From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753477AbeCFJNA (ORCPT ); Tue, 6 Mar 2018 04:13:00 -0500 Received: from cloudserver094114.home.pl ([79.96.170.134]:60016 "EHLO cloudserver094114.home.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753069AbeCFJMe (ORCPT ); Tue, 6 Mar 2018 04:12:34 -0500 From: "Rafael J. Wysocki" To: Peter Zijlstra , Linux PM Cc: Thomas Gleixner , Frederic Weisbecker , Paul McKenney , Thomas Ilsche , Doug Smythies , Rik van Riel , Aubrey Li , Mike Galbraith , LKML Subject: [RFC/RFT][PATCH v2 2/6] sched: idle: Do not stop the tick upfront in the idle loop Date: Tue, 06 Mar 2018 10:02:15 +0100 Message-ID: <3346281.BDGJiv2ZOp@aspire.rjw.lan> In-Reply-To: <2067762.1uWBf5RSRc@aspire.rjw.lan> References: <2067762.1uWBf5RSRc@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 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 --- -> 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); }