From: Rafael J. Wysocki <rafael.j.wyso...@intel.com> Push the decision whether or not to stop the tick somewhat deeper into the idle loop.
Stopping the tick upfront leads to unpleasant outcomes in case the idle governor doesn't agree with the timekeeping code on the duration of the upcoming idle period. Specifically, if the tick has been stopped and the idle governor predicts short idle, the situation is bad regardless of whether or not the prediction is accurate. If it is accurate, the tick has been stopped unnecessarily which means excessive overhead. If it is not accurate, the CPU is likely to spend too much time in the (shallow, because short idle has been predicted) idle state selected by the governor [1]. As the first step towards addressing this problem, change the code to make the tick stopping decision inside of the loop in do_idle(). In particular, do not stop the tick in the cpu_idle_poll() code path. Also don't do that in tick_nohz_irq_exit() which doesn't really have information to whether or not to stop the tick. Link: https://marc.info/?l=linux-pm&m=150116085925208&w=2 # [1] Link: https://tu-dresden.de/zih/forschung/ressourcen/dateien/projekte/haec/powernightmares.pdf Signed-off-by: Rafael J. Wysocki <rafael.j.wyso...@intel.com> --- -> v2: No changes. --- kernel/sched/idle.c | 13 ++++++++++--- kernel/time/tick-sched.c | 2 +- 2 files changed, 11 insertions(+), 4 deletions(-) Index: linux-pm/kernel/sched/idle.c =================================================================== --- linux-pm.orig/kernel/sched/idle.c +++ linux-pm/kernel/sched/idle.c @@ -220,13 +220,17 @@ static void do_idle(void) */ __current_set_polling(); - tick_nohz_idle_enter(); + tick_nohz_idle_prepare(); while (!need_resched()) { check_pgt_cache(); rmb(); if (cpu_is_offline(cpu)) { + local_irq_disable(); + tick_nohz_idle_go_idle(true); + local_irq_enable(); + cpuhp_report_idle_dead(); arch_cpu_idle_dead(); } @@ -240,10 +244,13 @@ static void do_idle(void) * broadcast device expired for us, we don't want to go deep * idle as we know that the IPI is going to arrive right away. */ - if (cpu_idle_force_poll || tick_check_broadcast_expired()) + if (cpu_idle_force_poll || tick_check_broadcast_expired()) { + tick_nohz_idle_go_idle(false); cpu_idle_poll(); - else + } else { + tick_nohz_idle_go_idle(true); cpuidle_idle_call(); + } arch_cpu_idle_exit(); } Index: linux-pm/kernel/time/tick-sched.c =================================================================== --- linux-pm.orig/kernel/time/tick-sched.c +++ linux-pm/kernel/time/tick-sched.c @@ -1007,7 +1007,7 @@ void tick_nohz_irq_exit(void) struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched); if (ts->inidle) - __tick_nohz_idle_enter(ts, true); + __tick_nohz_idle_enter(ts, false); else tick_nohz_full_update_tick(ts); }
