From c812fe636b6e1e3b3b409b316157f56054252e25 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Thu, 17 May 2012 17:15:29 +0200 Subject: sched/nohz: Fix rq->cpu_load calculations some more commit 5aaa0b7a2ed5b12692c9ffb5222182bd558d3146 upstream. Follow up on commit 556061b00 ("sched/nohz: Fix rq->cpu_load[] calculations") since while that fixed the busy case it regressed the mostly idle case. Add a callback from the nohz exit to also age the rq->cpu_load[] array. This closes the hole where either there was no nohz load balance pass during the nohz, or there was a 'significant' amount of idle time between the last nohz balance and the nohz exit. So we'll update unconditionally from the tick to not insert any accidental 0 load periods while busy, and we try and catch up from nohz idle balance and nohz exit. Both these are still prone to missing a jiffy, but that has always been the case. Signed-off-by: Peter Zijlstra Cc: pjt@google.com Cc: Venkatesh Pallipadi Link: http://lkml.kernel.org/n/tip-kt0trz0apodbf84ucjfdbr1a@git.kernel.org Signed-off-by: Ingo Molnar [bwh: Backported to 3.2: adjust filenames and context] Signed-off-by: Ben Hutchings --- kernel/sched.c | 53 +++++++++++++++++++++++++++++++++++++++--------- kernel/time/tick-sched.c | 1 + 2 files changed, 44 insertions(+), 10 deletions(-) (limited to 'kernel') diff --git a/kernel/sched.c b/kernel/sched.c index a409d81b7784..9cd8ca76bbac 100644 --- a/kernel/sched.c +++ b/kernel/sched.c @@ -3886,25 +3886,32 @@ static void __update_cpu_load(struct rq *this_rq, unsigned long this_load, sched_avg_update(this_rq); } +#ifdef CONFIG_NO_HZ +/* + * There is no sane way to deal with nohz on smp when using jiffies because the + * cpu doing the jiffies update might drift wrt the cpu doing the jiffy reading + * causing off-by-one errors in observed deltas; {0,2} instead of {1,1}. + * + * Therefore we cannot use the delta approach from the regular tick since that + * would seriously skew the load calculation. However we'll make do for those + * updates happening while idle (nohz_idle_balance) or coming out of idle + * (tick_nohz_idle_exit). + * + * This means we might still be one tick off for nohz periods. + */ + /* * Called from nohz_idle_balance() to update the load ratings before doing the * idle balance. */ static void update_idle_cpu_load(struct rq *this_rq) { - unsigned long curr_jiffies = jiffies; + unsigned long curr_jiffies = ACCESS_ONCE(jiffies); unsigned long load = this_rq->load.weight; unsigned long pending_updates; /* - * Bloody broken means of dealing with nohz, but better than nothing.. - * jiffies is updated by one cpu, another cpu can drift wrt the jiffy - * update and see 0 difference the one time and 2 the next, even though - * we ticked at roughtly the same rate. - * - * Hence we only use this from nohz_idle_balance() and skip this - * nonsense when called from the scheduler_tick() since that's - * guaranteed a stable rate. + * bail if there's load or we're actually up-to-date. */ if (load || curr_jiffies == this_rq->last_load_update_tick) return; @@ -3915,13 +3922,39 @@ static void update_idle_cpu_load(struct rq *this_rq) __update_cpu_load(this_rq, load, pending_updates); } +/* + * Called from tick_nohz_idle_exit() -- try and fix up the ticks we missed. + */ +void update_cpu_load_nohz(void) +{ + struct rq *this_rq = this_rq(); + unsigned long curr_jiffies = ACCESS_ONCE(jiffies); + unsigned long pending_updates; + + if (curr_jiffies == this_rq->last_load_update_tick) + return; + + raw_spin_lock(&this_rq->lock); + pending_updates = curr_jiffies - this_rq->last_load_update_tick; + if (pending_updates) { + this_rq->last_load_update_tick = curr_jiffies; + /* + * We were idle, this means load 0, the current load might be + * !0 due to remote wakeups and the sort. + */ + __update_cpu_load(this_rq, 0, pending_updates); + } + raw_spin_unlock(&this_rq->lock); +} +#endif /* CONFIG_NO_HZ */ + /* * Called from scheduler_tick() */ static void update_cpu_load_active(struct rq *this_rq) { /* - * See the mess in update_idle_cpu_load(). + * See the mess around update_idle_cpu_load() / update_cpu_load_nohz(). */ this_rq->last_load_update_tick = jiffies; __update_cpu_load(this_rq, this_rq->load.weight, 1); diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index 9955ebd7ab7d..793548cb5a95 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -549,6 +549,7 @@ void tick_nohz_restart_sched_tick(void) /* Update jiffies first */ select_nohz_load_balancer(0); tick_do_update_jiffies64(now); + update_cpu_load_nohz(); #ifndef CONFIG_VIRT_CPU_ACCOUNTING /* -- cgit v1.2.3