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=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 D74B5C742A8 for ; Fri, 12 Jul 2019 06:01:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B737320863 for ; Fri, 12 Jul 2019 06:01:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725932AbfGLGBT (ORCPT ); Fri, 12 Jul 2019 02:01:19 -0400 Received: from lgeamrelo11.lge.com ([156.147.23.51]:58537 "EHLO lgeamrelo11.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725267AbfGLGBT (ORCPT ); Fri, 12 Jul 2019 02:01:19 -0400 Received: from unknown (HELO lgemrelse7q.lge.com) (156.147.1.151) by 156.147.23.51 with ESMTP; 12 Jul 2019 15:01:16 +0900 X-Original-SENDERIP: 156.147.1.151 X-Original-MAILFROM: byungchul.park@lge.com Received: from unknown (HELO X58A-UD3R) (10.177.222.33) by 156.147.1.151 with ESMTP; 12 Jul 2019 15:01:16 +0900 X-Original-SENDERIP: 10.177.222.33 X-Original-MAILFROM: byungchul.park@lge.com Date: Fri, 12 Jul 2019 15:00:19 +0900 From: Byungchul Park To: "Paul E. McKenney" Cc: Joel Fernandes , josh@joshtriplett.org, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com, rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@lge.com Subject: Re: [PATCH] rcu: Make jiffies_till_sched_qs writable Message-ID: <20190712060019.GC7702@X58A-UD3R> References: <1562565609-12482-1-git-send-email-byungchul.park@lge.com> <20190708125013.GG26519@linux.ibm.com> <20190708130359.GA42888@google.com> <20190709055815.GA19459@X58A-UD3R> <20190709124102.GR26519@linux.ibm.com> <20190710012025.GA20711@X58A-UD3R> <20190711123052.GI26519@linux.ibm.com> <20190711130849.GA212044@google.com> <20190711150215.GK26519@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190711150215.GK26519@linux.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jul 11, 2019 at 08:02:15AM -0700, Paul E. McKenney wrote: > These would be the tunables controlling how quickly RCU takes its > various actions to encourage the current grace period to end quickly. Seriously one of the most interesting thing over all kernel works. > I would be happy to give you the exact list if you wish, but most of > them have appeared in this thread. Thank you. :) > The experiments should be designed to work out whether the current > default settings have configurations where they act badly. This might > also come up with advice for people attempting hand-tuning, or proposed > parameter-checking code to avoid bad combinations. Great. > For one example, setting the RCU CPU stall timeout too low will definitely > cause some unwanted splats. (Yes, one could argue that other things in > the kernel should change to allow this value to decrease, but things > like latency tracer and friends are probably more useful and important.) Agree. Thanks, Byungchul > > Thanx, Paul > > > thanks, > > > > - Joel > > > > > > > > > > Thanks, > > > > Byungchul > > > > > > > > ---8<--- > > > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt > > > > index e72c184..94b58f5 100644 > > > > --- a/Documentation/admin-guide/kernel-parameters.txt > > > > +++ b/Documentation/admin-guide/kernel-parameters.txt > > > > @@ -3792,10 +3792,6 @@ > > > > a value based on the most recent settings > > > > of rcutree.jiffies_till_first_fqs > > > > and rcutree.jiffies_till_next_fqs. > > > > - This calculated value may be viewed in > > > > - rcutree.jiffies_to_sched_qs. Any attempt to set > > > > - rcutree.jiffies_to_sched_qs will be cheerfully > > > > - overwritten. > > > > > > > > rcutree.kthread_prio= [KNL,BOOT] > > > > Set the SCHED_FIFO priority of the RCU per-CPU > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > > > index a2f8ba2..ad9dc86 100644 > > > > --- a/kernel/rcu/tree.c > > > > +++ b/kernel/rcu/tree.c > > > > @@ -421,10 +421,8 @@ static int rcu_is_cpu_rrupt_from_idle(void) > > > > * How long the grace period must be before we start recruiting > > > > * quiescent-state help from rcu_note_context_switch(). > > > > */ > > > > -static ulong jiffies_till_sched_qs = ULONG_MAX; > > > > +static ulong jiffies_till_sched_qs = ULONG_MAX; /* See adjust_jiffies_till_sched_qs(). */ > > > > module_param(jiffies_till_sched_qs, ulong, 0444); > > > > -static ulong jiffies_to_sched_qs; /* See adjust_jiffies_till_sched_qs(). */ > > > > -module_param(jiffies_to_sched_qs, ulong, 0444); /* Display only! */ > > > > > > > > /* > > > > * Make sure that we give the grace-period kthread time to detect any > > > > @@ -436,18 +434,13 @@ static void adjust_jiffies_till_sched_qs(void) > > > > { > > > > unsigned long j; > > > > > > > > - /* If jiffies_till_sched_qs was specified, respect the request. */ > > > > - if (jiffies_till_sched_qs != ULONG_MAX) { > > > > - WRITE_ONCE(jiffies_to_sched_qs, jiffies_till_sched_qs); > > > > - return; > > > > - } > > > > /* Otherwise, set to third fqs scan, but bound below on large system. */ > > > > j = READ_ONCE(jiffies_till_first_fqs) + > > > > 2 * READ_ONCE(jiffies_till_next_fqs); > > > > if (j < HZ / 10 + nr_cpu_ids / RCU_JIFFIES_FQS_DIV) > > > > j = HZ / 10 + nr_cpu_ids / RCU_JIFFIES_FQS_DIV; > > > > pr_info("RCU calculated value of scheduler-enlistment delay is %ld jiffies.\n", j); > > > > - WRITE_ONCE(jiffies_to_sched_qs, j); > > > > + WRITE_ONCE(jiffies_till_sched_qs, j); > > > > } > > > > > > > > static int param_set_first_fqs_jiffies(const char *val, const struct kernel_param *kp) > > > > @@ -1033,16 +1026,16 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp) > > > > > > > > /* > > > > * A CPU running for an extended time within the kernel can > > > > - * delay RCU grace periods: (1) At age jiffies_to_sched_qs, > > > > - * set .rcu_urgent_qs, (2) At age 2*jiffies_to_sched_qs, set > > > > + * delay RCU grace periods: (1) At age jiffies_till_sched_qs, > > > > + * set .rcu_urgent_qs, (2) At age 2*jiffies_till_sched_qs, set > > > > * both .rcu_need_heavy_qs and .rcu_urgent_qs. Note that the > > > > * unsynchronized assignments to the per-CPU rcu_need_heavy_qs > > > > * variable are safe because the assignments are repeated if this > > > > * CPU failed to pass through a quiescent state. This code > > > > - * also checks .jiffies_resched in case jiffies_to_sched_qs > > > > + * also checks .jiffies_resched in case jiffies_till_sched_qs > > > > * is set way high. > > > > */ > > > > - jtsq = READ_ONCE(jiffies_to_sched_qs); > > > > + jtsq = READ_ONCE(jiffies_till_sched_qs); > > > > ruqp = per_cpu_ptr(&rcu_data.rcu_urgent_qs, rdp->cpu); > > > > rnhqp = &per_cpu(rcu_data.rcu_need_heavy_qs, rdp->cpu); > > > > if (!READ_ONCE(*rnhqp) && > > > > @@ -3383,7 +3376,8 @@ static void __init rcu_init_geometry(void) > > > > jiffies_till_first_fqs = d; > > > > if (jiffies_till_next_fqs == ULONG_MAX) > > > > jiffies_till_next_fqs = d; > > > > - adjust_jiffies_till_sched_qs(); > > > > + if (jiffies_till_sched_qs == ULONG_MAX) > > > > + adjust_jiffies_till_sched_qs(); > > > > > > > > /* If the compile-time values are accurate, just leave. */ > > > > if (rcu_fanout_leaf == RCU_FANOUT_LEAF && > > >