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=-7.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 C5A41C3A589 for ; Thu, 15 Aug 2019 18:15:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 901182064A for ; Thu, 15 Aug 2019 18:15:20 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="p5/r3BDq" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730405AbfHOSPU (ORCPT ); Thu, 15 Aug 2019 14:15:20 -0400 Received: from mail-pg1-f194.google.com ([209.85.215.194]:46509 "EHLO mail-pg1-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730342AbfHOSPT (ORCPT ); Thu, 15 Aug 2019 14:15:19 -0400 Received: by mail-pg1-f194.google.com with SMTP id m3so1037359pgv.13 for ; Thu, 15 Aug 2019 11:15:19 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=wLsgqXU1lI5ukS7nuzUPXn2TwlmqZhK7yyq3cYkhveo=; b=p5/r3BDqyePLRWZQ4kkU58Z4NXUAlgxniz8FZG8FodttPDb6DqKAwtfqBI8v/T7JKf IKwqVHBxHrJsav3WVNp0XCsWhr3VNrX2dwvYrtE8BkHFbjfzVsVSOJvH8NDMtIz8jyi+ iGdcy7h2WlDyP2NxctB4VG8nteB3Nl7KObP+A= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=wLsgqXU1lI5ukS7nuzUPXn2TwlmqZhK7yyq3cYkhveo=; b=fbUiU//Tiw8mswKjyba52E7uc3VT01Rl1JW1julRLG8U3KRG7f7xthB2hKQhkLwAPZ rEK0L5I2XHsu5D1q/Vk9z6K3j493ow/ipdx/bX0phDrFROIcXREmkLdtmVOs4hf9chbz C2THDstQaWAhyWCmQLZLeXMg604xgZAWtklnrvRmouRXuCE6wKrp/zpT1XVrVl0zhSoK EJA76OaPKSaNqp48ziBberWl3ktKB56riAQj8biR2Pk/pv2+Nx3eoPpsutaxTWeIzXd1 JH7aWtchTRCir4iI8xJMEV6OBfGhPEe+J4LPIKnL0Y7B8gW0G1DIh81A9jNu+uoMQkzo jOYg== X-Gm-Message-State: APjAAAUOMAsMna/2abi4VRx995IrnByswWPnTU8DayTGBFHlsldgOO6N 0ch4l8Oav7j98QgWM/1d9I/N6w== X-Google-Smtp-Source: APXvYqxTFqdb8fffqZFeuX0zZUg9CtmHSvW4suc+gHzFWwMwUJHi6Czj1sHJLviOjZfcOi9pT6vJGg== X-Received: by 2002:aa7:93aa:: with SMTP id x10mr6846163pff.83.1565892918857; Thu, 15 Aug 2019 11:15:18 -0700 (PDT) Received: from localhost ([172.19.216.18]) by smtp.gmail.com with ESMTPSA id a128sm3419007pfb.185.2019.08.15.11.15.17 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 15 Aug 2019 11:15:17 -0700 (PDT) Date: Thu, 15 Aug 2019 14:15:00 -0400 From: Joel Fernandes To: "Paul E. McKenney" Cc: Frederic Weisbecker , rcu@vger.kernel.org, linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com Subject: Re: [PATCH RFC tip/core/rcu 14/14] rcu/nohz: Make multi_cpu_stop() enable tick on all online CPUs Message-ID: <20190815181500.GC12078@google.com> References: <20190802151435.GA1081@linux.ibm.com> <20190802151501.13069-14-paulmck@linux.ibm.com> <20190812210232.GA3648@lenoir> <20190812232316.GT28441@linux.ibm.com> <20190813123016.GA11455@lenoir> <20190813144809.GB28441@linux.ibm.com> <20190814175546.GB68498@google.com> <20190814220516.GY28441@linux.ibm.com> <20190815150735.GA12078@google.com> <20190815172351.GI28441@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190815172351.GI28441@linux.ibm.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Thu, Aug 15, 2019 at 10:23:51AM -0700, Paul E. McKenney wrote: > On Thu, Aug 15, 2019 at 11:07:35AM -0400, Joel Fernandes wrote: > > On Wed, Aug 14, 2019 at 03:05:16PM -0700, Paul E. McKenney wrote: > > [snip] > > > > > If so, perhaps that monitoring could periodically invoke an RCU function > > > > > that I provide for deciding when to turn the tick on. We would also need > > > > > to work out how to turn the tick off in a timely fashion once the CPU got > > > > > out of kernel mode, perhaps in rcu_user_enter() or rcu_nmi_exit_common(). > > > > > > > > > > If this would be called only every second or so, the separate grace-period > > > > > checking is still needed for its shorter timespan, though. > > > > > > > > > > Thoughts? > > > > > > > > Do you want me to test the below patch to see if it fixes the issue with my > > > > other test case (where I had a nohz full CPU holding up a grace period). > > > > > > Please! > > > > I tried the patch below, but it did not seem to make a difference to the > > issue I was seeing. My test tree is here in case you can spot anything I did > > not do right: https://github.com/joelagnel/linux-kernel/commits/rcu/nohz-test > > The main patch is here: > > https://github.com/joelagnel/linux-kernel/commit/4dc282b559d918a0be826936f997db0bdad7abb3 > > That is more aggressive that rcutorture's rcu_torture_fwd_prog_nr(), so > I am guessing that I need to up rcu_torture_fwd_prog_nr()'s game. I am > currently testing that. > > > On the trace output, I grep something like: egrep "(rcu_perf|cpu 3|3d)". I > > see a few ticks after 300ms, but then there are no more ticks and just a > > periodic resched_cpu() from rcu_implicit_dynticks_qs(): > > > > [ 19.534107] rcu_perf-165 12.... 2276436us : rcu_perf_writer: Start of rcuperf test > > [ 19.557968] rcu_pree-10 0d..1 2287973us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 > > [ 20.136222] rcu_perf-165 3d.h. 2591894us : rcu_sched_clock_irq: sched-tick > > [ 20.137185] rcu_perf-165 3d.h2 2591906us : rcu_sched_clock_irq: sched-tick > > [ 20.138149] rcu_perf-165 3d.h. 2591911us : rcu_sched_clock_irq: sched-tick > > [ 20.139106] rcu_perf-165 3d.h. 2591915us : rcu_sched_clock_irq: sched-tick [snip] > > [ 20.147797] rcu_perf-165 3d.h. 2591953us : rcu_sched_clock_irq: sched-tick > > [ 20.148759] rcu_perf-165 3d.h. 2591957us : rcu_sched_clock_irq: sched-tick > > [ 20.151655] rcu_pree-10 0d..1 2591979us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 > > [ 20.732938] rcu_pree-10 0d..1 2895960us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 [snip] > > [ 26.566100] rcu_pree-10 0d..1 5935982us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 > > [ 27.144497] rcu_pree-10 0d..1 6239973us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 > > [ 27.192661] rcu_perf-165 3d.h. 6276923us : rcu_sched_clock_irq: sched-tick > > [ 27.705789] rcu_pree-10 0d..1 6541901us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 > > [ 28.292155] rcu_pree-10 0d..1 6845974us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 > > [ 28.874049] rcu_pree-10 0d..1 7149972us : rcu_implicit_dynticks_qs: Sending urgent resched to cpu 3 > > [ 29.112646] rcu_perf-165 3.... 7275951us : rcu_perf_writer: End of rcuperf test > > That would be due to my own stupidity. I forgot to clear ->rcu_forced_tick > in rcu_disable_tick_upon_qs() inside the "if" statement. This of course > prevents rcu_nmi_exit_common() from ever re-enabling it. > > Excellent catch! Thank you for testing this!!! Ah I missed it too. Happy to help! I tried setting it as below but getting same results: +/* + * If the scheduler-clock interrupt was enabled on a nohz_full CPU + * in order to get to a quiescent state, disable it. + */ +void rcu_disable_tick_upon_qs(struct rcu_data *rdp) +{ + if (tick_nohz_full_cpu(rdp->cpu) && rdp->rcu_forced_tick) + tick_dep_clear_cpu(rdp->cpu, TICK_DEP_MASK_RCU); + rdp->rcu_forced_tick = false; +} + > > [snip] > > > > > if (rnp->qsmask & mask) { /* RCU waiting on incoming CPU? */ > > > > > + rcu_disable_tick_upon_qs(rdp); > > > > > /* Report QS -after- changing ->qsmaskinitnext! */ > > > > > rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags); > > > > > > > > Just curious about the existing code. If a CPU is just starting up (after > > > > bringing it online), how can RCU be waiting on it? I thought RCU would not be > > > > watching offline CPUs. > > > > > > Well, neither grace periods nor CPU-hotplug operations are atomic, > > > and each can take significant time to complete. > > > > > > So suppose we have a large system with multiple leaf rcu_node structures > > > (not that 17 CPUs is all that many these days, but please bear with me). > > > Suppose just after a new grace period initializes a given leaf rcu_node > > > structure, one of its CPUs goes offline (yes, that CPU would have to > > > have waited on a grace period, but that might have been the previous > > > grace period). But before the FQS scan notices that RCU is waiting on > > > an offline CPU, the CPU comes back online. > > > > > > That situation is exactly what the above code is intended to handle. > > > > That makes sense! > > > > > Without that code, RCU can give false-positive splats at various points > > > in its processing. ("Wait! How can a task be blocked waiting on a > > > grace period that hasn't even started yet???") > > > > I did not fully understand the question in brackets though, a task can be on > > a different CPU though which has nothing to do with the CPU that's going > > offline/online so it could totally be waiting on a grace period right? > > > > Also waiting on a grace period that hasn't even started is totally possible: > > > > GP1 GP2 > > |<--------->|<-------->| > > ^ ^ > > | |____ task gets unblocked > > task blocks > > on synchronize_rcu > > but is waiting on > > GP2 which hasn't started > > > > Or did I misunderstand the question? > > There is a ->gp_tasks field in the leaf rcu_node structures that > references a list of tasks blocking the current grace period. When there > is no grace period in progress (as is the case from the end of GP1 to > the beginning of GP2, the RCU code expects ->gp_tasks to be NULL. > Without the curiosity code you pointed out above, ->gp_tasks could > in fact end up being non-NULL when no grace period was in progress. > > And did end up being non-NULL from time to time, initially every few > hundred hours of a particular rcutorture scenario. Oh ok! I will think more about it. I am not yet able to connect the gp_tasks being non-NULL to the CPU going offline/online scenario though. Maybe I should delete this code, run an experiment and trace for this condition (gp_tasks != NULL)? I love it how you found these issues by heavy testing and fixed them. thanks, - Joel