From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751919AbeCTQoH (ORCPT ); Tue, 20 Mar 2018 12:44:07 -0400 Received: from cloudserver094114.home.pl ([79.96.170.134]:51268 "EHLO cloudserver094114.home.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751755AbeCTQnr (ORCPT ); Tue, 20 Mar 2018 12:43:47 -0400 From: "Rafael J. Wysocki" To: Peter Zijlstra , Linux PM Cc: Frederic Weisbecker , Thomas Gleixner , Paul McKenney , Thomas Ilsche , Doug Smythies , Rik van Riel , Aubrey Li , Mike Galbraith , LKML Subject: [RFT][PATCH v7 2/8] sched: idle: Do not stop the tick upfront in the idle loop Date: Tue, 20 Mar 2018 16:15:24 +0100 Message-ID: <2802114.rPO26tdOrH@aspire.rjw.lan> In-Reply-To: <2390019.oHdSGtR3EE@aspire.rjw.lan> References: <2390019.oHdSGtR3EE@aspire.rjw.lan> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Rafael J. Wysocki 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 nohz 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 enough information on 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 Suggested-by: Frederic Weisbecker Signed-off-by: Rafael J. Wysocki --- include/linux/tick.h | 2 ++ kernel/sched/idle.c | 9 ++++++--- kernel/time/tick-sched.c | 26 ++++++++++++++++++-------- 3 files changed, 26 insertions(+), 11 deletions(-) Index: linux-pm/kernel/sched/idle.c =================================================================== --- linux-pm.orig/kernel/sched/idle.c +++ linux-pm/kernel/sched/idle.c @@ -221,13 +221,13 @@ static void do_idle(void) __current_set_polling(); tick_nohz_idle_enter(); - tick_nohz_idle_stop_tick_protected(); while (!need_resched()) { check_pgt_cache(); rmb(); if (cpu_is_offline(cpu)) { + tick_nohz_idle_stop_tick_protected(); cpuhp_report_idle_dead(); arch_cpu_idle_dead(); } @@ -241,10 +241,13 @@ 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_restart_tick(); cpu_idle_poll(); - else + } else { + tick_nohz_idle_stop_tick(); 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 @@ -984,12 +984,10 @@ void tick_nohz_irq_exit(void) { struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched); - if (ts->inidle) { + if (ts->inidle) tick_nohz_start_idle(ts); - __tick_nohz_idle_stop_tick(ts); - } else { + else tick_nohz_full_update_tick(ts); - } } /** @@ -1050,6 +1048,20 @@ static void tick_nohz_account_idle_ticks #endif } +static void __tick_nohz_idle_restart_tick(struct tick_sched *ts, ktime_t now) +{ + tick_nohz_restart_sched_tick(ts, now); + tick_nohz_account_idle_ticks(ts); +} + +void tick_nohz_idle_restart_tick(void) +{ + struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched); + + if (ts->tick_stopped) + __tick_nohz_idle_restart_tick(ts, ktime_get()); +} + /** * tick_nohz_idle_exit - restart the idle tick from the idle task * @@ -1074,10 +1086,8 @@ void tick_nohz_idle_exit(void) if (ts->idle_active) tick_nohz_stop_idle(ts, now); - if (ts->tick_stopped) { - tick_nohz_restart_sched_tick(ts, now); - tick_nohz_account_idle_ticks(ts); - } + if (ts->tick_stopped) + __tick_nohz_idle_restart_tick(ts, now); local_irq_enable(); } Index: linux-pm/include/linux/tick.h =================================================================== --- linux-pm.orig/include/linux/tick.h +++ linux-pm/include/linux/tick.h @@ -115,6 +115,7 @@ enum tick_dep_bits { extern bool tick_nohz_enabled; extern int tick_nohz_tick_stopped(void); extern void tick_nohz_idle_stop_tick(void); +extern void tick_nohz_idle_restart_tick(void); extern void tick_nohz_idle_enter(void); extern void tick_nohz_idle_exit(void); extern void tick_nohz_irq_exit(void); @@ -135,6 +136,7 @@ static inline void tick_nohz_idle_stop_t #define tick_nohz_enabled (0) static inline int tick_nohz_tick_stopped(void) { return 0; } static inline void tick_nohz_idle_stop_tick(void) { } +static inline void tick_nohz_idle_restart_tick(void) { } static inline void tick_nohz_idle_enter(void) { } static inline void tick_nohz_idle_exit(void) { }