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Frederic Weisbecker4eacdf12013-01-16 17:16:37 +01001/*
2 * Context tracking: Probe on high level context boundaries such as kernel
3 * and userspace. This includes syscalls and exceptions entry/exit.
4 *
5 * This is used by RCU to remove its dependency on the timer tick while a CPU
6 * runs in userspace.
7 *
8 * Started by Frederic Weisbecker:
9 *
10 * Copyright (C) 2012 Red Hat, Inc., Frederic Weisbecker <fweisbec@redhat.com>
11 *
12 * Many thanks to Gilad Ben-Yossef, Paul McKenney, Ingo Molnar, Andrew Morton,
13 * Steven Rostedt, Peter Zijlstra for suggestions and improvements.
14 *
15 */
16
Frederic Weisbecker91d1aa432012-11-27 19:33:25 +010017#include <linux/context_tracking.h>
18#include <linux/rcupdate.h>
19#include <linux/sched.h>
Frederic Weisbecker91d1aa432012-11-27 19:33:25 +010020#include <linux/hardirq.h>
Frederic Weisbecker6a616712012-12-16 20:00:34 +010021#include <linux/export.h>
Frederic Weisbecker91d1aa432012-11-27 19:33:25 +010022
Frederic Weisbecker1b6a2592013-07-11 20:27:43 +020023#define CREATE_TRACE_POINTS
24#include <trace/events/context_tracking.h>
25
Frederic Weisbecker65f382f2013-07-11 19:12:32 +020026struct static_key context_tracking_enabled = STATIC_KEY_INIT_FALSE;
Frederic Weisbecker48d6a812013-07-10 02:44:35 +020027EXPORT_SYMBOL_GPL(context_tracking_enabled);
Frederic Weisbecker65f382f2013-07-11 19:12:32 +020028
29DEFINE_PER_CPU(struct context_tracking, context_tracking);
Frederic Weisbecker48d6a812013-07-10 02:44:35 +020030EXPORT_SYMBOL_GPL(context_tracking);
Frederic Weisbecker91d1aa432012-11-27 19:33:25 +010031
Frederic Weisbecker2e709332013-07-10 00:55:25 +020032void context_tracking_cpu_set(int cpu)
33{
Frederic Weisbecker65f382f2013-07-11 19:12:32 +020034 if (!per_cpu(context_tracking.active, cpu)) {
35 per_cpu(context_tracking.active, cpu) = true;
36 static_key_slow_inc(&context_tracking_enabled);
37 }
Frederic Weisbecker2e709332013-07-10 00:55:25 +020038}
39
Frederic Weisbecker4eacdf12013-01-16 17:16:37 +010040/**
Frederic Weisbeckerad657822013-07-10 02:44:35 +020041 * context_tracking_user_enter - Inform the context tracking that the CPU is going to
42 * enter userspace mode.
Frederic Weisbecker4eacdf12013-01-16 17:16:37 +010043 *
44 * This function must be called right before we switch from the kernel
45 * to userspace, when it's guaranteed the remaining kernel instructions
46 * to execute won't use any RCU read side critical section because this
47 * function sets RCU in extended quiescent state.
48 */
Frederic Weisbeckerad657822013-07-10 02:44:35 +020049void context_tracking_user_enter(void)
Frederic Weisbecker91d1aa432012-11-27 19:33:25 +010050{
51 unsigned long flags;
52
53 /*
Frederic Weisbecker0c06a5d2013-09-10 00:54:17 +020054 * Repeat the user_enter() check here because some archs may be calling
55 * this from asm and if no CPU needs context tracking, they shouldn't
Frederic Weisbecker58135f52013-11-06 14:45:57 +010056 * go further. Repeat the check here until they support the inline static
57 * key check.
Frederic Weisbecker0c06a5d2013-09-10 00:54:17 +020058 */
Frederic Weisbecker58135f52013-11-06 14:45:57 +010059 if (!context_tracking_is_enabled())
Frederic Weisbecker0c06a5d2013-09-10 00:54:17 +020060 return;
61
62 /*
Frederic Weisbecker91d1aa432012-11-27 19:33:25 +010063 * Some contexts may involve an exception occuring in an irq,
64 * leading to that nesting:
65 * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit()
66 * This would mess up the dyntick_nesting count though. And rcu_irq_*()
67 * helpers are enough to protect RCU uses inside the exception. So
68 * just return immediately if we detect we are in an IRQ.
69 */
70 if (in_interrupt())
71 return;
72
Frederic Weisbecker4eacdf12013-01-16 17:16:37 +010073 /* Kernel threads aren't supposed to go to userspace */
Frederic Weisbecker91d1aa432012-11-27 19:33:25 +010074 WARN_ON_ONCE(!current->mm);
75
76 local_irq_save(flags);
Frederic Weisbeckerd65ec122013-07-11 23:59:33 +020077 if ( __this_cpu_read(context_tracking.state) != IN_USER) {
78 if (__this_cpu_read(context_tracking.active)) {
Frederic Weisbecker1b6a2592013-07-11 20:27:43 +020079 trace_user_enter(0);
Frederic Weisbeckerd65ec122013-07-11 23:59:33 +020080 /*
81 * At this stage, only low level arch entry code remains and
82 * then we'll run in userspace. We can assume there won't be
83 * any RCU read-side critical section until the next call to
84 * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency
85 * on the tick.
86 */
87 vtime_user_enter(current);
88 rcu_user_enter();
89 }
Frederic Weisbecker4eacdf12013-01-16 17:16:37 +010090 /*
Frederic Weisbeckerd65ec122013-07-11 23:59:33 +020091 * Even if context tracking is disabled on this CPU, because it's outside
92 * the full dynticks mask for example, we still have to keep track of the
93 * context transitions and states to prevent inconsistency on those of
94 * other CPUs.
95 * If a task triggers an exception in userspace, sleep on the exception
96 * handler and then migrate to another CPU, that new CPU must know where
97 * the exception returns by the time we call exception_exit().
98 * This information can only be provided by the previous CPU when it called
99 * exception_enter().
100 * OTOH we can spare the calls to vtime and RCU when context_tracking.active
101 * is false because we know that CPU is not tickless.
Frederic Weisbecker4eacdf12013-01-16 17:16:37 +0100102 */
Frederic Weisbeckerabf917c2012-07-25 07:56:04 +0200103 __this_cpu_write(context_tracking.state, IN_USER);
Frederic Weisbecker91d1aa432012-11-27 19:33:25 +0100104 }
105 local_irq_restore(flags);
106}
107
Steven Rostedt29bb9e52013-05-24 15:23:40 -0400108#ifdef CONFIG_PREEMPT
109/**
110 * preempt_schedule_context - preempt_schedule called by tracing
111 *
112 * The tracing infrastructure uses preempt_enable_notrace to prevent
113 * recursion and tracing preempt enabling caused by the tracing
114 * infrastructure itself. But as tracing can happen in areas coming
115 * from userspace or just about to enter userspace, a preempt enable
116 * can occur before user_exit() is called. This will cause the scheduler
117 * to be called when the system is still in usermode.
118 *
119 * To prevent this, the preempt_enable_notrace will use this function
120 * instead of preempt_schedule() to exit user context if needed before
121 * calling the scheduler.
122 */
Peter Zijlstrabdb43802013-09-10 12:15:23 +0200123asmlinkage void __sched notrace preempt_schedule_context(void)
Steven Rostedt29bb9e52013-05-24 15:23:40 -0400124{
Steven Rostedt29bb9e52013-05-24 15:23:40 -0400125 enum ctx_state prev_ctx;
126
Frederic Weisbeckerfbb00b52013-06-19 23:56:22 +0200127 if (likely(!preemptible()))
Steven Rostedt29bb9e52013-05-24 15:23:40 -0400128 return;
129
130 /*
131 * Need to disable preemption in case user_exit() is traced
132 * and the tracer calls preempt_enable_notrace() causing
133 * an infinite recursion.
134 */
135 preempt_disable_notrace();
136 prev_ctx = exception_enter();
137 preempt_enable_no_resched_notrace();
138
139 preempt_schedule();
140
141 preempt_disable_notrace();
142 exception_exit(prev_ctx);
143 preempt_enable_notrace();
144}
145EXPORT_SYMBOL_GPL(preempt_schedule_context);
146#endif /* CONFIG_PREEMPT */
Frederic Weisbecker4eacdf12013-01-16 17:16:37 +0100147
148/**
Frederic Weisbeckerad657822013-07-10 02:44:35 +0200149 * context_tracking_user_exit - Inform the context tracking that the CPU is
150 * exiting userspace mode and entering the kernel.
Frederic Weisbecker4eacdf12013-01-16 17:16:37 +0100151 *
152 * This function must be called after we entered the kernel from userspace
153 * before any use of RCU read side critical section. This potentially include
154 * any high level kernel code like syscalls, exceptions, signal handling, etc...
155 *
156 * This call supports re-entrancy. This way it can be called from any exception
157 * handler without needing to know if we came from userspace or not.
158 */
Frederic Weisbeckerad657822013-07-10 02:44:35 +0200159void context_tracking_user_exit(void)
Frederic Weisbecker91d1aa432012-11-27 19:33:25 +0100160{
161 unsigned long flags;
162
Frederic Weisbecker58135f52013-11-06 14:45:57 +0100163 if (!context_tracking_is_enabled())
Frederic Weisbecker0c06a5d2013-09-10 00:54:17 +0200164 return;
165
Frederic Weisbecker91d1aa432012-11-27 19:33:25 +0100166 if (in_interrupt())
167 return;
168
169 local_irq_save(flags);
170 if (__this_cpu_read(context_tracking.state) == IN_USER) {
Frederic Weisbeckerd65ec122013-07-11 23:59:33 +0200171 if (__this_cpu_read(context_tracking.active)) {
172 /*
173 * We are going to run code that may use RCU. Inform
174 * RCU core about that (ie: we may need the tick again).
175 */
176 rcu_user_exit();
177 vtime_user_exit(current);
Frederic Weisbecker1b6a2592013-07-11 20:27:43 +0200178 trace_user_exit(0);
Frederic Weisbeckerd65ec122013-07-11 23:59:33 +0200179 }
Frederic Weisbeckerabf917c2012-07-25 07:56:04 +0200180 __this_cpu_write(context_tracking.state, IN_KERNEL);
Frederic Weisbecker91d1aa432012-11-27 19:33:25 +0100181 }
182 local_irq_restore(flags);
183}
184
Frederic Weisbecker4eacdf12013-01-16 17:16:37 +0100185/**
Frederic Weisbecker73d424f2013-07-11 19:42:13 +0200186 * __context_tracking_task_switch - context switch the syscall callbacks
Frederic Weisbecker4eacdf12013-01-16 17:16:37 +0100187 * @prev: the task that is being switched out
188 * @next: the task that is being switched in
189 *
190 * The context tracking uses the syscall slow path to implement its user-kernel
191 * boundaries probes on syscalls. This way it doesn't impact the syscall fast
192 * path on CPUs that don't do context tracking.
193 *
194 * But we need to clear the flag on the previous task because it may later
195 * migrate to some CPU that doesn't do the context tracking. As such the TIF
196 * flag may not be desired there.
197 */
Frederic Weisbecker73d424f2013-07-11 19:42:13 +0200198void __context_tracking_task_switch(struct task_struct *prev,
199 struct task_struct *next)
Frederic Weisbecker91d1aa432012-11-27 19:33:25 +0100200{
Frederic Weisbeckerd65ec122013-07-11 23:59:33 +0200201 clear_tsk_thread_flag(prev, TIF_NOHZ);
202 set_tsk_thread_flag(next, TIF_NOHZ);
Frederic Weisbecker91d1aa432012-11-27 19:33:25 +0100203}
Frederic Weisbecker65f382f2013-07-11 19:12:32 +0200204
205#ifdef CONFIG_CONTEXT_TRACKING_FORCE
206void __init context_tracking_init(void)
207{
208 int cpu;
209
210 for_each_possible_cpu(cpu)
211 context_tracking_cpu_set(cpu);
212}
213#endif