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Date: Sat, 18 Jul 2009 16:14:56 +0200
From: Ingo Molnar <mingo@...e.hu>
To: Martin Schwidefsky <schwidefsky@...ibm.com>,
Frédéric Weisbecker <fweisbec@...il.com>
Cc: linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
john stultz <johnstul@...ibm.com>
Subject: Re: [RFC][PATCH] reuse ktime in sub-functions of tick_check_idle.
* Martin Schwidefsky <schwidefsky@...ibm.com> wrote:
> From: Martin Schwidefsky <schwidefsky@...ibm.com>
>
> On a system with NOHZ=y tick_check_idle calls tick_nohz_stop_idle and
> tick_nohz_update_jiffies. Given the right conditions (ts->idle_active
> and/or ts->tick_stopped) both function get a time stamp with ktime_get.
> The same time stamp can be reused if both function require one.
>
> On s390 this change has the additional benefit that gcc inlines the
> tick_nohz_stop_idle function into tick_check_idle. The number of
> instructions to execute tick_check_idle drops from 225 to 144
> (without the ktime_get optimization it is 367 vs 215 instructions).
>
> before:
>
> 0) | tick_check_idle() {
> 0) | tick_nohz_stop_idle() {
> 0) | ktime_get() {
> 0) | read_tod_clock() {
> 0) 0.601 us | }
> 0) 1.765 us | }
> 0) 3.047 us | }
> 0) | ktime_get() {
> 0) | read_tod_clock() {
> 0) 0.570 us | }
> 0) 1.727 us | }
> 0) | tick_do_update_jiffies64() {
> 0) 0.609 us | }
> 0) 8.055 us | }
>
> after:
>
> 0) | tick_check_idle() {
> 0) | ktime_get() {
> 0) | read_tod_clock() {
> 0) 0.617 us | }
> 0) 1.773 us | }
> 0) | tick_do_update_jiffies64() {
> 0) 0.593 us | }
> 0) 4.477 us | }
Nice!
> Cc: Ingo Molnar <mingo@...e.hu>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: john stultz <johnstul@...ibm.com>
> Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com>
> ---
>
> kernel/time/tick-sched.c | 56 ++++++++++++++++++++++++-----------------------
> 1 file changed, 29 insertions(+), 27 deletions(-)
>
> Index: git-linux-2.6/kernel/time/tick-sched.c
> ===================================================================
> --- git-linux-2.6.orig/kernel/time/tick-sched.c
> +++ git-linux-2.6/kernel/time/tick-sched.c
> @@ -134,18 +134,13 @@ __setup("nohz=", setup_tick_nohz);
> * value. We do this unconditionally on any cpu, as we don't know whether the
> * cpu, which has the update task assigned is in a long sleep.
> */
> -static void tick_nohz_update_jiffies(void)
> +static void tick_nohz_update_jiffies(ktime_t now)
> {
> int cpu = smp_processor_id();
> struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
> unsigned long flags;
> - ktime_t now;
> -
> - if (!ts->tick_stopped)
> - return;
>
> cpumask_clear_cpu(cpu, nohz_cpu_mask);
> - now = ktime_get();
> ts->idle_waketime = now;
>
> local_irq_save(flags);
> @@ -155,20 +150,17 @@ static void tick_nohz_update_jiffies(voi
> touch_softlockup_watchdog();
> }
>
> -static void tick_nohz_stop_idle(int cpu)
> +static void tick_nohz_stop_idle(int cpu, ktime_t now)
> {
> struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
> + ktime_t delta;
>
> - if (ts->idle_active) {
> - ktime_t now, delta;
> - now = ktime_get();
> - delta = ktime_sub(now, ts->idle_entrytime);
> - ts->idle_lastupdate = now;
> - ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
> - ts->idle_active = 0;
> + delta = ktime_sub(now, ts->idle_entrytime);
> + ts->idle_lastupdate = now;
> + ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);
> + ts->idle_active = 0;
>
> - sched_clock_idle_wakeup_event(0);
> - }
> + sched_clock_idle_wakeup_event(0);
> }
>
> static ktime_t tick_nohz_start_idle(struct tick_sched *ts)
> @@ -431,7 +423,11 @@ void tick_nohz_restart_sched_tick(void)
> ktime_t now;
>
> local_irq_disable();
> - tick_nohz_stop_idle(cpu);
> + if (ts->idle_active || (ts->inidle && ts->tick_stopped))
> + now = ktime_get();
> +
> + if (ts->idle_active)
> + tick_nohz_stop_idle(cpu, now);
>
> if (!ts->inidle || !ts->tick_stopped) {
> ts->inidle = 0;
> @@ -445,7 +441,6 @@ void tick_nohz_restart_sched_tick(void)
>
> /* Update jiffies first */
> select_nohz_load_balancer(0);
> - now = ktime_get();
> tick_do_update_jiffies64(now);
> cpumask_clear_cpu(cpu, nohz_cpu_mask);
>
> @@ -579,22 +574,18 @@ static void tick_nohz_switch_to_nohz(voi
> * timer and do not touch the other magic bits which need to be done
> * when idle is left.
> */
> -static void tick_nohz_kick_tick(int cpu)
> +static void tick_nohz_kick_tick(int cpu, ktime_t now)
> {
> #if 0
hm?
> /* Switch back to 2.6.27 behaviour */
>
> struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
> - ktime_t delta, now;
> -
> - if (!ts->tick_stopped)
> - return;
> + ktime_t delta;
>
> /*
> * Do not touch the tick device, when the next expiry is either
> * already reached or less/equal than the tick period.
> */
> - now = ktime_get();
> delta = ktime_sub(hrtimer_get_expires(&ts->sched_timer), now);
> if (delta.tv64 <= tick_period.tv64)
> return;
> @@ -614,11 +605,22 @@ static inline void tick_nohz_switch_to_n
> */
> void tick_check_idle(int cpu)
> {
> +#ifdef CONFIG_NO_HZ
> + struct tick_sched *ts;
> +#endif
> +
> tick_check_oneshot_broadcast(cpu);
> #ifdef CONFIG_NO_HZ
> - tick_nohz_stop_idle(cpu);
> - tick_nohz_update_jiffies();
> - tick_nohz_kick_tick(cpu);
> + ts = &per_cpu(tick_cpu_sched, cpu);
> + if (ts->idle_active || ts->tick_stopped) {
> + ktime_t now = ktime_get();
> + if (ts->idle_active)
> + tick_nohz_stop_idle(cpu, now);
> + if (ts->tick_stopped) {
> + tick_nohz_update_jiffies(now);
> + tick_nohz_kick_tick(cpu, now);
> + }
> + }
> #endif
Those ifdefs look quite ugly, dont they?
Ingo
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