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58135f574f
Use a function with a meaningful name to check the global context tracking state. static_key_false() is a bit confusing for reviewers. Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@kernel.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Oleg Nesterov <oleg@redhat.com> Cc: Steven Rostedt <rostedt@goodmis.org>
214 lines
6.8 KiB
C
214 lines
6.8 KiB
C
/*
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* Context tracking: Probe on high level context boundaries such as kernel
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* and userspace. This includes syscalls and exceptions entry/exit.
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*
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* This is used by RCU to remove its dependency on the timer tick while a CPU
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* runs in userspace.
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*
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* Started by Frederic Weisbecker:
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*
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* Copyright (C) 2012 Red Hat, Inc., Frederic Weisbecker <fweisbec@redhat.com>
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*
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* Many thanks to Gilad Ben-Yossef, Paul McKenney, Ingo Molnar, Andrew Morton,
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* Steven Rostedt, Peter Zijlstra for suggestions and improvements.
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*
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*/
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#include <linux/context_tracking.h>
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#include <linux/rcupdate.h>
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#include <linux/sched.h>
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#include <linux/hardirq.h>
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#include <linux/export.h>
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#define CREATE_TRACE_POINTS
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#include <trace/events/context_tracking.h>
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struct static_key context_tracking_enabled = STATIC_KEY_INIT_FALSE;
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EXPORT_SYMBOL_GPL(context_tracking_enabled);
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DEFINE_PER_CPU(struct context_tracking, context_tracking);
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EXPORT_SYMBOL_GPL(context_tracking);
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void context_tracking_cpu_set(int cpu)
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{
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if (!per_cpu(context_tracking.active, cpu)) {
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per_cpu(context_tracking.active, cpu) = true;
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static_key_slow_inc(&context_tracking_enabled);
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}
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}
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/**
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* context_tracking_user_enter - Inform the context tracking that the CPU is going to
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* enter userspace mode.
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*
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* This function must be called right before we switch from the kernel
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* to userspace, when it's guaranteed the remaining kernel instructions
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* to execute won't use any RCU read side critical section because this
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* function sets RCU in extended quiescent state.
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*/
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void context_tracking_user_enter(void)
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{
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unsigned long flags;
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/*
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* Repeat the user_enter() check here because some archs may be calling
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* this from asm and if no CPU needs context tracking, they shouldn't
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* go further. Repeat the check here until they support the inline static
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* key check.
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*/
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if (!context_tracking_is_enabled())
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return;
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/*
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* Some contexts may involve an exception occuring in an irq,
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* leading to that nesting:
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* rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit()
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* This would mess up the dyntick_nesting count though. And rcu_irq_*()
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* helpers are enough to protect RCU uses inside the exception. So
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* just return immediately if we detect we are in an IRQ.
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*/
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if (in_interrupt())
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return;
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/* Kernel threads aren't supposed to go to userspace */
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WARN_ON_ONCE(!current->mm);
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local_irq_save(flags);
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if ( __this_cpu_read(context_tracking.state) != IN_USER) {
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if (__this_cpu_read(context_tracking.active)) {
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trace_user_enter(0);
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/*
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* At this stage, only low level arch entry code remains and
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* then we'll run in userspace. We can assume there won't be
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* any RCU read-side critical section until the next call to
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* user_exit() or rcu_irq_enter(). Let's remove RCU's dependency
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* on the tick.
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*/
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vtime_user_enter(current);
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rcu_user_enter();
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}
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/*
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* Even if context tracking is disabled on this CPU, because it's outside
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* the full dynticks mask for example, we still have to keep track of the
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* context transitions and states to prevent inconsistency on those of
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* other CPUs.
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* If a task triggers an exception in userspace, sleep on the exception
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* handler and then migrate to another CPU, that new CPU must know where
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* the exception returns by the time we call exception_exit().
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* This information can only be provided by the previous CPU when it called
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* exception_enter().
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* OTOH we can spare the calls to vtime and RCU when context_tracking.active
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* is false because we know that CPU is not tickless.
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*/
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__this_cpu_write(context_tracking.state, IN_USER);
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}
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local_irq_restore(flags);
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}
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#ifdef CONFIG_PREEMPT
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/**
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* preempt_schedule_context - preempt_schedule called by tracing
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*
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* The tracing infrastructure uses preempt_enable_notrace to prevent
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* recursion and tracing preempt enabling caused by the tracing
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* infrastructure itself. But as tracing can happen in areas coming
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* from userspace or just about to enter userspace, a preempt enable
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* can occur before user_exit() is called. This will cause the scheduler
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* to be called when the system is still in usermode.
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*
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* To prevent this, the preempt_enable_notrace will use this function
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* instead of preempt_schedule() to exit user context if needed before
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* calling the scheduler.
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*/
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asmlinkage void __sched notrace preempt_schedule_context(void)
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{
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enum ctx_state prev_ctx;
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if (likely(!preemptible()))
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return;
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/*
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* Need to disable preemption in case user_exit() is traced
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* and the tracer calls preempt_enable_notrace() causing
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* an infinite recursion.
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*/
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preempt_disable_notrace();
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prev_ctx = exception_enter();
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preempt_enable_no_resched_notrace();
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preempt_schedule();
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preempt_disable_notrace();
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exception_exit(prev_ctx);
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preempt_enable_notrace();
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}
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EXPORT_SYMBOL_GPL(preempt_schedule_context);
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#endif /* CONFIG_PREEMPT */
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/**
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* context_tracking_user_exit - Inform the context tracking that the CPU is
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* exiting userspace mode and entering the kernel.
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*
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* This function must be called after we entered the kernel from userspace
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* before any use of RCU read side critical section. This potentially include
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* any high level kernel code like syscalls, exceptions, signal handling, etc...
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*
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* This call supports re-entrancy. This way it can be called from any exception
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* handler without needing to know if we came from userspace or not.
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*/
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void context_tracking_user_exit(void)
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{
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unsigned long flags;
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if (!context_tracking_is_enabled())
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return;
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if (in_interrupt())
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return;
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local_irq_save(flags);
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if (__this_cpu_read(context_tracking.state) == IN_USER) {
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if (__this_cpu_read(context_tracking.active)) {
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/*
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* We are going to run code that may use RCU. Inform
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* RCU core about that (ie: we may need the tick again).
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*/
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rcu_user_exit();
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vtime_user_exit(current);
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trace_user_exit(0);
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}
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__this_cpu_write(context_tracking.state, IN_KERNEL);
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}
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local_irq_restore(flags);
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}
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/**
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* __context_tracking_task_switch - context switch the syscall callbacks
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* @prev: the task that is being switched out
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* @next: the task that is being switched in
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*
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* The context tracking uses the syscall slow path to implement its user-kernel
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* boundaries probes on syscalls. This way it doesn't impact the syscall fast
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* path on CPUs that don't do context tracking.
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*
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* But we need to clear the flag on the previous task because it may later
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* migrate to some CPU that doesn't do the context tracking. As such the TIF
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* flag may not be desired there.
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*/
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void __context_tracking_task_switch(struct task_struct *prev,
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struct task_struct *next)
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{
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clear_tsk_thread_flag(prev, TIF_NOHZ);
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set_tsk_thread_flag(next, TIF_NOHZ);
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}
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#ifdef CONFIG_CONTEXT_TRACKING_FORCE
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void __init context_tracking_init(void)
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{
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int cpu;
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for_each_possible_cpu(cpu)
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context_tracking_cpu_set(cpu);
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}
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#endif
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