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author | Mike Galbraith <mgalbraith@suse.de> | 2012-05-08 12:20:58 +0200 |
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committer | Thomas Gleixner <tglx@linutronix.de> | 2012-05-25 01:44:50 +0200 |
commit | 5307c9556bc17e3cd26d4e94fc3b2565921834de (patch) | |
tree | e51937060ccd5e293dec7246a7961b0ae5509a53 /kernel/time/tick-sched.c | |
parent | ce004178be1bbaa292e9e6497939e2970300095a (diff) | |
download | linux-5307c9556bc17e3cd26d4e94fc3b2565921834de.tar.gz |
tick: Add tick skew boot option
Let the user decide whether power consumption or jitter is the more important consideration for their machines. Quoting removal commit af5ab277ded04bd9bc6b048c5a2f0e7d70ef0867: "Historically, Linux has tried to make the regular timer tick on the various CPUs not happen at the same time, to avoid contention on xtime_lock. Nowadays, with the tickless kernel, this contention no longer happens since time keeping and updating are done differently. In addition, this skew is actually hurting power consumption in a measurable way on many-core systems." Problems: - Contrary to the above, systems do encounter contention on both xtime_lock and RCU structure locks when the tick is synchronized. - Moderate sized RT systems suffer intolerable jitter due to the tick being synchronized. - SGI reports the same for their large systems. - Fully utilized systems reap no power saving benefit from skew removal, but do suffer from resulting induced lock contention. - 0209f649 rcu: limit rcu_node leaf-level fanout This patch was born to combat lock contention which testing showed to have been _induced by_ skew removal. Skew the tick, contention disappeared virtually completely. Signed-off-by: Mike Galbraith <mgalbraith@suse.de> Link: http://lkml.kernel.org/r/1336472458.21924.78.camel@marge.simpson.net Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Diffstat (limited to 'kernel/time/tick-sched.c')
-rw-r--r-- | kernel/time/tick-sched.c | 18 |
1 files changed, 18 insertions, 0 deletions
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index 6a3a5b9ff561..4eddbb5ea9c5 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -814,6 +814,8 @@ static enum hrtimer_restart tick_sched_timer(struct hrtimer *timer) return HRTIMER_RESTART; } +static int sched_skew_tick; + /** * tick_setup_sched_timer - setup the tick emulation timer */ @@ -831,6 +833,14 @@ void tick_setup_sched_timer(void) /* Get the next period (per cpu) */ hrtimer_set_expires(&ts->sched_timer, tick_init_jiffy_update()); + /* Offset the tick to avert xtime_lock contention. */ + if (sched_skew_tick) { + u64 offset = ktime_to_ns(tick_period) >> 1; + do_div(offset, num_possible_cpus()); + offset *= smp_processor_id(); + hrtimer_add_expires_ns(&ts->sched_timer, offset); + } + for (;;) { hrtimer_forward(&ts->sched_timer, now, tick_period); hrtimer_start_expires(&ts->sched_timer, @@ -910,3 +920,11 @@ int tick_check_oneshot_change(int allow_nohz) tick_nohz_switch_to_nohz(); return 0; } + +static int __init skew_tick(char *str) +{ + get_option(&str, &sched_skew_tick); + + return 0; +} +early_param("skew_tick", skew_tick); |