[PATCH 7/9] timer: Spare timer softirq until next expiry

Frederic Weisbecker
2020-07-07T01:33:31Z

Now that the core timer infrastructure doesn't depend anymore on
periodic base->clk increments, even when the CPU is not in NO_HZ mode,
we can delay the timer softirqs until we have actual timers to expire.

Some spurious softirqs can still remain since base->next_expiry doesn't
keep track of canceled timers but we are still way ahead of the
unconditional periodic softirqs (~15 times less of them with 1000 Hz
and ~5 times less with 100 Hz).

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Juri Lelli <juri.lelli@redhat.com>
---
kernel/time/timer.c | 49 ++++++++-------------------------------------
1 file changed, 8 insertions(+), 41 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index ffa2c956d968..cbc5ac7f772d 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1457,10 +1457,10 @@ static void expire_timers(struct timer_base *base, struct hlist_head *head)
}
}

-static int __collect_expired_timers(struct timer_base *base,
- struct hlist_head *heads)
+static int collect_expired_timers(struct timer_base *base,
+ struct hlist_head *heads)
{
- unsigned long clk = base->clk;
+ unsigned long clk = base->clk = base->next_expiry;
struct hlist_head *vec;
int i, levels = 0;
unsigned int idx;
@@ -1683,40 +1683,6 @@ void timer_clear_idle(void)
*/
base->is_idle = false;
}
-
-static int collect_expired_timers(struct timer_base *base,
- struct hlist_head *heads)
-{
- unsigned long now = READ_ONCE(jiffies);
-
- /*
- * NOHZ optimization. After a long idle sleep we need to forward the
- * base to current jiffies. Avoid a loop by searching the bitfield for
- * the next expiring timer.
- */
- if ((long)(now - base->clk) > 2) {
- /*
- * If the next timer is ahead of time forward to current
- * jiffies, otherwise forward to the next expiry time:
- */
- if (time_after(base->next_expiry, now)) {
- /*
- * The call site will increment base->clk and then
- * terminate the expiry loop immediately.
- */
- base->clk = now;
- return 0;
- }
- base->clk = base->next_expiry;
- }
- return __collect_expired_timers(base, heads);
-}
-#else
-static inline int collect_expired_timers(struct timer_base *base,
- struct hlist_head *heads)
-{
- return __collect_expired_timers(base, heads);
-}
#endif

/*
@@ -1749,7 +1715,7 @@ static inline void __run_timers(struct timer_base *base)
struct hlist_head heads[LVL_DEPTH];
int levels;

- if (!time_after_eq(jiffies, base->clk))
+ if (time_before(jiffies, base->next_expiry))
return;

timer_base_lock_expiry(base);
@@ -1762,7 +1728,8 @@ static inline void __run_timers(struct timer_base *base)
*/
base->must_forward_clk = false;

- while (time_after_eq(jiffies, base->clk)) {
+ while (time_after_eq(jiffies, base->clk) &&
+ time_after_eq(jiffies, base->next_expiry)) {

levels = collect_expired_timers(base, heads);
base->clk++;
@@ -1797,12 +1764,12 @@ void run_local_timers(void)

hrtimer_run_queues();
/* Raise the softirq only if required. */
- if (time_before(jiffies, base->clk)) {
+ if (time_before(jiffies, base->next_expiry)) {
if (!IS_ENABLED(CONFIG_NO_HZ_COMMON))
return;
/* CPU is awake, so check the deferrable base. */
base++;
- if (time_before(jiffies, base->clk))
+ if (time_before(jiffies, base->next_expiry))
return;
}
raise_softirq(TIMER_SOFTIRQ);
--
2.26.2