/* * 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 <fweisbec@redhat.com> * * Many thanks to Gilad Ben-Yossef, Paul McKenney, Ingo Molnar, Andrew Morton, * Steven Rostedt, Peter Zijlstra for suggestions and improvements. * */ #include <linux/context_tracking.h> #include <linux/rcupdate.h> #include <linux/sched.h> #include <linux/hardirq.h> #include <linux/export.h> #define CREATE_TRACE_POINTS #include <trace/events/context_tracking.h> 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_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 context_tracking_user_enter(void) { 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) != IN_USER) { if (__this_cpu_read(context_tracking.active)) { trace_user_enter(0); /* * 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. */ 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, IN_USER); } local_irq_restore(flags); } #ifdef CONFIG_PREEMPT /** * preempt_schedule_context - preempt_schedule called by tracing * * The tracing infrastructure uses preempt_enable_notrace to prevent * recursion and tracing preempt enabling caused by the tracing * infrastructure itself. But as tracing can happen in areas coming * from userspace or just about to enter userspace, a preempt enable * can occur before user_exit() is called. This will cause the scheduler * to be called when the system is still in usermode. * * To prevent this, the preempt_enable_notrace will use this function * instead of preempt_schedule() to exit user context if needed before * calling the scheduler. */ asmlinkage void __sched notrace preempt_schedule_context(void) { enum ctx_state prev_ctx; if (likely(!preemptible())) return; /* * Need to disable preemption in case user_exit() is traced * and the tracer calls preempt_enable_notrace() causing * an infinite recursion. */ preempt_disable_notrace(); prev_ctx = exception_enter(); preempt_enable_no_resched_notrace(); preempt_schedule(); preempt_disable_notrace(); exception_exit(prev_ctx); preempt_enable_notrace(); } EXPORT_SYMBOL_GPL(preempt_schedule_context); #endif /* CONFIG_PREEMPT */ /** * context_tracking_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 context_tracking_user_exit(void) { unsigned long flags; if (!context_tracking_is_enabled()) return; if (in_interrupt()) return; local_irq_save(flags); if (__this_cpu_read(context_tracking.state) == IN_USER) { 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(); vtime_user_exit(current); trace_user_exit(0); } __this_cpu_write(context_tracking.state, IN_KERNEL); } local_irq_restore(flags); } /** * __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