 8 && delta < LVL_START(8)) { - idx = calc_index(expires, 7, bucket_expiry); + return 7; + } + + return -1; +} + +static int calc_wheel_index(unsigned long expires, unsigned long clk, + unsigned long *bucket_expiry) +{ + unsigned long delta = expires - clk; + unsigned int idx; + int lvl = get_wheel_lvl(delta); + + if (lvl >= 0) { + idx = calc_index(expires, lvl, bucket_expiry); } else if ((long) delta < 0) { idx = clk & LVL_MASK; *bucket_expiry = clk; @@ -545,6 +555,38 @@ static int calc_wheel_index(unsigned long expires, unsigned long clk, return idx; } +/** + * upper_bound_timeout - return granularity-adjusted timeout + * @timeout: timeout value in jiffies + * + * This function return supplied timeout adjusted based on timer wheel + * granularity effectively making supplied value an upper bound at which the + * timer will expire. Due to the way timer wheel works timeouts smaller than + * LVL_GRAN on their respecrive levels will be _at least_ + * LVL_GRAN(lvl) - LVL_GRAN(lvl -1)) jiffies early. + */ +unsigned long upper_bound_timeout(unsigned long timeout) +{ + int lvl = get_wheel_lvl(timeout); + + if (lvl < 0) { + if ((long) timeout < 0) { + /* + * This will expire immediately so no adjustment + * needed. + */ + return timeout; + } else { + if (timeout > WHEEL_TIMEOUT_CUTOFF) + timeout = WHEEL_TIMEOUT_CUTOFF; + lvl = LVL_DEPTH - 1; + } + } + + return LVL_GRAN(lvl) > timeout ? 0 : timeout - LVL_GRAN(lvl); +} +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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