The patch titled
tick-sched.c: suppress needless timer reprogramming
has been added to the -mm tree. Its filename is
tick-schedc-suppress-needless-timer-reprogramming.patch
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The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/
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Subject: tick-sched.c: suppress needless timer reprogramming
From: "Woodruff, Richard" <r-woodruff2@xxxxxx>
In my device I get many interrupts from a high speed USB device in a very
short period of time. The system spends a lot of time reprogramming the
hardware timer which is in a slower timing domain as compared to the CPU.
This results in the CPU spending a huge amount of time waiting for the
timer posting to be done. All of this reprogramming is useless as the
wake up time has not changed.
As measured using ETM trace this drops my reprogramming penalty from
almost 60% CPU load down to 15% during high interrupt rate. I can send
traces to show this.
Suppress setting of duplicate timer event when timer already stopped.
Timer programming can be very costly and can result in long cpu stall/wait
times.
Signed-off-by: Richard Woodruff <r-woodruff2@xxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/time/tick-sched.c | 19 +++++++++++++------
1 file changed, 13 insertions(+), 6 deletions(-)
diff -puN kernel/time/tick-sched.c~tick-schedc-suppress-needless-timer-reprogramming kernel/time/tick-sched.c
--- a/kernel/time/tick-sched.c~tick-schedc-suppress-needless-timer-reprogramming
+++ a/kernel/time/tick-sched.c
@@ -261,6 +261,17 @@ void tick_nohz_stop_sched_tick(void)
/* Schedule the tick, if we are at least one jiffie off */
if ((long)delta_jiffies >= 1) {
+ /*
+ * calculate the expiry time for the next timer wheel
+ * timer
+ */
+ expires = ktime_add_ns(last_update, tick_period.tv64 *
+ delta_jiffies);
+
+ /* Skip reprogram of event if its not changed */
+ if(ts->tick_stopped && ktime_equal(expires, dev->next_event))
+ goto out2;
+
if (delta_jiffies > 1)
cpu_set(cpu, nohz_cpu_mask);
/*
@@ -311,12 +322,7 @@ void tick_nohz_stop_sched_tick(void)
goto out;
}
- /*
- * calculate the expiry time for the next timer wheel
- * timer
- */
- expires = ktime_add_ns(last_update, tick_period.tv64 *
- delta_jiffies);
+ /* Mark expiries */
ts->idle_expires = expires;
if (ts->nohz_mode == NOHZ_MODE_HIGHRES) {
@@ -335,6 +341,7 @@ void tick_nohz_stop_sched_tick(void)
tick_do_update_jiffies64(ktime_get());
cpu_clear(cpu, nohz_cpu_mask);
}
+out2:
raise_softirq_irqoff(TIMER_SOFTIRQ);
out:
ts->next_jiffies = next_jiffies;
_
Patches currently in -mm which might be from r-woodruff2@xxxxxx are
tick-schedc-suppress-needless-timer-reprogramming.patch
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