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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 1D1F7C7618F for ; Fri, 19 Jul 2019 01:11:17 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E69972084D for ; Fri, 19 Jul 2019 01:11:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726248AbfGSBLQ (ORCPT ); Thu, 18 Jul 2019 21:11:16 -0400 Received: from lgeamrelo11.lge.com ([156.147.23.51]:43008 "EHLO lgeamrelo11.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726072AbfGSBLQ (ORCPT ); Thu, 18 Jul 2019 21:11:16 -0400 Received: from unknown (HELO lgeamrelo01.lge.com) (156.147.1.125) by 156.147.23.51 with ESMTP; 19 Jul 2019 10:11:15 +0900 X-Original-SENDERIP: 156.147.1.125 X-Original-MAILFROM: byungchul.park@lge.com Received: from unknown (HELO X58A-UD3R) (10.177.222.33) by 156.147.1.125 with ESMTP; 19 Jul 2019 10:11:15 +0900 X-Original-SENDERIP: 10.177.222.33 X-Original-MAILFROM: byungchul.park@lge.com Date: Fri, 19 Jul 2019 10:10:11 +0900 From: Byungchul Park To: Joel Fernandes Cc: "Paul E. McKenney" , Byungchul Park , rcu , LKML , kernel-team@lge.com Subject: Re: [PATCH] rcu: Make jiffies_till_sched_qs writable Message-ID: <20190719011011.GC28226@X58A-UD3R> References: <20190712063240.GD7702@X58A-UD3R> <20190712125116.GB92297@google.com> <20190713151330.GE26519@linux.ibm.com> <20190713154257.GE133650@google.com> <20190713174111.GG26519@linux.ibm.com> <20190719003942.GA28226@X58A-UD3R> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Thu, Jul 18, 2019 at 08:52:52PM -0400, Joel Fernandes wrote: > On Thu, Jul 18, 2019 at 8:40 PM Byungchul Park wrote: > [snip] > > > - There is a bug in the CPU stopper machinery itself preventing it > > > from scheduling the stopper on Y. Even though Y is not holding up the > > > grace period. > > > > Or any thread on Y is busy with preemption/irq disabled preventing the > > stopper from being scheduled on Y. > > > > Or something is stuck in ttwu() to wake up the stopper on Y due to any > > scheduler locks such as pi_lock or rq->lock or something. > > > > I think what you mentioned can happen easily. > > > > Basically we would need information about preemption/irq disabled > > sections on Y and scheduler's current activity on every cpu at that time. > > I think all that's needed is an NMI backtrace on all CPUs. An ARM we > don't have NMI solutions and only IPI or interrupt based backtrace > works which should at least catch and the preempt disable and softirq > disable cases. > > But yeah I don't see why just the stacks of those CPUs that are > blocking the CPU X would not suffice for the trivial cases where a > piece of misbehaving code disable interrupts / preemption and > prevented the stopper thread from executing. Right. So it makes more interesting tho! :-) > May be once the test case is ready (no rush!) , then it will be more > clear what can help. Yes. I'm really happy to help things about RCU that I love, fixed or improved. And with the the test case or a real issue, I believe I can do more helpful work. Looking forward to it, too (no rush!). Thanks, Byungchul