From 57f0f512b273f60d52568b8c6b77e17f5636edc0 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Andr=C3=A9=20Fabian=20Silva=20Delgado?= Date: Wed, 5 Aug 2015 17:04:01 -0300 Subject: Initial import --- kernel/context_tracking.c | 195 ++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 195 insertions(+) create mode 100644 kernel/context_tracking.c (limited to 'kernel/context_tracking.c') diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c new file mode 100644 index 000000000..72d59a1a6 --- /dev/null +++ b/kernel/context_tracking.c @@ -0,0 +1,195 @@ +/* + * 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 +#include +#include +#include + +#define CREATE_TRACE_POINTS +#include + +struct static_key context_tracking_enabled = STATIC_KEY_INIT_FALSE; +EXPORT_SYMBOL_GPL(context_tracking_enabled); + +DEFINE_PER_CPU(struct context_tracking, context_tracking); +EXPORT_SYMBOL_GPL(context_tracking); + +void context_tracking_cpu_set(int cpu) +{ + if (!per_cpu(context_tracking.active, cpu)) { + per_cpu(context_tracking.active, cpu) = true; + static_key_slow_inc(&context_tracking_enabled); + } +} + +/** + * context_tracking_enter - Inform the context tracking that the CPU is going + * enter user or guest space mode. + * + * This function must be called right before we switch from the kernel + * to user or guest space, 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 context_tracking_enter(enum ctx_state state) +{ + unsigned long flags; + + /* + * Repeat the user_enter() check here because some archs may be calling + * this from asm and if no CPU needs context tracking, they shouldn't + * go further. Repeat the check here until they support the inline static + * key check. + */ + if (!context_tracking_is_enabled()) + return; + + /* + * 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; + + /* Kernel threads aren't supposed to go to userspace */ + WARN_ON_ONCE(!current->mm); + + local_irq_save(flags); + if ( __this_cpu_read(context_tracking.state) != state) { + if (__this_cpu_read(context_tracking.active)) { + /* + * 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. + */ + if (state == CONTEXT_USER) { + trace_user_enter(0); + vtime_user_enter(current); + } + rcu_user_enter(); + } + /* + * Even if context tracking is disabled on this CPU, because it's outside + * the full dynticks mask for example, we still have to keep track of the + * context transitions and states to prevent inconsistency on those of + * other CPUs. + * If a task triggers an exception in userspace, sleep on the exception + * handler and then migrate to another CPU, that new CPU must know where + * the exception returns by the time we call exception_exit(). + * This information can only be provided by the previous CPU when it called + * exception_enter(). + * OTOH we can spare the calls to vtime and RCU when context_tracking.active + * is false because we know that CPU is not tickless. + */ + __this_cpu_write(context_tracking.state, state); + } + local_irq_restore(flags); +} +NOKPROBE_SYMBOL(context_tracking_enter); +EXPORT_SYMBOL_GPL(context_tracking_enter); + +void context_tracking_user_enter(void) +{ + context_tracking_enter(CONTEXT_USER); +} +NOKPROBE_SYMBOL(context_tracking_user_enter); + +/** + * context_tracking_exit - Inform the context tracking that the CPU is + * exiting user or guest mode and entering the kernel. + * + * This function must be called after we entered the kernel from user or + * guest space 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 context_tracking_exit(enum ctx_state state) +{ + unsigned long flags; + + if (!context_tracking_is_enabled()) + return; + + if (in_interrupt()) + return; + + local_irq_save(flags); + if (__this_cpu_read(context_tracking.state) == state) { + if (__this_cpu_read(context_tracking.active)) { + /* + * 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(); + if (state == CONTEXT_USER) { + vtime_user_exit(current); + trace_user_exit(0); + } + } + __this_cpu_write(context_tracking.state, CONTEXT_KERNEL); + } + local_irq_restore(flags); +} +NOKPROBE_SYMBOL(context_tracking_exit); +EXPORT_SYMBOL_GPL(context_tracking_exit); + +void context_tracking_user_exit(void) +{ + context_tracking_exit(CONTEXT_USER); +} +NOKPROBE_SYMBOL(context_tracking_user_exit); + +/** + * __context_tracking_task_switch - context switch the syscall callbacks + * @prev: the task that is being switched out + * @next: the task that is being switched in + * + * The context tracking uses the syscall slow path to implement its user-kernel + * boundaries probes 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) +{ + clear_tsk_thread_flag(prev, TIF_NOHZ); + set_tsk_thread_flag(next, TIF_NOHZ); +} + +#ifdef CONFIG_CONTEXT_TRACKING_FORCE +void __init context_tracking_init(void) +{ + int cpu; + + for_each_possible_cpu(cpu) + context_tracking_cpu_set(cpu); +} +#endif -- cgit v1.2.3