It looks like an old version of this patch is included in v4.9*-rt*
kernels -- E.g. commit 032f93ca in v4.9.68-rt60. There's nothing
functionally wrong with the included version to the best of my
knowledge. However, I posted a newer V3 [1][2] based on Thomas' feedback
that's substantially cleaner and likely more efficient (haven't measured
yet). I think we should include the latter version instead, if only for
the cosmetic benefits. Thoughts?
[1] https://patchwork.kernel.org/patch/9879825/ [PATCH v3,1/2]
[2] https://patchwork.kernel.org/patch/9879827/ [PATCH v3,2/2]
-- Haris
On 08/03/2017 04:06 PM, Haris Okanovic wrote:
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 <[email protected]>
---
[PATCH v3]
- Split block_softirq into separate commit
https://github.com/harisokanovic/linux/tree/dev/hokanovi/timer-peek-v5
---
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 078027d8a866..f0ef9675abdf 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -208,6 +208,7 @@ struct timer_base {
bool migration_enabled;
bool nohz_active;
bool is_idle;
+ bool block_softirq;
DECLARE_BITMAP(pending_map, WHEEL_SIZE);
struct hlist_head vectors[WHEEL_SIZE];
struct hlist_head expired_lists[LVL_DEPTH];
@@ -1376,9 +1377,11 @@ 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;
@@ -1702,6 +1705,9 @@ static __latent_entropy void run_timer_softirq(struct
softirq_action *h)
__run_timers(base);
if (IS_ENABLED(CONFIG_NO_HZ_COMMON) && base->nohz_active)
__run_timers(this_cpu_ptr(&timer_bases[BASE_DEF]));
+
+ /* Allow new TIMER_SOFTIRQs to get scheduled by run_local_timers() */
+ base->block_softirq = false;
}
/*
@@ -1712,6 +1718,14 @@ 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) || !base->nohz_active)
@@ -1720,7 +1734,10 @@ 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);
}
