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,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 7D6B0C76195 for ; Fri, 19 Jul 2019 00:55:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 57EDD20665 for ; Fri, 19 Jul 2019 00:55:10 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726042AbfGSAzK (ORCPT ); Thu, 18 Jul 2019 20:55:10 -0400 Received: from lgeamrelo12.lge.com ([156.147.23.52]:58054 "EHLO lgeamrelo11.lge.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726072AbfGSAzK (ORCPT ); Thu, 18 Jul 2019 20:55:10 -0400 Received: from unknown (HELO lgeamrelo02.lge.com) (156.147.1.126) by 156.147.23.52 with ESMTP; 19 Jul 2019 09:55:07 +0900 X-Original-SENDERIP: 156.147.1.126 X-Original-MAILFROM: byungchul.park@lge.com Received: from unknown (HELO X58A-UD3R) (10.177.222.33) by 156.147.1.126 with ESMTP; 19 Jul 2019 09:55:07 +0900 X-Original-SENDERIP: 10.177.222.33 X-Original-MAILFROM: byungchul.park@lge.com Date: Fri, 19 Jul 2019 09:54:03 +0900 From: Byungchul Park To: "Paul E. McKenney" Cc: Joel Fernandes , Byungchul Park , rcu , LKML , kernel-team@lge.com Subject: Re: [PATCH] rcu: Make jiffies_till_sched_qs writable Message-ID: <20190719005403.GB28226@X58A-UD3R> References: <20190711195839.GA163275@google.com> <20190712063240.GD7702@X58A-UD3R> <20190712125116.GB92297@google.com> <20190713151330.GE26519@linux.ibm.com> <20190713154257.GE133650@google.com> <20190713174111.GG26519@linux.ibm.com> <20190718213419.GV14271@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190718213419.GV14271@linux.ibm.com> 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 02:34:19PM -0700, Paul E. McKenney wrote: > On Thu, Jul 18, 2019 at 12:14:22PM -0400, Joel Fernandes wrote: > > Trimming the list a bit to keep my noise level low, > > > > On Sat, Jul 13, 2019 at 1:41 PM Paul E. McKenney wrote: > > [snip] > > > > It still feels like you guys are hyperfocusing on this one particular > > > > > knob. I instead need you to look at the interrelating knobs as a group. > > > > > > > > Thanks for the hints, we'll do that. > > > > > > > > > On the debugging side, suppose someone gives you an RCU bug report. > > > > > What information will you need? How can you best get that information > > > > > without excessive numbers of over-and-back interactions with the guy > > > > > reporting the bug? As part of this last question, what information is > > > > > normally supplied with the bug? Alternatively, what information are > > > > > bug reporters normally expected to provide when asked? > > > > > > > > I suppose I could dig out some of our Android bug reports of the past where > > > > there were RCU issues but if there's any fires you are currently fighting do > > > > send it our way as debugging homework ;-) > > > > > > Suppose that you were getting RCU CPU stall > > > warnings featuring multi_cpu_stop() called from cpu_stopper_thread(). > > > Of course, this really means that some other CPU/task is holding up > > > multi_cpu_stop() without also blocking the current grace period. > > > > > > > So I took a shot at this trying to learn how CPU stoppers work in > > relation to this problem. > > > > I am assuming here say CPU X has entered MULTI_STOP_DISABLE_IRQ state > > in multi_cpu_stop() but another CPU Y has not yet entered this state. > > So CPU X is stalling RCU but it is really because of CPU Y. Now in the > > problem statement, you mentioned CPU Y is not holding up the grace > > period, which means Y doesn't have any of IRQ, BH or preemption > > disabled ; but is still somehow stalling RCU indirectly by troubling > > X. > > > > This can only happen if : > > - CPU Y has a thread executing on it that is higher priority than CPU > > X's stopper thread which prevents it from getting scheduled. - but the > > CPU stopper thread (migration/..) is highest priority RT so this would > > be some kind of an odd scheduler bug. > > - 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. > > - CPU Y might have already passed through its quiescent state for > the current grace period, then disabled IRQs indefinitely. Or for a longer time than the period that rcu considers as a stall. Or preemption disabled for that long time. Or the stopper on Y even has yet to be woken up inside scheduler because of any reasons but maybe locks. > Now, CPU Y would block a later grace period, but CPU X is > preventing the current grace period from ending, so no such > later grace period can start. > > > Did I get that right? Would be exciting to run the rcutorture test > > once Paul has it available to reproduce this problem. > > Working on it! Slow, I know! > > Thanx, Paul