From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-yw0-f199.google.com (mail-yw0-f199.google.com [209.85.161.199]) by kanga.kvack.org (Postfix) with ESMTP id 766CC6B0006 for ; Wed, 11 Apr 2018 10:09:17 -0400 (EDT) Received: by mail-yw0-f199.google.com with SMTP id m75so880293ywd.23 for ; Wed, 11 Apr 2018 07:09:17 -0700 (PDT) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id u127sor283897ywg.104.2018.04.11.07.09.16 for (Google Transport Security); Wed, 11 Apr 2018 07:09:16 -0700 (PDT) Date: Wed, 11 Apr 2018 07:09:13 -0700 From: Tejun Heo Subject: Re: [PATCH] Revert mm/vmstat.c: fix vmstat_update() preemption BUG Message-ID: <20180411140913.GE793541@devbig577.frc2.facebook.com> References: <20180411095757.28585-1-bigeasy@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Vlastimil Babka Cc: Sebastian Andrzej Siewior , linux-mm@kvack.org, linux-kernel@vger.kernel.org, tglx@linutronix.de, "Steven J . Hill" , Andrew Morton , Christoph Lameter Hello, On Wed, Apr 11, 2018 at 03:56:43PM +0200, Vlastimil Babka wrote: > > vmstat_update() is invoked by a kworker on a specific CPU. This worker > > it bound to this CPU. The name of the worker was "kworker/1:1" so it > > should have been a worker which was bound to CPU1. A worker which can > > run on any CPU would have a `u' before the first digit. > > Oh my, and I have just been assured by Tejun that his cannot happen :) > And yet, in the original report [1] I see: > > CPU: 0 PID: 269 Comm: kworker/1:1 Not tainted > > So is this perhaps related to the cpu hotplug that [1] mentions? e.g. is > the cpu being hotplugged cpu 1, the worker started too early before > stuff can be scheduled on the CPU, so it has to run on different than > designated CPU? > > [1] https://marc.info/?l=linux-mm&m=152088260625433&w=2 The report says that it happens when hotplug is attempted. Per-cpu doesn't pin the cpu alive, so if the cpu goes down while a work item is in flight or a work item is queued while a cpu is offline it'll end up executing on some other cpu. So, if a piece of code doesn't want that happening, it gotta interlock itself - ie. start queueing when the cpu comes online and flush and prevent further queueing when its cpu goes down. Thanks. -- tejun