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_PASS,URIBL_BLOCKED,USER_AGENT_MUTT 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 418FDC43144 for ; Mon, 25 Jun 2018 08:22:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C4A4B255D1 for ; Mon, 25 Jun 2018 08:22:09 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C4A4B255D1 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=lge.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754295AbeFYIWG (ORCPT ); Mon, 25 Jun 2018 04:22:06 -0400 Received: from lgeamrelo13.lge.com ([156.147.23.53]:41218 "EHLO lgeamrelo11.lge.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752237AbeFYIWE (ORCPT ); Mon, 25 Jun 2018 04:22:04 -0400 Received: from unknown (HELO lgemrelse6q.lge.com) (156.147.1.121) by 156.147.23.53 with ESMTP; 25 Jun 2018 17:22:02 +0900 X-Original-SENDERIP: 156.147.1.121 X-Original-MAILFROM: byungchul.park@lge.com Received: from unknown (HELO X58A-UD3R) (10.177.222.33) by 156.147.1.121 with ESMTP; 25 Jun 2018 17:22:02 +0900 X-Original-SENDERIP: 10.177.222.33 X-Original-MAILFROM: byungchul.park@lge.com Date: Mon, 25 Jun 2018 17:21:54 +0900 From: Byungchul Park To: "Paul E. McKenney" Cc: jiangshanlai@gmail.com, josh@joshtriplett.org, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, linux-kernel@vger.kernel.org, kernel-team@lge.com, joel@joelfernandes.org Subject: Re: [PATCH] rcu: Refactor rcu_{nmi,irq}_{enter,exit}() Message-ID: <20180625082154.GA21377@X58A-UD3R> References: <1529647926-24427-1-git-send-email-byungchul.park@lge.com> <20180622062351.GC17010@X58A-UD3R> <20180623174954.GA3584@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180623174954.GA3584@linux.vnet.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 Sat, Jun 23, 2018 at 10:49:54AM -0700, Paul E. McKenney wrote: > On Fri, Jun 22, 2018 at 03:23:51PM +0900, Byungchul Park wrote: > > On Fri, Jun 22, 2018 at 03:12:06PM +0900, Byungchul Park wrote: > > > When passing through irq or NMI contexts, the current code uses > > > ->dynticks_nmi_nesting to detect if it's in the ourmost at the moment. > > > > > > Here, the thing is that all the related functions, rcu_irq_enter(), > > > rcu_nmi_enter(), rcu_irq_exit() and rcu_nmi_exit() are carrying out > > > the check within each under the following call relationship so there > > > must be redundant conditional branches: > > > > > > rcu_irq_enter() > > > /* A conditional branch with ->dynticks_nmi_nesting */ > > > rcu_nmi_enter() > > > /* A conditional branch with ->dynticks_nmi_nesting */ > > ^ > > Precisely, ->dynticks > > > /* A conditional branch with ->dynticks_nmi_nesting */ > > > > > > rcu_irq_exit() > > > /* A conditional branch with ->dynticks_nmi_nesting */ > > > rcu_nmi_exit() > > > /* A conditional branch with ->dynticks_nmi_nesting */ > > > /* A conditional branch with ->dynticks_nmi_nesting */ > > > > > > rcu_nmi_enter() > > > /* A conditional branch with ->dynticks_nmi_nesting */ > > ^ > > Precisely, ->dynticks > > > > > > rcu_nmi_exit() > > > /* A conditional branch with ->dynticks_nmi_nesting */ > > > > > > In order to make the code more readable and remove redundant > > > conditional branches, we need to refactor rcu_{nmi,irq}_{enter,exit}() > > > so they use a common function for each like: > > > > > > rcu_irq_enter() inlining rcu_nmi_enter_common(irq) > > > /* A conditional branch with ->dynticks_nmi_nesting */ > > ^ > > Precisely, ->dynticks > > > > > > rcu_irq_exit() inlining rcu_nmi_exit_common(irq) > > > /* A conditional branch with ->dynticks_nmi_nesting */ > > > > > > rcu_nmi_enter() inlining rcu_nmi_enter_common(nmi) > > > /* A conditional branch with ->dynticks_nmi_nesting */ > > ^ > > Precisely, ->dynticks > > > > > > rcu_nmi_exit() inlining rcu_nmi_exit_common(nmi) > > > /* A conditional branch with ->dynticks_nmi_nesting */ > > I queued this for testing and further review, updating the commit log > as follows. Please let me know if I messed up something. Much better. Thanks a lot. > Thanx, Paul > > ------------------------------------------------------------------------ > > commit 5e5ea52645b197fb7ae2f59f7927079b91e91aa0 > Author: Byungchul Park > Date: Fri Jun 22 15:12:06 2018 +0900 > > rcu: Refactor rcu_{nmi,irq}_{enter,exit}() > > When entering or exiting irq or NMI handlers, the current code uses > ->dynticks_nmi_nesting to detect if it is in the outermost handler, > that is, the one interrupting or returning to an RCU-idle context (the > idle loop or nohz_full usermode execution). When entering the outermost > handler via an interrupt (as opposed to NMI), it is necessary to invoke > rcu_dynticks_task_exit() just before the CPU is marked non-idle from an > RCU perspective and to invoke rcu_cleanup_after_idle() just after the > CPU is marked non-idle. Similarly, when exiting the outermost handler > via an interrupt, it is necessary to invoke rcu_prepare_for_idle() just > before marking the CPU idle and to invoke rcu_dynticks_task_enter() > just after marking the CPU idle. > > The decision to execute these four functions is currently taken in > rcu_irq_enter() and rcu_irq_exit() as follows: > > rcu_irq_enter() > /* A conditional branch with ->dynticks_nmi_nesting */ > rcu_nmi_enter() > /* A conditional branch with ->dynticks */ > /* A conditional branch with ->dynticks_nmi_nesting */ > > rcu_irq_exit() > /* A conditional branch with ->dynticks_nmi_nesting */ > rcu_nmi_exit() > /* A conditional branch with ->dynticks_nmi_nesting */ > /* A conditional branch with ->dynticks_nmi_nesting */ > > rcu_nmi_enter() > /* A conditional branch with ->dynticks */ > > rcu_nmi_exit() > /* A conditional branch with ->dynticks_nmi_nesting */ > > This works, but the conditional branches in rcu_irq_enter() and > rcu_irq_exit() are redundant with those in rcu_nmi_enter() and > rcu_nmi_exit(), respectively. Redundant branches are not something > we want in the to/from-idle fastpaths, so this commit refactors > rcu_{nmi,irq}_{enter,exit}() so they use a common inlined function passed > a constant argument as follows: > > rcu_irq_enter() inlining rcu_nmi_enter_common(irq=true) > /* A conditional branch with ->dynticks */ > > rcu_irq_exit() inlining rcu_nmi_exit_common(irq=true) > /* A conditional branch with ->dynticks_nmi_nesting */ > > rcu_nmi_enter() inlining rcu_nmi_enter_common(irq=false) > /* A conditional branch with ->dynticks */ > > rcu_nmi_exit() inlining rcu_nmi_exit_common(irq=false) > /* A conditional branch with ->dynticks_nmi_nesting */ > > The combination of the constant function argument and the inlining allows > the compiler to discard the conditionals that previously controlled > execution of rcu_dynticks_task_exit(), rcu_cleanup_after_idle(), > rcu_prepare_for_idle(), and rcu_dynticks_task_enter(). This reduces both > the to-idle and from-idle path lengths by two conditional branches each, > and improves readability as well. > > Suggested-by: Paul E. McKenney > Signed-off-by: Byungchul Park > Signed-off-by: Paul E. McKenney > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 8788ddbc0d13..4ed74f10c852 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -773,17 +773,17 @@ void rcu_user_enter(void) > #endif /* CONFIG_NO_HZ_FULL */ > > /** > - * rcu_nmi_exit - inform RCU of exit from NMI context > + * rcu_nmi_exit_common - inform RCU of exit from NMI context > * > * If we are returning from the outermost NMI handler that interrupted an > * RCU-idle period, update rdtp->dynticks and rdtp->dynticks_nmi_nesting > * to let the RCU grace-period handling know that the CPU is back to > * being RCU-idle. > * > - * If you add or remove a call to rcu_nmi_exit(), be sure to test > + * If you add or remove a call to rcu_nmi_exit_common(), be sure to test > * with CONFIG_RCU_EQS_DEBUG=y. > */ > -void rcu_nmi_exit(void) > +static __always_inline void rcu_nmi_exit_common(bool irq) > { > struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks); > > @@ -809,7 +809,22 @@ void rcu_nmi_exit(void) > /* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */ > trace_rcu_dyntick(TPS("Startirq"), rdtp->dynticks_nmi_nesting, 0, rdtp->dynticks); > WRITE_ONCE(rdtp->dynticks_nmi_nesting, 0); /* Avoid store tearing. */ > + > + if (irq) > + rcu_prepare_for_idle(); > + > rcu_dynticks_eqs_enter(); > + > + if (irq) > + rcu_dynticks_task_enter(); > +} > + > +/** > + * rcu_nmi_exit - inform RCU of exit from NMI context > + */ > +void rcu_nmi_exit(void) > +{ > + rcu_nmi_exit_common(false); > } > > /** > @@ -833,14 +848,8 @@ void rcu_nmi_exit(void) > */ > void rcu_irq_exit(void) > { > - struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks); > - > lockdep_assert_irqs_disabled(); > - if (rdtp->dynticks_nmi_nesting == 1) > - rcu_prepare_for_idle(); > - rcu_nmi_exit(); > - if (rdtp->dynticks_nmi_nesting == 0) > - rcu_dynticks_task_enter(); > + rcu_nmi_exit_common(true); > } > > /* > @@ -923,7 +932,7 @@ void rcu_user_exit(void) > #endif /* CONFIG_NO_HZ_FULL */ > > /** > - * rcu_nmi_enter - inform RCU of entry to NMI context > + * rcu_nmi_enter_common - inform RCU of entry to NMI context > * > * If the CPU was idle from RCU's viewpoint, update rdtp->dynticks and > * rdtp->dynticks_nmi_nesting to let the RCU grace-period handling know > @@ -931,10 +940,10 @@ void rcu_user_exit(void) > * long as the nesting level does not overflow an int. (You will probably > * run out of stack space first.) > * > - * If you add or remove a call to rcu_nmi_enter(), be sure to test > + * If you add or remove a call to rcu_nmi_enter_common(), be sure to test > * with CONFIG_RCU_EQS_DEBUG=y. > */ > -void rcu_nmi_enter(void) > +static __always_inline void rcu_nmi_enter_common(bool irq) > { > struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks); > long incby = 2; > @@ -951,7 +960,15 @@ void rcu_nmi_enter(void) > * period (observation due to Andy Lutomirski). > */ > if (rcu_dynticks_curr_cpu_in_eqs()) { > + > + if (irq) > + rcu_dynticks_task_exit(); > + > rcu_dynticks_eqs_exit(); > + > + if (irq) > + rcu_cleanup_after_idle(); > + > incby = 1; > } > trace_rcu_dyntick(incby == 1 ? TPS("Endirq") : TPS("++="), > @@ -962,6 +979,14 @@ void rcu_nmi_enter(void) > barrier(); > } > > +/** > + * rcu_nmi_enter - inform RCU of entry to NMI context > + */ > +void rcu_nmi_enter(void) > +{ > + rcu_nmi_enter_common(false); > +} > + > /** > * rcu_irq_enter - inform RCU that current CPU is entering irq away from idle > * > @@ -986,14 +1011,8 @@ void rcu_nmi_enter(void) > */ > void rcu_irq_enter(void) > { > - struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks); > - > lockdep_assert_irqs_disabled(); > - if (rdtp->dynticks_nmi_nesting == 0) > - rcu_dynticks_task_exit(); > - rcu_nmi_enter(); > - if (rdtp->dynticks_nmi_nesting == 1) > - rcu_cleanup_after_idle(); > + rcu_nmi_enter_common(true); > } > > /*