[tip:timers/core] nohz: Track last do_timer() cpu

tip-bot for Thomas Gleixner
2009-11-13T19:50:23Z

Commit-ID: 27185016b806d5a1181ff501cae120582b2b27dd
Gitweb: http://git.kernel.org/tip/27185016b806d5a1181ff501cae120582b2b27dd
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Thu, 12 Nov 2009 22:12:06 +0100
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitDate: Fri, 13 Nov 2009 20:46:24 +0100

nohz: Track last do_timer() cpu

The previous patch which limits the sleep time to the maximum
deferment time of the time keeping clocksource has some limitations on
SMP machines: if all CPUs are idle then for all CPUs the maximum sleep
time is limited.

Solve this by keeping track of which cpu had the do_timer() duty
assigned last and limit the sleep time only for this cpu.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
LKML-Reference: <new-submission>
Cc: Jon Hunter <jon-hunter@ti.com>
Cc: John Stultz <johnstul@us.ibm.com>
---
include/linux/tick.h | 2 +
kernel/time/tick-sched.c | 52 ++++++++++++++++++++++++---------------------
2 files changed, 30 insertions(+), 24 deletions(-)

diff --git a/include/linux/tick.h b/include/linux/tick.h
index 8dc0821..d2ae79e 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -43,6 +43,7 @@ enum tick_nohz_mode {
* @idle_exittime: Time when the idle state was left
* @idle_sleeptime: Sum of the time slept in idle with sched tick stopped
* @sleep_length: Duration of the current idle sleep
+ * @do_timer_lst: CPU was the last one doing do_timer before going idle
*/
struct tick_sched {
struct hrtimer sched_timer;
@@ -64,6 +65,7 @@ struct tick_sched {
unsigned long last_jiffies;
unsigned long next_jiffies;
ktime_t idle_expires;
+ int do_timer_last;
};

extern void __init tick_init(void);
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index a80b464..df133bc 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -263,17 +263,7 @@ void tick_nohz_stop_sched_tick(int inidle)
seq = read_seqbegin(&xtime_lock);
last_update = last_jiffies_update;
last_jiffies = jiffies;
-
- /*
- * On SMP we really should only care for the CPU which
- * has the do_timer duty assigned. All other CPUs can
- * sleep as long as they want.
- */
- if (cpu == tick_do_timer_cpu ||
- tick_do_timer_cpu == TICK_DO_TIMER_NONE)
- time_delta = timekeeping_max_deferment();
- else
- time_delta = KTIME_MAX;
+ time_delta = timekeeping_max_deferment();
} while (read_seqretry(&xtime_lock, seq));

if (rcu_needs_cpu(cpu) || printk_needs_cpu(cpu) ||
@@ -296,6 +286,29 @@ void tick_nohz_stop_sched_tick(int inidle)
if ((long)delta_jiffies >= 1) {

/*
+ * If this cpu is the one which updates jiffies, then
+ * give up the assignment and let it be taken by the
+ * cpu which runs the tick timer next, which might be
+ * this cpu as well. If we don't drop this here the
+ * jiffies might be stale and do_timer() never
+ * invoked. Keep track of the fact that it was the one
+ * which had the do_timer() duty last. If this cpu is
+ * the one which had the do_timer() duty last, we
+ * limit the sleep time to the timekeeping
+ * max_deferement value which we retrieved
+ * above. Otherwise we can sleep as long as we want.
+ */
+ if (cpu == tick_do_timer_cpu) {
+ tick_do_timer_cpu = TICK_DO_TIMER_NONE;
+ ts->do_timer_last = 1;
+ } else if (tick_do_timer_cpu != TICK_DO_TIMER_NONE) {
+ time_delta = KTIME_MAX;
+ ts->do_timer_last = 0;
+ } else if (!ts->do_timer_last) {
+ time_delta = KTIME_MAX;
+ }
+
+ /*
* calculate the expiry time for the next timer wheel
* timer. delta_jiffies >= NEXT_TIMER_MAX_DELTA signals
* that there is no timer pending or at least extremely
@@ -312,21 +325,12 @@ void tick_nohz_stop_sched_tick(int inidle)
*/
time_delta = min_t(u64, time_delta,
tick_period.tv64 * delta_jiffies);
- expires = ktime_add_ns(last_update, time_delta);
- } else {
- expires.tv64 = KTIME_MAX;
}

- /*
- * If this cpu is the one which updates jiffies, then
- * give up the assignment and let it be taken by the
- * cpu which runs the tick timer next, which might be
- * this cpu as well. If we don't drop this here the
- * jiffies might be stale and do_timer() never
- * invoked.
- */
- if (cpu == tick_do_timer_cpu)
- tick_do_timer_cpu = TICK_DO_TIMER_NONE;
+ if (time_delta < KTIME_MAX)
+ expires = ktime_add_ns(last_update, time_delta);
+ else
+ expires.tv64 = KTIME_MAX;

if (delta_jiffies > 1)
cpumask_set_cpu(cpu, nohz_cpu_mask);