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From: "Woodruff, Richard" <r-woodruff2@ti.com>
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.
[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Richard Woodruff <r-woodruff2@ti.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
On Wed, 24 Sep 2008 18:31:29 +0200 (CEST) Thomas Gleixner <tglx@linutronix.de> wrote:
> No, we only fall trrough into raise_softirq() when the reprogram code
> detects that the event already expired. So you change the flow :)
>
> It does also not deal with delta_jiffies >= NEXT_TIMER_MAX_DELTA :(
>
> I have a closer look on that.
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
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
@@ -282,6 +282,17 @@ void tick_nohz_stop_sched_tick(int inidl
/* 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);
/*
@@ -332,12 +343,7 @@ void tick_nohz_stop_sched_tick(int inidl
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) {
@@ -356,6 +362,7 @@ void tick_nohz_stop_sched_tick(int inidl
tick_do_update_jiffies64(ktime_get());
cpu_clear(cpu, nohz_cpu_mask);
}
+out2:
raise_softirq_irqoff(TIMER_SOFTIRQ);
out:
ts->next_jiffies = next_jiffies;
_
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