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Subject[PATCH v4 2/2] timers: Don't search for expired timers while TIMER_SOFTIRQ is scheduled
Date
This change avoid needlessly searching for more timers in
run_local_timers() (hard interrupt context) when they can't fire.
For example, when ktimersoftd/run_timer_softirq() is scheduled but
preempted due to cpu contention. When it runs, run_timer_softirq() will
discover newly expired timers up to current jiffies in addition to
firing previously expired timers.

However, this change also adds an edge case where non-hrtimer firing
is sometimes delayed by an additional tick. This is acceptable since we
don't make latency guarantees for non-hrtimers and would prefer to
minimize hard interrupt time instead.

Signed-off-by: Haris Okanovic <haris.okanovic@ni.com>
---
[PATCH v3]
- Split block_softirq into separate commit
[PATCH v4]
- Rebase onto v4.14.20-rt17

https://github.com/harisokanovic/linux/tree/dev/hokanovi/timer-peek-v6
---
kernel/time/timer.c | 21 +++++++++++++++++++--
1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 98e952a6428d..5687e9bcf378 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -207,6 +207,7 @@ struct timer_base {
unsigned int cpu;
bool is_idle;
bool must_forward_clk;
+ bool block_softirq;
DECLARE_BITMAP(pending_map, WHEEL_SIZE);
struct hlist_head vectors[WHEEL_SIZE];
struct hlist_head expired_lists[LVL_DEPTH];
@@ -1404,9 +1405,11 @@ static int __collect_expired_timers(struct timer_base *base)

/*
* expire_timers() must be called at least once before we can
- * collect more timers.
+ * collect more timers. We should never hit this case unless
+ * TIMER_SOFTIRQ got raised without expired timers.
*/
- if (base->expired_levels)
+ if (WARN_ONCE(base->expired_levels,
+ "Must expire collected timers before collecting more"))
return base->expired_levels;

clk = base->clk;
@@ -1748,6 +1751,9 @@ static __latent_entropy void run_timer_softirq(struct softirq_action *h)
__run_timers(base);
if (IS_ENABLED(CONFIG_NO_HZ_COMMON))
__run_timers(this_cpu_ptr(&timer_bases[BASE_DEF]));
+
+ /* Allow new TIMER_SOFTIRQs to get scheduled by run_local_timers() */
+ base->block_softirq = false;
}

/*
@@ -1758,6 +1764,14 @@ void run_local_timers(void)
struct timer_base *base = this_cpu_ptr(&timer_bases[BASE_STD]);

hrtimer_run_queues();
+
+ /*
+ * Skip if TIMER_SOFTIRQ is already running on this CPU, since it
+ * will find and expire all timers up to current jiffies.
+ */
+ if (base->block_softirq)
+ return;
+
/* Raise the softirq only if required. */
if (time_before(jiffies, base->clk) || !tick_find_expired(base)) {
if (!IS_ENABLED(CONFIG_NO_HZ_COMMON))
@@ -1766,7 +1780,10 @@ void run_local_timers(void)
base++;
if (time_before(jiffies, base->clk) || !tick_find_expired(base))
return;
+ base--;
}
+
+ base->block_softirq = true;
raise_softirq(TIMER_SOFTIRQ);
}

--
2.15.1
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