From patchwork Thu Apr 7 23:42:58 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Chen Yu X-Patchwork-Id: 12805403 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id DFA34C43219 for ; Thu, 7 Apr 2022 15:44:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1345055AbiDGPqX (ORCPT ); Thu, 7 Apr 2022 11:46:23 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42880 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1345029AbiDGPqP (ORCPT ); Thu, 7 Apr 2022 11:46:15 -0400 Received: from mga09.intel.com (mga09.intel.com [134.134.136.24]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id BCF26C31DD; Thu, 7 Apr 2022 08:44:11 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1649346251; x=1680882251; h=from:to:cc:subject:date:message-id:mime-version: content-transfer-encoding; bh=pNOhE9/kv8vrHZD2BZf+FPGAZBIr/4yQ5tgZOcRzFbg=; b=OyWlzofPCp/B/M+veNTK4V9IJcJBRfIfk1h8wUxa0a2trquQKUp9SqbA L1EFZfCKhUN4CmoOaAFDj6UV19Kre5sWmgDQnkzrkJCvVwxyZTez4Wesx 89KfwQwHEqsx49iQNZ6rLQ9ziJ4y5BP5JMx64rhfv7R0WTwajftR3YlF9 yElErfMvrIRthAEhADrYVj8jnMKODJLzk0DqTrRnt4fLcdicT+VQtpRVK FIn5p4r89uMrj5Tux1AzhewR5QbqGqfoDe/Okc92Rf0aQCVz4/1re3za7 34GcnBpGRs2WoipBuhb5qmYeT0FYcbIXfJQ+aMrMP1Kmy7T/axCne65Gm Q==; X-IronPort-AV: E=McAfee;i="6400,9594,10310"; a="261051683" X-IronPort-AV: E=Sophos;i="5.90,242,1643702400"; d="scan'208";a="261051683" Received: from orsmga007.jf.intel.com ([10.7.209.58]) by orsmga102.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 07 Apr 2022 08:43:54 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.90,242,1643702400"; d="scan'208";a="550114073" Received: from chenyu-dev.sh.intel.com ([10.239.158.170]) by orsmga007.jf.intel.com with ESMTP; 07 Apr 2022 08:43:51 -0700 From: Chen Yu To: linux-pm@vger.kernel.org Cc: "Rafael J. Wysocki" , Srinivas Pandruvada , Vincent Guittot , Peter Zijlstra , Len Brown , Tim Chen , Giovanni Gherdovich , Chen Yu , linux-kernel@vger.kernel.org, Zhang Rui , Chen Yu Subject: [PATCH] cpufreq: intel_pstate: Handle no_turbo in frequency invariance Date: Fri, 8 Apr 2022 07:42:58 +0800 Message-Id: <20220407234258.569681-1-yu.c.chen@intel.com> X-Mailer: git-send-email 2.25.1 MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-pm@vger.kernel.org Problem statement: Once the user has disabled turbo frequency by echo 1 > /sys/devices/system/cpu/intel_pstate/no_turbo, the cfs_rq's util_avg becomes quite small when compared with CPU capacity. Step to reproduce: echo 1 > /sys/devices/system/cpu/intel_pstate/no_turbo ./x86_cpuload --count 1 --start 3 --timeout 100 --busy 99 would launch 1 thread and bind it to CPU3, lasting for 100 seconds, with a CPU utilization of 99%. [1] top result: %Cpu3 : 98.4 us, 0.0 sy, 0.0 ni, 1.6 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st check util_avg: cat /sys/kernel/debug/sched/debug | grep "cfs_rq\[3\]" -A 20 | grep util_avg .util_avg : 611 So the util_avg/cpu capacity is 611/1024, which is much smaller than 98.4% shown in the top result. This might impact some logic in the scheduler. For example, group_is_overloaded() would compare the group_capacity and group_util in the sched group, to check if this sched group is overloaded or not. With this gap, even when there is a nearly 100% workload, the sched group will not be regarded as overloaded. Besides group_is_overloaded(), there are also other victims. There is a ongoing work that aims to optimize the task wakeup in a LLC domain. The main idea is to stop searching idle CPUs if the sched domain is overloaded[2]. This proposal also relies on the util_avg/CPU capacity to decide whether the LLC domain is overloaded. Analysis: CPU frequency invariance has caused this difference. In summary, the util_sum of cfs rq would decay quite fast when the CPU is in idle, when the CPU frequency invariance is enabled. The detail is as followed: As depicted in update_rq_clock_pelt(), when the frequency invariance is enabled, there would be two clock variables on each rq, clock_task and clock_pelt: The clock_pelt scales the time to reflect the effective amount of computation done during the running delta time but then syncs back to clock_task when rq is idle. absolute time | 1| 2| 3| 4| 5| 6| 7| 8| 9|10|11|12|13|14|15|16 @ max frequency ------******---------------******--------------- @ half frequency ------************---------************--------- clock pelt | 1| 2| 3| 4| 7| 8| 9| 10| 11|14|15|16 The fast decay of util_sum during idle is due to: 1. rq->clock_pelt is always behind rq->clock_task 2. rq->last_update is updated to rq->clock_pelt' after invoking ___update_load_sum() 3. Then the CPU becomes idle, the rq->clock_pelt' would be suddenly increased a lot to rq->clock_task 4. Enters ___update_load_sum() again, the idle period is calculated by rq->clock_task - rq->last_update, AKA, rq->clock_task - rq->clock_pelt'. The lower the CPU frequency is, the larger the delta = rq->clock_task - rq->clock_pelt' will be. Since the idle period will be used to decay the util_sum only, the util_sum drops significantly during idle period. Proposal: This symptom is not only caused by disabling turbo frequency, but it would also appear if the user limits the max frequency at runtime. Because if the frequency is always lower than the max frequency, CPU frequency invariance would decay the util_sum quite fast during idle. As some end users would disable turbo after boot up, this patch aims to present this symptom and deals with turbo scenarios for now. It might be ideal if CPU frequency invariance is aware of the max CPU frequency (user specified) at runtime in the future. [Previous patch seems to be lost on LKML, this is a resend, sorry for any inconvenience] Link: https://github.com/yu-chen-surf/x86_cpuload.git #1 Link: https://lore.kernel.org/lkml/20220310005228.11737-1-yu.c.chen@intel.com/ #2 Signed-off-by: Chen Yu Acked-by: Peter Zijlstra (Intel) Reviewed-by: Giovanni Gherdovich --- drivers/cpufreq/intel_pstate.c | 1 + 1 file changed, 1 insertion(+) diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c index 846bb3a78788..2216b24b6f84 100644 --- a/drivers/cpufreq/intel_pstate.c +++ b/drivers/cpufreq/intel_pstate.c @@ -1322,6 +1322,7 @@ static ssize_t store_no_turbo(struct kobject *a, struct kobj_attribute *b, mutex_unlock(&intel_pstate_limits_lock); intel_pstate_update_policies(); + arch_set_max_freq_ratio(global.no_turbo); mutex_unlock(&intel_pstate_driver_lock);