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 X-Spam-Level: X-Spam-Status: No, score=-3.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, UNPARSEABLE_RELAY,URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E34E3C07E9C for ; Thu, 8 Jul 2021 03:11:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CA94C61CD8 for ; Thu, 8 Jul 2021 03:11:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230414AbhGHDOO convert rfc822-to-8bit (ORCPT ); Wed, 7 Jul 2021 23:14:14 -0400 Received: from out30-43.freemail.mail.aliyun.com ([115.124.30.43]:57561 "EHLO out30-43.freemail.mail.aliyun.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230244AbhGHDON (ORCPT ); Wed, 7 Jul 2021 23:14:13 -0400 X-Alimail-AntiSpam: AC=PASS;BC=-1|-1;BR=01201311R861e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=e01e04423;MF=zhaoyan.liao@linux.alibaba.com;NM=1;PH=DS;RN=11;SR=0;TI=SMTPD_---0Uf4GV0k_1625713889; Received: from 30.250.200.142(mailfrom:zhaoyan.liao@linux.alibaba.com fp:SMTPD_---0Uf4GV0k_1625713889) by smtp.aliyun-inc.com(127.0.0.1); Thu, 08 Jul 2021 11:11:30 +0800 Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (Mac OS X Mail 14.0 \(3654.40.0.2.32\)) Subject: Re: [PATCH] use 64bit timer for hpet From: Linux In-Reply-To: <875yxmqw2s.ffs@nanos.tec.linutronix.de> Date: Thu, 8 Jul 2021 11:11:28 +0800 Cc: mingo@redhat.com, hpa@zytor.com, dwmw@amazon.co.uk, linux-kernel@vger.kernel.org, kernel-janitors@vger.kernel.org, songmuchun@bytedance.com, likunkun@bytedance.com, guancheng.rjk@alibaba-inc.com, duanxiongchun@bytedance.com, wenan.mao@linux.alibaba.com Content-Transfer-Encoding: 8BIT Message-Id: <8A96C0F7-FBE4-4B23-8565-E814401BF927@linux.alibaba.com> References: <1625213625-25745-1-git-send-email-zhaoyan.liao@linux.alibaba.com> <875yxmqw2s.ffs@nanos.tec.linutronix.de> To: Thomas Gleixner X-Mailer: Apple Mail (2.3654.40.0.2.32) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Gleixner, > Seriously? The wrap-around time for 32bit HPET @24MHz is ~3 minutes. In some cases, our system will be very busy, and the timeout of 3 minutes is not an exaggeration. Then, the system considers that the tsc clock is inaccurate and switches the tsc clock to the hpet clock, which brings greater performance overhead. > Aside of that the reason why the kernel does not support 64bit HPET is > that there are HPETs which advertise 64bit support, but the > implementation is buggy. Can you tell me what is the buggy with the 64-bit hpet clock? In my opinion, it is unreasonable to use a lower-bit width clock to calibrate a higher-bit width clock, and the hardware already supports the higher-bit width. > 2021年7月7日 下午6:04，Thomas Gleixner 写道： > > Liao, > > On Fri, Jul 02 2021 at 16:13, zhaoyan liao wrote: >> The kernel judges whether the tsc clock is accurate in the >> clocksource_watchdog background thread function. The hpet clock source >> is 32-bit, but tsc is 64-bit. Therefore, when the system is busy and the >> clocksource_watchdog cannot be scheduled in time, the hpet clock may >> overflow and cause the system to misjudge tsc as unreliable. > > Seriously? The wrap-around time for 32bit HPET @24MHz is ~3 minutes. > >> In this case, we recommend that the kernel adopts the 64-bit hpet clock >> by default to keep the width of the two clock sources the same to reduce >> misjudgment. Some CPU models may not support 64-bit hpet, but according >> to the description of the CPU's register manual, it does not affect our >> reading action. > > So much for the theory. > >> -#define HPET_MASK CLOCKSOURCE_MASK(32) >> +#define HPET_MASK CLOCKSOURCE_MASK(64) > > How is that valid for a 32bit HPET? This breaks the clocksource. > >> +inline unsigned long hpet_readq(unsigned int a) >> +{ >> + return readq(hpet_virt_address + a); > > Breaks 32bit build immediately. > > Aside of that the reason why the kernel does not support 64bit HPET is > that there are HPETs which advertise 64bit support, but the > implementation is buggy. > > IOW, while this works for your hardware this breaks quite some parts of > the universe. Not really a good approach. > > Thanks, > > tglx