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=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS 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 01BE9C072B5 for ; Fri, 24 May 2019 17:28:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C2B6A217F9 for ; Fri, 24 May 2019 17:28:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731988AbfEXR2t (ORCPT ); Fri, 24 May 2019 13:28:49 -0400 Received: from mx1.redhat.com ([209.132.183.28]:44724 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726674AbfEXR2t (ORCPT ); Fri, 24 May 2019 13:28:49 -0400 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 028C3305B16F; Fri, 24 May 2019 17:28:49 +0000 (UTC) Received: from llong.remote.csb (dhcp-17-85.bos.redhat.com [10.18.17.85]) by smtp.corp.redhat.com (Postfix) with ESMTP id CD5EB5D71B; Fri, 24 May 2019 17:28:47 +0000 (UTC) Subject: Re: [PATCH v2] locking/lock_events: Use this_cpu_add() when necessary To: Will Deacon Cc: Peter Zijlstra , Ingo Molnar , Thomas Gleixner , Borislav Petkov , "H. Peter Anvin" , linux-kernel@vger.kernel.org, x86@kernel.org, Davidlohr Bueso , Linus Torvalds , Tim Chen , huang ying References: <20190524165346.26373-1-longman@redhat.com> <20190524171939.GA9120@fuggles.cambridge.arm.com> From: Waiman Long Organization: Red Hat Message-ID: Date: Fri, 24 May 2019 13:28:47 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20190524171939.GA9120@fuggles.cambridge.arm.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.49]); Fri, 24 May 2019 17:28:49 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/24/19 1:19 PM, Will Deacon wrote: > On Fri, May 24, 2019 at 12:53:46PM -0400, Waiman Long wrote: >> The kernel test robot has reported that the use of __this_cpu_add() >> causes bug messages like: >> >> BUG: using __this_cpu_add() in preemptible [00000000] code: ... >> >> This is only an issue on preempt kernel where preemption can happen in >> the middle of a percpu operation. We are still using __this_cpu_*() for >> !preempt kernel to avoid additional overhead in case CONFIG_PREEMPT_COUNT >> is set. >> >> v2: Simplify the condition to just preempt or !preempt. >> >> Fixes: a8654596f0371 ("locking/rwsem: Enable lock event counting") >> Signed-off-by: Waiman Long >> --- >> kernel/locking/lock_events.h | 23 +++++++++++++++++++++-- >> 1 file changed, 21 insertions(+), 2 deletions(-) >> >> diff --git a/kernel/locking/lock_events.h b/kernel/locking/lock_events.h >> index feb1acc54611..05f34068ec06 100644 >> --- a/kernel/locking/lock_events.h >> +++ b/kernel/locking/lock_events.h >> @@ -30,13 +30,32 @@ enum lock_events { >> */ >> DECLARE_PER_CPU(unsigned long, lockevents[lockevent_num]); >> >> +/* >> + * The purpose of the lock event counting subsystem is to provide a low >> + * overhead way to record the number of specific locking events by using >> + * percpu counters. It is the percpu sum that matters, not specifically >> + * how many of them happens in each cpu. >> + * >> + * In !preempt kernel, we can just use __this_cpu_*() as preemption >> + * won't happen in the middle of the percpu operation. In preempt kernel, >> + * preemption happens in the middle of the percpu operation may produce >> + * incorrect result. >> + */ >> +#ifdef CONFIG_PREEMPT >> +#define lockevent_percpu_inc(x) this_cpu_inc(x) >> +#define lockevent_percpu_add(x, v) this_cpu_add(x, v) >> +#else >> +#define lockevent_percpu_inc(x) __this_cpu_inc(x) >> +#define lockevent_percpu_add(x, v) __this_cpu_add(x, v) > Are you sure this works wrt IRQs? For example, if I take an interrupt when > trying to update the counter, and then the irq handler takes a qspinlock > which in turn tries to update the counter. Would I lose an update in that > scenario? > > Will Good point! But this will be an issue even if we use the non-underscore version as I don't think it will disable interrupt. Also it is only a problem if the percpu operation is more than 1 instruction. It is a single instruction for x86. Other architectures may require more than 1 instruction. In those cases, we may lose count, but it is still better than getting the count from one CPU and put it into another CPU. Cheers, Longman