From: "Woodruff, Richard" 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 Cc: Thomas Gleixner On Wed, 24 Sep 2008 18:31:29 +0200 (CEST) Thomas Gleixner 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 --- 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; _