From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753164Ab2AQSMz (ORCPT ); Tue, 17 Jan 2012 13:12:55 -0500 Received: from mx1.redhat.com ([209.132.183.28]:31281 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750949Ab2AQSMy (ORCPT ); Tue, 17 Jan 2012 13:12:54 -0500 From: Prarit Bhargava To: linux-kernel@vger.kernel.org Cc: Prarit Bhargava , Len Brown Subject: [PATCH] Fix c-state transitions when !NOHZ (nohz=off kernel parameter) Date: Tue, 17 Jan 2012 13:12:48 -0500 Message-Id: <1326823968-3482-1-git-send-email-prarit@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Some users do still boot with "nohz=off" which re-enables the old "tickfull" kernel and still want the (diminished) power savings of c-states. When examining the c-state transitions for !NOHZ it was noticed that the processors never leaving the polling state. ie) powertop shows 98-99% residency in POLLING for all cpus This occurs because the value of the current tick_sched's sleep_length is never updated from its initialized value if !NOHZ. sleep_length is used by the cpuidle code (via a call to tick_nohz_get_sleep_length()) to determine how long a cpu should sleep for and in what c-state. Admittedly I am not overly familiar with this code and there may be some unknown repercussion from doing this. I can say that experimentally across several systems I do see the cpus going into deeper c-states after making this change, and there does not appear to be anything wrong with the system. ----8<---- Fix c-state transitions when !NOHZ (nohz=off kernel parameter) The value of sleep_length needs to be updated for the !NOHZ case in order for processor c-state transitions to work. Cc: Len Brown --- kernel/time/tick-sched.c | 4 +++- 1 files changed, 3 insertions(+), 1 deletions(-) diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index 7656642..d2e2782 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -300,8 +300,10 @@ static void tick_nohz_stop_sched_tick(struct tick_sched *ts) tick_do_timer_cpu = TICK_DO_TIMER_NONE; } - if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE)) + if (unlikely(ts->nohz_mode == NOHZ_MODE_INACTIVE)) { + ts->sleep_length = ktime_sub(dev->next_event, now); return; + } if (need_resched()) return; -- 1.7.1