Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752729AbdI1LjS (ORCPT ); Thu, 28 Sep 2017 07:39:18 -0400 Received: from merlin.infradead.org ([205.233.59.134]:53048 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751058AbdI1LjR (ORCPT ); Thu, 28 Sep 2017 07:39:17 -0400 Date: Thu, 28 Sep 2017 13:38:55 +0200 From: Peter Zijlstra To: Sergey Senozhatsky Cc: "Levin, Alexander (Sasha Levin)" , Sasha Levin , Thomas Gleixner , Ingo Molnar , "linux-kernel@vger.kernel.org List" , "pmladek@suse.com" , "sergey.senozhatsky@gmail.com" , "rostedt@goodmis.org" Subject: Re: sched: serial port lockdep warning when offlining CPUs Message-ID: <20170928113855.btpqkjvy2odhc6wx@hirez.programming.kicks-ass.net> References: <20170928093847.zgcgvxb7ajiaywbr@hirez.programming.kicks-ass.net> <20170928110428.ew5ukfihb3bdiszr@sasha-lappy> <20170928111325.GA30818@jagdpanzerIV.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170928111325.GA30818@jagdpanzerIV.localdomain> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1185 Lines: 30 On Thu, Sep 28, 2017 at 08:13:26PM +0900, Sergey Senozhatsky wrote: > On (09/28/17 11:04), Levin, Alexander (Sasha Levin) wrote: > > On Thu, Sep 28, 2017 at 11:38:47AM +0200, Peter Zijlstra wrote: > > >On Thu, Sep 28, 2017 at 02:19:46AM -0700, Sasha Levin wrote: > > >> Hi all, > > >> > > >> I seem to be hitting the following warning when offlining CPUs on the > > >> latest -next kernel: > > >> > > >> [289683102.607076] Unregister pv shared memory for cpu 8 > > >> [289683102.622922] select_fallback_rq: 3 callbacks suppressed > > > > > >This is because you hit a printk() from the scheduler, that's known > > >broken (along with the rest of printk). > > > > > >You forgot to actually include that printk() though I suspect it reads > > >like: > > > > > > "process %d (%s) no longer affine to cpu%d\n" > > > > > >Now that uses printk_deferred() which _should_ work lots better, but > > >clearly the printk() stuff went wobbly again. > > > > Hm, so is this an actual possible lockup, or just a false positive? > > a possible one. Yeah, printk() can deadlock when used from scheduler locks / NMI context and a few other crucial places. printk_deferred() used to be 'safe' though.