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=-16.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=ham 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 BB44AC433F5 for ; Fri, 17 Sep 2021 15:08:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A51EC61164 for ; Fri, 17 Sep 2021 15:08:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233632AbhIQPJ4 (ORCPT ); Fri, 17 Sep 2021 11:09:56 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:39753 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230429AbhIQPJu (ORCPT ); Fri, 17 Sep 2021 11:09:50 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1631891308; 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: in-reply-to:in-reply-to:references:references; bh=BDOPunLui+O3AxAR6Z76Qgr43mUFIQ9hp6aQ6KbOUvg=; b=Ak5Fu7Yau4XkSwL+r3xegtn5avbxiOKpTJlPHcuSrFY/GzaPRHz2I2r3f44vZ/NuhPSwmy CT56pB7WbP9wmsxN2CgdEhTyfQW3jEZ4Hi1+GrOqDEfw1V9zO+bLsFs7OnBIVqwhVYhlOh 3L2p6zqHb3g60VjXQhSc63VU1EJnYUw= 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-494-CrElNu7_PNekYFzVga5gBQ-1; Fri, 17 Sep 2021 11:08:26 -0400 X-MC-Unique: CrElNu7_PNekYFzVga5gBQ-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 4E40E1084683; Fri, 17 Sep 2021 15:08:25 +0000 (UTC) Received: from piliu.users.ipa.redhat.com (ovpn-8-17.pek2.redhat.com [10.72.8.17]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 28F5069FAD; Fri, 17 Sep 2021 15:08:19 +0000 (UTC) Date: Fri, 17 Sep 2021 23:08:13 +0800 From: Pingfan Liu To: Petr Mladek Cc: Peter Zijlstra , Pingfan Liu , linux-kernel@vger.kernel.org, Andrew Morton , Wang Qing , Santosh Sivaraj , Sumit Garg , Will Deacon , Mark Rutland Subject: Re: [PATCH 2/5] kernel/watchdog_hld: clarify the condition in hardlockup_detector_event_create() Message-ID: References: <20210915035103.15586-1-kernelfans@gmail.com> <20210915035103.15586-3-kernelfans@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 16, 2021 at 10:02:27AM +0200, Petr Mladek wrote: > On Thu 2021-09-16 11:57:44, Pingfan Liu wrote: > > On Wed, Sep 15, 2021 at 03:45:06PM +0200, Peter Zijlstra wrote: > > > On Wed, Sep 15, 2021 at 11:51:00AM +0800, Pingfan Liu wrote: > > > > hardlockup_detector_event_create() indirectly calls > > > > kmem_cache_alloc_node(), which is blockable. > > > > > > > > So here, the really planned context is is_percpu_thread(). > > > > > > > > Signed-off-by: Pingfan Liu > > > > Cc: Petr Mladek > > > > Cc: Andrew Morton > > > > Cc: Wang Qing > > > > Cc: "Peter Zijlstra (Intel)" > > > > Cc: Santosh Sivaraj > > > > Cc: Sumit Garg > > > > Cc: Will Deacon > > > > Cc: Mark Rutland > > > > To: linux-kernel@vger.kernel.org > > > > --- > > > > kernel/watchdog_hld.c | 5 ++++- > > > > 1 file changed, 4 insertions(+), 1 deletion(-) > > > > > > > > diff --git a/kernel/watchdog_hld.c b/kernel/watchdog_hld.c > > > > index 247bf0b1582c..6876e796dbf5 100644 > > > > --- a/kernel/watchdog_hld.c > > > > +++ b/kernel/watchdog_hld.c > > > > @@ -165,10 +165,13 @@ static void watchdog_overflow_callback(struct perf_event *event, > > > > > > > > static int hardlockup_detector_event_create(void) > > > > { > > > > - unsigned int cpu = smp_processor_id(); > > > > + unsigned int cpu; > > > > struct perf_event_attr *wd_attr; > > > > struct perf_event *evt; > > > > > > > > + /* This function plans to execute in cpu bound kthread */ > > > > + BUG_ON(!is_percpu_thread()); > > > > + cpu = raw_smp_processor_id(); > > > > wd_attr = &wd_hw_attr; > > > > wd_attr->sample_period = hw_nmi_get_sample_period(watchdog_thresh); > > > > > > This patch makes no sense. > > > > This patch aims to disable any attempt such as using get_cpu()/put_cpu() to > > shut up the check_preemption_disabled(). > > I have to say that the description of the problem is really cryptic. > Please, provide more context, code paths, sample code, next time. > Sorry, I will be more straight forward. And I had thought commit log had mentioned it. > Well, I probably got it. The code might sleep. But it should run on the And you do get it. > same CPU even after waking up. You try to achieve this by running > the code in a process that is bound to a single CPU. > > IMHO, this is not reliable. Anyone could change the affinity > of the process in the meantime. > No, it is not. As the code says: PF_NO_SETAFFINITY. static inline bool is_percpu_thread(void) { #ifdef CONFIG_SMP return (current->flags & PF_NO_SETAFFINITY) && (current->nr_cpus_allowed == 1); #else return true; #endif } This is critical for cpuhp_* (kernel/cpu.c). And hardlockup_detector_event_create() should be planned to run on such a kthread. Thanks, Pingfan > I see two solutions. Either avoid the sleep or making sure > that the code access per-CPU variables on the same CPU > all the time. > > For example, you might use > > *per_cpu_ptr(watchdog_ev, cpu) = evt; > > instead of > > this_cpu_write(watchdog_ev, evt); > > Best Regards, > Petr