2 * Context tracking: Probe on high level context boundaries such as kernel
3 * and userspace. This includes syscalls and exceptions entry/exit.
5 * This is used by RCU to remove its dependency on the timer tick while a CPU
8 * Started by Frederic Weisbecker:
10 * Copyright (C) 2012 Red Hat, Inc., Frederic Weisbecker <fweisbec@redhat.com>
12 * Many thanks to Gilad Ben-Yossef, Paul McKenney, Ingo Molnar, Andrew Morton,
13 * Steven Rostedt, Peter Zijlstra for suggestions and improvements.
17 #include <linux/context_tracking.h>
18 #include <linux/rcupdate.h>
19 #include <linux/sched.h>
20 #include <linux/hardirq.h>
21 #include <linux/export.h>
22 #include <linux/kprobes.h>
24 #define CREATE_TRACE_POINTS
25 #include <trace/events/context_tracking.h>
27 struct static_key context_tracking_enabled = STATIC_KEY_INIT_FALSE;
28 EXPORT_SYMBOL_GPL(context_tracking_enabled);
30 DEFINE_PER_CPU(struct context_tracking, context_tracking);
31 EXPORT_SYMBOL_GPL(context_tracking);
33 static bool context_tracking_recursion_enter(void)
37 recursion = __this_cpu_inc_return(context_tracking.recursion);
41 WARN_ONCE((recursion < 1), "Invalid context tracking recursion value %d\n", recursion);
42 __this_cpu_dec(context_tracking.recursion);
47 static void context_tracking_recursion_exit(void)
49 __this_cpu_dec(context_tracking.recursion);
53 * context_tracking_enter - Inform the context tracking that the CPU is going
54 * enter user or guest space mode.
56 * This function must be called right before we switch from the kernel
57 * to user or guest space, when it's guaranteed the remaining kernel
58 * instructions to execute won't use any RCU read side critical section
59 * because this function sets RCU in extended quiescent state.
61 void context_tracking_enter(enum ctx_state state)
66 * Some contexts may involve an exception occuring in an irq,
67 * leading to that nesting:
68 * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit()
69 * This would mess up the dyntick_nesting count though. And rcu_irq_*()
70 * helpers are enough to protect RCU uses inside the exception. So
71 * just return immediately if we detect we are in an IRQ.
76 /* Kernel threads aren't supposed to go to userspace */
77 WARN_ON_ONCE(!current->mm);
79 local_irq_save(flags);
80 if (!context_tracking_recursion_enter())
83 if ( __this_cpu_read(context_tracking.state) != state) {
84 if (__this_cpu_read(context_tracking.active)) {
86 * At this stage, only low level arch entry code remains and
87 * then we'll run in userspace. We can assume there won't be
88 * any RCU read-side critical section until the next call to
89 * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency
92 if (state == CONTEXT_USER) {
94 vtime_user_enter(current);
99 * Even if context tracking is disabled on this CPU, because it's outside
100 * the full dynticks mask for example, we still have to keep track of the
101 * context transitions and states to prevent inconsistency on those of
103 * If a task triggers an exception in userspace, sleep on the exception
104 * handler and then migrate to another CPU, that new CPU must know where
105 * the exception returns by the time we call exception_exit().
106 * This information can only be provided by the previous CPU when it called
108 * OTOH we can spare the calls to vtime and RCU when context_tracking.active
109 * is false because we know that CPU is not tickless.
111 __this_cpu_write(context_tracking.state, state);
113 context_tracking_recursion_exit();
115 local_irq_restore(flags);
117 NOKPROBE_SYMBOL(context_tracking_enter);
118 EXPORT_SYMBOL_GPL(context_tracking_enter);
120 void context_tracking_user_enter(void)
124 NOKPROBE_SYMBOL(context_tracking_user_enter);
127 * context_tracking_exit - Inform the context tracking that the CPU is
128 * exiting user or guest mode and entering the kernel.
130 * This function must be called after we entered the kernel from user or
131 * guest space before any use of RCU read side critical section. This
132 * potentially include any high level kernel code like syscalls, exceptions,
133 * signal handling, etc...
135 * This call supports re-entrancy. This way it can be called from any exception
136 * handler without needing to know if we came from userspace or not.
138 void context_tracking_exit(enum ctx_state state)
145 local_irq_save(flags);
146 if (!context_tracking_recursion_enter())
147 goto out_irq_restore;
149 if (__this_cpu_read(context_tracking.state) == state) {
150 if (__this_cpu_read(context_tracking.active)) {
152 * We are going to run code that may use RCU. Inform
153 * RCU core about that (ie: we may need the tick again).
156 if (state == CONTEXT_USER) {
157 vtime_user_exit(current);
161 __this_cpu_write(context_tracking.state, CONTEXT_KERNEL);
163 context_tracking_recursion_exit();
165 local_irq_restore(flags);
167 NOKPROBE_SYMBOL(context_tracking_exit);
168 EXPORT_SYMBOL_GPL(context_tracking_exit);
170 void context_tracking_user_exit(void)
174 NOKPROBE_SYMBOL(context_tracking_user_exit);
176 void __init context_tracking_cpu_set(int cpu)
178 static __initdata bool initialized = false;
180 if (!per_cpu(context_tracking.active, cpu)) {
181 per_cpu(context_tracking.active, cpu) = true;
182 static_key_slow_inc(&context_tracking_enabled);
189 * Set TIF_NOHZ to init/0 and let it propagate to all tasks through fork
190 * This assumes that init is the only task at this early boot stage.
192 set_tsk_thread_flag(&init_task, TIF_NOHZ);
193 WARN_ON_ONCE(!tasklist_empty());
198 #ifdef CONFIG_CONTEXT_TRACKING_FORCE
199 void __init context_tracking_init(void)
203 for_each_possible_cpu(cpu)
204 context_tracking_cpu_set(cpu);