From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756283Ab2LNP2R (ORCPT ); Fri, 14 Dec 2012 10:28:17 -0500 Received: from mail-bk0-f46.google.com ([209.85.214.46]:37970 "EHLO mail-bk0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756166Ab2LNP2P (ORCPT ); Fri, 14 Dec 2012 10:28:15 -0500 From: Frederic Weisbecker To: "Paul E. McKenney" , Ingo Molnar , Andrew Morton Cc: LKML , Frederic Weisbecker , Gilad Ben-Yossef , Thomas Gleixner , Steven Rostedt , Peter Zijlstra , Li Zhong Subject: [PATCH v2] context_tracking: Add comments on interface and internals Date: Fri, 14 Dec 2012 16:27:48 +0100 Message-Id: <1355498868-8337-1-git-send-email-fweisbec@gmail.com> X-Mailer: git-send-email 1.7.5.4 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This subsystem lacks many explanations on its purpose and design. Add these missing comments. v2: Address comments from Andrew Reported-by: Andrew Morton Signed-off-by: Frederic Weisbecker Cc: Gilad Ben-Yossef Cc: Thomas Gleixner Cc: Andrew Morton Cc: Paul E. McKenney Cc: Ingo Molnar Cc: Steven Rostedt Cc: Peter Zijlstra Cc: Li Zhong --- kernel/context_tracking.c | 73 ++++++++++++++++++++++++++++++++++++++------ 1 files changed, 63 insertions(+), 10 deletions(-) diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c index e0e07fd..9f6c38f 100644 --- a/kernel/context_tracking.c +++ b/kernel/context_tracking.c @@ -1,3 +1,19 @@ +/* + * Context tracking: Probe on high level context boundaries such as kernel + * and userspace. This includes syscalls and exceptions entry/exit. + * + * This is used by RCU to remove its dependency on the timer tick while a CPU + * runs in userspace. + * + * Started by Frederic Weisbecker: + * + * Copyright (C) 2012 Red Hat, Inc., Frederic Weisbecker + * + * Many thanks to Gilad Ben-Yossef, Paul McKenney, Ingo Molnar, Andrew Morton, + * Steven Rostedt, Peter Zijlstra for suggestions and improvements. + * + */ + #include #include #include @@ -6,8 +22,8 @@ struct context_tracking { /* - * When active is false, hooks are not set to - * minimize overhead: TIF flags are cleared + * When active is false, hooks are unset in order + * to minimize overhead: TIF flags are cleared * and calls to user_enter/exit are ignored. This * may be further optimized using static keys. */ @@ -24,6 +40,15 @@ static DEFINE_PER_CPU(struct context_tracking, context_tracking) = { #endif }; +/** + * user_enter - Inform the context tracking that the CPU is going to + * enter userspace mode. + * + * This function must be called right before we switch from the kernel + * to userspace, when it's guaranteed the remaining kernel instructions + * to execute won't use any RCU read side critical section because this + * function sets RCU in extended quiescent state. + */ void user_enter(void) { unsigned long flags; @@ -39,40 +64,68 @@ void user_enter(void) if (in_interrupt()) return; + /* Kernel threads aren't supposed to go to userspace */ WARN_ON_ONCE(!current->mm); local_irq_save(flags); if (__this_cpu_read(context_tracking.active) && __this_cpu_read(context_tracking.state) != IN_USER) { __this_cpu_write(context_tracking.state, IN_USER); + /* + * At this stage, only low level arch entry code remains and + * then we'll run in userspace. We can assume there won't be + * any RCU read-side critical section until the next call to + * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency + * on the tick. + */ rcu_user_enter(); } local_irq_restore(flags); } + +/** + * user_exit - Inform the context tracking that the CPU is + * exiting userspace mode and entering the kernel. + * + * This function must be called after we entered the kernel from userspace + * before any use of RCU read side critical section. This potentially include + * any high level kernel code like syscalls, exceptions, signal handling, etc... + * + * This call supports re-entrancy. This way it can be called from any exception + * handler without needing to know if we came from userspace or not. + */ void user_exit(void) { unsigned long flags; - /* - * Some contexts may involve an exception occuring in an irq, - * leading to that nesting: - * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit() - * This would mess up the dyntick_nesting count though. And rcu_irq_*() - * helpers are enough to protect RCU uses inside the exception. So - * just return immediately if we detect we are in an IRQ. - */ if (in_interrupt()) return; local_irq_save(flags); if (__this_cpu_read(context_tracking.state) == IN_USER) { __this_cpu_write(context_tracking.state, IN_KERNEL); + /* + * We are going to run code that may use RCU. Inform + * RCU core about that (ie: we may need the tick again). + */ rcu_user_exit(); } local_irq_restore(flags); } + +/** + * context_tracking_task_switch - context switch the syscall hooks + * + * The context tracking uses the syscall slow path to implement its user-kernel + * boundaries hooks on syscalls. This way it doesn't impact the syscall fast + * path on CPUs that don't do context tracking. + * + * But we need to clear the flag on the previous task because it may later + * migrate to some CPU that doesn't do the context tracking. As such the TIF + * flag may not be desired there. + */ void context_tracking_task_switch(struct task_struct *prev, struct task_struct *next) { -- 1.7.5.4