From mboxrd@z Thu Jan 1 00:00:00 1970 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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D5B8FC4332F for ; Mon, 15 Nov 2021 23:41:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BB2FC61AFF for ; Mon, 15 Nov 2021 23:41:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1355458AbhKOXnV (ORCPT ); Mon, 15 Nov 2021 18:43:21 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:34508 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232742AbhKOTXF (ORCPT ); Mon, 15 Nov 2021 14:23:05 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1637004008; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=qmwKehppkwWgIW0CJQQmyBVMOkBKnionBiXfk6rMmJ0=; b=UlCP56hkgg42xT+0o8aifsFvoMnMM7QOS6Bmu7I1fDNsNAZbH9vEQTQnbT+SecdvT9nqi3 pVUedNnjZGwXgxNxPWhzndo4KPEICzSQWPp59/9vatXjxaBtFw37ca+VIHpXVHgo+vA2Rt WAmdfHi5J/VM7DfzupBnG3W7+cWJTD8= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-314-Kq5aWUEKNAWsz2-tY5v98Q-1; Mon, 15 Nov 2021 14:20:03 -0500 X-MC-Unique: Kq5aWUEKNAWsz2-tY5v98Q-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 3BA0C1808308; Mon, 15 Nov 2021 19:20:01 +0000 (UTC) Received: from [10.22.33.148] (unknown [10.22.33.148]) by smtp.corp.redhat.com (Postfix) with ESMTP id E9A6210190A7; Mon, 15 Nov 2021 19:19:59 +0000 (UTC) Message-ID: Date: Mon, 15 Nov 2021 14:19:59 -0500 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0 Subject: Re: [PATCH 1/2] clocksource: Avoid accidental unstable marking of clocksources Content-Language: en-US To: Feng Tang Cc: "Paul E. McKenney" , John Stultz , Thomas Gleixner , Stephen Boyd , linux-kernel@vger.kernel.org, Peter Zijlstra , Cassio Neri , Linus Walleij , Colin Ian King , Frederic Weisbecker References: <20211110221732.272986-1-longman@redhat.com> <20211110221732.272986-2-longman@redhat.com> <20211111045703.GA15896@shbuild999.sh.intel.com> <20211111144311.GK641268@paulmck-ThinkPad-P17-Gen-1> <20211112054417.GA29845@shbuild999.sh.intel.com> <889b16c6-b6cc-63d7-a6de-8cec42c7d78c@redhat.com> <20211114155407.GB641268@paulmck-ThinkPad-P17-Gen-1> <20211115020851.GB29845@shbuild999.sh.intel.com> <20211115075915.GA34844@shbuild999.sh.intel.com> From: Waiman Long In-Reply-To: <20211115075915.GA34844@shbuild999.sh.intel.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/15/21 02:59, Feng Tang wrote: > On Sun, Nov 14, 2021 at 10:24:56PM -0500, Waiman Long wrote: >> On 11/14/21 21:08, Feng Tang wrote: >>> Or did you have something else in mind? >>>>>> I'm not sure the detail in Waiman's cases, and in our cases (stress-ng) >>>>>> the delay between watchdog's (HPET here) read were not linear, that >>>>>> from debug data, sometimes the 3-2 difference could be bigger or much >>>>>> bigger than the 2-1 difference. >>>>>> >>>>>> The reason could be the gap between 2 reads depends hugely on the system >>>>>> pressure at that time that 3 HPET read happens. On our test box (a >>>>>> 2-Socket Cascade Lake AP server), the 2-1 and 3-2 difference are stably >>>>>> about 2.5 us, while under the stress it could be bumped to from 6 us >>>>>> to 2800 us. >>>>>> >>>>>> So I think checking the 3-2 difference plus increasing the max retries >>>>>> to 10 may be a simple way, if the watchdog read is found to be >>>>>> abnormally long, we skip this round of check. >>>>> On one of the test system, I had measured that normal delay >>>>> (hpet->tsc->hpet) was normally a bit over 2us. It was a bit more than 4us at >>>>> bootup time. However, the same system under stress could have a delay of >>>>> over 200us at bootup time. When I measured the consecutive hpet delay, it >>>>> was about 180us. So hpet read did dominate the total clocksource read delay. >>>> Thank you both for the data! >>>> >>>>> I would not suggest increasing the max retries as it may still fail in most >>>>> cases because the system stress will likely not be going away within a short >>>>> time. So we are likely just wasting cpu times. I believe we should just skip >>>>> it if it is the watchdog read that is causing most of the delay. >>>> If anything, adding that extra read would cause me to -reduce- the number >>>> of retries to avoid increasing the per-watchdog overhead. >>> I understand Waiman's concern here, and in our test patch, the 2 >>> consecutive watchdog read delay check is done inside this retrying >>> loop accompanying the 'cs' read, and once an abnormal delay is found, >>> the watchdog check is skipped without waiting for the max-retries to >>> complete. >>> >>> Our test data shows the consecutive delay is not always big even when >>> the system is much stressed, that's why I suggest to increase the >>> retries. >> If we need a large number of retries to avoid triggering the unstable TSC >> message, we should consider increase the threshod instead. Right? >> >> That is why my patch 2 makes the max skew value a configurable option so >> that we can tune it if necessary. > I'm fine with it, though the ideal case I expected is with carefully > picked values for max_retries/screw_threshhold, we could save the users > from configuring these. But given the complexity of all HWs out there, > it's not an easy goal. > > And I still suggest to put the consecutive watchdog read check inside > the retry loop, so that it could bail out early when detecting the > abnormal delay. Yes, I think that may sense. > > Another thing is we may need to set the 'watchdog_reset_pending', as > under the stress, there could be consecutive many times of "skipping" > watchdog check, and the saved value of 'cs' and 'watchdog' should be > reset. I agree with that too. IOW, we combine the best part of the patches together. I will post an updated patch for that. Cheers, Longman