146 lines
4.2 KiB
C
146 lines
4.2 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/kvm_host.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_PER_CPU(struct context_tracking, context_tracking) = {
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#ifdef CONFIG_CONTEXT_TRACKING_FORCE
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.active = true,
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#endif
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};
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/**
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* 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 user_enter(void)
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{
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unsigned long flags;
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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.active) &&
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__this_cpu_read(context_tracking.state) != IN_USER) {
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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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__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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/**
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* 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 user_exit(void)
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{
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unsigned long flags;
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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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/*
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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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__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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void guest_enter(void)
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{
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if (vtime_accounting_enabled())
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vtime_guest_enter(current);
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else
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__guest_enter();
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}
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EXPORT_SYMBOL_GPL(guest_enter);
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void guest_exit(void)
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{
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if (vtime_accounting_enabled())
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vtime_guest_exit(current);
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else
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__guest_exit();
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}
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EXPORT_SYMBOL_GPL(guest_exit);
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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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if (__this_cpu_read(context_tracking.active)) {
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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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}
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