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=-5.6 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_PASS,USER_AGENT_MUTT 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 37543C43381 for ; Thu, 21 Mar 2019 12:07:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id F13FE21874 for ; Thu, 21 Mar 2019 12:07:01 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="Ad6j8D/F" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728013AbfCUMHA (ORCPT ); Thu, 21 Mar 2019 08:07:00 -0400 Received: from mail-qt1-f196.google.com ([209.85.160.196]:43841 "EHLO mail-qt1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727878AbfCUMHA (ORCPT ); Thu, 21 Mar 2019 08:07:00 -0400 Received: by mail-qt1-f196.google.com with SMTP id v32so6213308qtc.10 for ; Thu, 21 Mar 2019 05:06:58 -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=tmLxNvHglUGHNpHUKmRi7iiO4J3SVPDfcA7k8hgaYTo=; b=Ad6j8D/FENFz01hanNDFZpe794+GYeb1V9F3BlCHlcumhZQtVg34QkFupg/zCEca+4 qHY8hcySqKD43FNnCk88GS4UwAopJIte+uStD9LRsfBYrRE2mV+S37epQkuNJyyayV8/ ha4FC+zytQQ6IUQj6GoRGIvN9mnuTZgtXD/3A= 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=tmLxNvHglUGHNpHUKmRi7iiO4J3SVPDfcA7k8hgaYTo=; b=HKVqw+KGFCd77QeS/79F1PFOmdGXAZ50OGvbt/Ay7EW/YuXYwi1zQkLzuZ/SeWBhHS doaPSxNSsenbzlQroNn71xGOORiPeTJMV3mhixHcJJtrNWJBW7VG7JMnZEJ27JwEXstW LRWCo3P5ou+ia3Qq42e6539dQRBGWQskiC0qlSUhNSniXgOfXhERwHgNt1l5oqFytF3y 7tQHkHFcfNxHgpmlTYmCgpXPUNVMNBoVeK27M+hcmbzLRZ7joMP2hct+sblnT2tcgQ90 qMppqBQzIwRhkgKTFkMxDK3ocnb7U+jbgGtnqcDOtQ8PKhyBEjkaClM7nRxrFQO2BAsp vNlw== X-Gm-Message-State: APjAAAUt6lGRdGsC8Mbx8eMdmq1xh08US9t2eItdu5/PBmTvh+YYutJX Jhy5IBfmNju998fwRzxL7ngmHA== X-Google-Smtp-Source: APXvYqxPzC1Wjh3zAfziZLhhzWEh01AGt5v1yLe2YbUlIFHOzu3LDN/T95y65Ydvx/1As/NI5KR99A== X-Received: by 2002:ac8:21c7:: with SMTP id 7mr1270433qtz.66.1553170018348; Thu, 21 Mar 2019 05:06:58 -0700 (PDT) Received: from localhost ([2620:0:1004:1100:cca9:fccc:8667:9bdc]) by smtp.gmail.com with ESMTPSA id u24sm3102693qku.85.2019.03.21.05.06.56 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Thu, 21 Mar 2019 05:06:57 -0700 (PDT) Date: Thu, 21 Mar 2019 08:06:56 -0400 From: Joel Fernandes To: Sebastian Andrzej Siewior Cc: linux-kernel@vger.kernel.org, Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , tglx@linutronix.de, "Paul E. McKenney" , Mike Galbraith , rcu@vger.kernel.org Subject: Re: [PATCH] rcu: Allow to eliminate softirq processing from rcutree Message-ID: <20190321120656.GA61489@google.com> References: <20190315111130.4902-1-bigeasy@linutronix.de> <20190320002613.GA129907@google.com> <20190320112835.prq22vsto3ecckff@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190320112835.prq22vsto3ecckff@linutronix.de> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 20, 2019 at 12:28:35PM +0100, Sebastian Andrzej Siewior wrote: > On 2019-03-19 20:26:13 [-0400], Joel Fernandes wrote: > > > @@ -2769,19 +2782,121 @@ static void invoke_rcu_callbacks(struct rcu_data *rdp) > > > { > > > if (unlikely(!READ_ONCE(rcu_scheduler_fully_active))) > > > return; > > > - if (likely(!rcu_state.boost)) { > > > - rcu_do_batch(rdp); > > > - return; > > > - } > > > - invoke_rcu_callbacks_kthread(); > > > + rcu_do_batch(rdp); > > > > Looks like a nice change, but one question... > > > > Consider the case where rcunosoftirq boot option is not passed. > > > > Before, if RCU_BOOST=y, then callbacks would be invoked in rcuc threads if > > possible, by those threads being woken up from within the softirq context > > (in invoke_rcu_callbacks). > > > > Now, if RCU_BOOST=y, then callbacks would only be invoked in softirq context > > and not in the threads at all. Because rcu_softirq_enabled = false, so the > > path executes: > > rcu_read_unlock_special() -> > > raise_softirq_irqsoff() -> > > rcu_process_callbacks_si() -> > > rcu_process_callbacks() -> > > invoke_rcu_callbacks() -> > > rcu_do_batch() > > > > This seems like a behavioral change to me. This makes the callbacks always > > execute from the softirq context and not the threads when boosting is > > configured. IMO in the very least, such behavioral change should be > > documented in the change. > > > > One way to fix this I think could be, if boosting is enabled, then set > > rcu_softirq_enabled to false by default so the callbacks are still executed > > in the rcuc threads. > > > > Did I miss something? Sorry if I did, thanks! > > So with all the swaps and reorder we talking about this change: > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 0a719f726e149..82810483bfc6c 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -2306,20 +2306,6 @@ static void rcu_core_si(struct softirq_action *h) > rcu_core(); > } > > -/* > - * Schedule RCU callback invocation. If the running implementation of RCU > - * does not support RCU priority boosting, just do a direct call, otherwise > - * wake up the per-CPU kernel kthread. Note that because we are running > - * on the current CPU with softirqs disabled, the rcu_cpu_kthread_task > - * cannot disappear out from under us. > - */ > -static void invoke_rcu_callbacks(struct rcu_data *rdp) > -{ > - if (unlikely(!READ_ONCE(rcu_scheduler_fully_active))) > - return; > - rcu_do_batch(rdp); > -} > - > static void rcu_wake_cond(struct task_struct *t, int status) > { > /* > @@ -2330,6 +2316,19 @@ static void rcu_wake_cond(struct task_struct *t, int status) > wake_up_process(t); > } > > +static void invoke_rcu_core_kthread(void) > +{ > + struct task_struct *t; > + unsigned long flags; > + > + local_irq_save(flags); > + __this_cpu_write(rcu_data.rcu_cpu_has_work, 1); > + t = __this_cpu_read(rcu_data.rcu_cpu_kthread_task); > + if (t != NULL && t != current) > + rcu_wake_cond(t, __this_cpu_read(rcu_data.rcu_cpu_kthread_status)); > + local_irq_restore(flags); > +} > + > static bool rcu_softirq_enabled = true; > > static int __init rcunosoftirq_setup(char *str) > @@ -2339,26 +2338,33 @@ static int __init rcunosoftirq_setup(char *str) > } > __setup("rcunosoftirq", rcunosoftirq_setup); > > +/* > + * Schedule RCU callback invocation. If the running implementation of RCU > + * does not support RCU priority boosting, just do a direct call, otherwise > + * wake up the per-CPU kernel kthread. Note that because we are running > + * on the current CPU with softirqs disabled, the rcu_cpu_kthread_task > + * cannot disappear out from under us. > + */ > +static void invoke_rcu_callbacks(struct rcu_data *rdp) > +{ > + if (unlikely(!READ_ONCE(rcu_scheduler_fully_active))) > + return; > + if (rcu_state.boost || rcu_softirq_enabled) > + invoke_rcu_core_kthread(); Here shouldn't it be this? if (rcu_state.boost || !rcu_softirq_enabled) Also the rcu/dev branch has the following hunk where we unconditionally invoke rcu_do_batch even when boosting which would still have the issue I pointed. I would suggest Sebastian to post the latest v4 or v5 with all diff squashed, and then we do another round of review with latest patch, thanks! @@ -2306,18 +2320,110 @@ static void invoke_rcu_callbacks(struct rcu_data *rdp) { if (unlikely(!READ_ONCE(rcu_scheduler_fully_active))) return; - if (likely(!rcu_state.boost)) { - rcu_do_batch(rdp); - return; - } - invoke_rcu_callbacks_kthread(); + rcu_do_batch(rdp); +} + thanks, - Joel > + rcu_do_batch(rdp); > +} > + > /* > * Wake up this CPU's rcuc kthread to do RCU core processing. > */ > static void invoke_rcu_core(void) > { > - unsigned long flags; > - struct task_struct *t; > - > if (!cpu_online(smp_processor_id())) > return; > - if (rcu_softirq_enabled) { > + if (rcu_softirq_enabled) > raise_softirq(RCU_SOFTIRQ); > - } else { > - local_irq_save(flags); > - __this_cpu_write(rcu_data.rcu_cpu_has_work, 1); > - t = __this_cpu_read(rcu_data.rcu_cpu_kthread_task); > - if (t != NULL && t != current) > - rcu_wake_cond(t, __this_cpu_read(rcu_data.rcu_cpu_kthread_status)); > - local_irq_restore(flags); > - } > + else > + invoke_rcu_core_kthread(); > } > > static void rcu_cpu_kthread_park(unsigned int cpu) > @@ -2426,7 +2432,8 @@ static int __init rcu_spawn_core_kthreads(void) > per_cpu(rcu_data.rcu_cpu_has_work, cpu) = 0; > if (!IS_ENABLED(CONFIG_RCU_BOOST) && !rcu_softirq_enabled) > return 0; > - WARN_ONCE(smpboot_register_percpu_thread(&rcu_cpu_thread_spec), "%s: Could not start rcub kthread, OOM is now expected behavior\n", __func__); > + WARN_ONCE(smpboot_register_percpu_thread(&rcu_cpu_thread_spec), > + "%s: Could not start rcuc kthread, OOM is now expected behavior\n", __func__); > return 0; > } > early_initcall(rcu_spawn_core_kthreads); > -- > 2.20.1 > > > - Joel > > Sebastian