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4eacdf18 FW |
1 | /* |
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 | ||
91d1aa43 FW |
17 | #include <linux/context_tracking.h> |
18 | #include <linux/rcupdate.h> | |
19 | #include <linux/sched.h> | |
91d1aa43 | 20 | #include <linux/hardirq.h> |
6a61671b | 21 | #include <linux/export.h> |
91d1aa43 | 22 | |
1b6a259a FW |
23 | #define CREATE_TRACE_POINTS |
24 | #include <trace/events/context_tracking.h> | |
25 | ||
65f382fd | 26 | struct static_key context_tracking_enabled = STATIC_KEY_INIT_FALSE; |
48d6a816 | 27 | EXPORT_SYMBOL_GPL(context_tracking_enabled); |
65f382fd FW |
28 | |
29 | DEFINE_PER_CPU(struct context_tracking, context_tracking); | |
48d6a816 | 30 | EXPORT_SYMBOL_GPL(context_tracking); |
91d1aa43 | 31 | |
2e709338 FW |
32 | void context_tracking_cpu_set(int cpu) |
33 | { | |
65f382fd FW |
34 | if (!per_cpu(context_tracking.active, cpu)) { |
35 | per_cpu(context_tracking.active, cpu) = true; | |
36 | static_key_slow_inc(&context_tracking_enabled); | |
37 | } | |
2e709338 FW |
38 | } |
39 | ||
4eacdf18 | 40 | /** |
ad65782f FW |
41 | * context_tracking_user_enter - Inform the context tracking that the CPU is going to |
42 | * enter userspace mode. | |
4eacdf18 FW |
43 | * |
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 | */ | |
ad65782f | 49 | void context_tracking_user_enter(void) |
91d1aa43 FW |
50 | { |
51 | unsigned long flags; | |
52 | ||
53 | /* | |
54 | * Some contexts may involve an exception occuring in an irq, | |
55 | * leading to that nesting: | |
56 | * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit() | |
57 | * This would mess up the dyntick_nesting count though. And rcu_irq_*() | |
58 | * helpers are enough to protect RCU uses inside the exception. So | |
59 | * just return immediately if we detect we are in an IRQ. | |
60 | */ | |
61 | if (in_interrupt()) | |
62 | return; | |
63 | ||
4eacdf18 | 64 | /* Kernel threads aren't supposed to go to userspace */ |
91d1aa43 FW |
65 | WARN_ON_ONCE(!current->mm); |
66 | ||
67 | local_irq_save(flags); | |
d65ec121 FW |
68 | if ( __this_cpu_read(context_tracking.state) != IN_USER) { |
69 | if (__this_cpu_read(context_tracking.active)) { | |
1b6a259a | 70 | trace_user_enter(0); |
d65ec121 FW |
71 | /* |
72 | * At this stage, only low level arch entry code remains and | |
73 | * then we'll run in userspace. We can assume there won't be | |
74 | * any RCU read-side critical section until the next call to | |
75 | * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency | |
76 | * on the tick. | |
77 | */ | |
78 | vtime_user_enter(current); | |
79 | rcu_user_enter(); | |
80 | } | |
4eacdf18 | 81 | /* |
d65ec121 FW |
82 | * Even if context tracking is disabled on this CPU, because it's outside |
83 | * the full dynticks mask for example, we still have to keep track of the | |
84 | * context transitions and states to prevent inconsistency on those of | |
85 | * other CPUs. | |
86 | * If a task triggers an exception in userspace, sleep on the exception | |
87 | * handler and then migrate to another CPU, that new CPU must know where | |
88 | * the exception returns by the time we call exception_exit(). | |
89 | * This information can only be provided by the previous CPU when it called | |
90 | * exception_enter(). | |
91 | * OTOH we can spare the calls to vtime and RCU when context_tracking.active | |
92 | * is false because we know that CPU is not tickless. | |
4eacdf18 | 93 | */ |
abf917cd | 94 | __this_cpu_write(context_tracking.state, IN_USER); |
91d1aa43 FW |
95 | } |
96 | local_irq_restore(flags); | |
97 | } | |
98 | ||
29bb9e5a SR |
99 | #ifdef CONFIG_PREEMPT |
100 | /** | |
101 | * preempt_schedule_context - preempt_schedule called by tracing | |
102 | * | |
103 | * The tracing infrastructure uses preempt_enable_notrace to prevent | |
104 | * recursion and tracing preempt enabling caused by the tracing | |
105 | * infrastructure itself. But as tracing can happen in areas coming | |
106 | * from userspace or just about to enter userspace, a preempt enable | |
107 | * can occur before user_exit() is called. This will cause the scheduler | |
108 | * to be called when the system is still in usermode. | |
109 | * | |
110 | * To prevent this, the preempt_enable_notrace will use this function | |
111 | * instead of preempt_schedule() to exit user context if needed before | |
112 | * calling the scheduler. | |
113 | */ | |
114 | void __sched notrace preempt_schedule_context(void) | |
115 | { | |
29bb9e5a SR |
116 | enum ctx_state prev_ctx; |
117 | ||
fbb00b56 | 118 | if (likely(!preemptible())) |
29bb9e5a SR |
119 | return; |
120 | ||
121 | /* | |
122 | * Need to disable preemption in case user_exit() is traced | |
123 | * and the tracer calls preempt_enable_notrace() causing | |
124 | * an infinite recursion. | |
125 | */ | |
126 | preempt_disable_notrace(); | |
127 | prev_ctx = exception_enter(); | |
128 | preempt_enable_no_resched_notrace(); | |
129 | ||
130 | preempt_schedule(); | |
131 | ||
132 | preempt_disable_notrace(); | |
133 | exception_exit(prev_ctx); | |
134 | preempt_enable_notrace(); | |
135 | } | |
136 | EXPORT_SYMBOL_GPL(preempt_schedule_context); | |
137 | #endif /* CONFIG_PREEMPT */ | |
4eacdf18 FW |
138 | |
139 | /** | |
ad65782f FW |
140 | * context_tracking_user_exit - Inform the context tracking that the CPU is |
141 | * exiting userspace mode and entering the kernel. | |
4eacdf18 FW |
142 | * |
143 | * This function must be called after we entered the kernel from userspace | |
144 | * before any use of RCU read side critical section. This potentially include | |
145 | * any high level kernel code like syscalls, exceptions, signal handling, etc... | |
146 | * | |
147 | * This call supports re-entrancy. This way it can be called from any exception | |
148 | * handler without needing to know if we came from userspace or not. | |
149 | */ | |
ad65782f | 150 | void context_tracking_user_exit(void) |
91d1aa43 FW |
151 | { |
152 | unsigned long flags; | |
153 | ||
91d1aa43 FW |
154 | if (in_interrupt()) |
155 | return; | |
156 | ||
157 | local_irq_save(flags); | |
158 | if (__this_cpu_read(context_tracking.state) == IN_USER) { | |
d65ec121 FW |
159 | if (__this_cpu_read(context_tracking.active)) { |
160 | /* | |
161 | * We are going to run code that may use RCU. Inform | |
162 | * RCU core about that (ie: we may need the tick again). | |
163 | */ | |
164 | rcu_user_exit(); | |
165 | vtime_user_exit(current); | |
1b6a259a | 166 | trace_user_exit(0); |
d65ec121 | 167 | } |
abf917cd | 168 | __this_cpu_write(context_tracking.state, IN_KERNEL); |
91d1aa43 FW |
169 | } |
170 | local_irq_restore(flags); | |
171 | } | |
172 | ||
4eacdf18 | 173 | /** |
73d424f9 | 174 | * __context_tracking_task_switch - context switch the syscall callbacks |
4eacdf18 FW |
175 | * @prev: the task that is being switched out |
176 | * @next: the task that is being switched in | |
177 | * | |
178 | * The context tracking uses the syscall slow path to implement its user-kernel | |
179 | * boundaries probes on syscalls. This way it doesn't impact the syscall fast | |
180 | * path on CPUs that don't do context tracking. | |
181 | * | |
182 | * But we need to clear the flag on the previous task because it may later | |
183 | * migrate to some CPU that doesn't do the context tracking. As such the TIF | |
184 | * flag may not be desired there. | |
185 | */ | |
73d424f9 FW |
186 | void __context_tracking_task_switch(struct task_struct *prev, |
187 | struct task_struct *next) | |
91d1aa43 | 188 | { |
d65ec121 FW |
189 | clear_tsk_thread_flag(prev, TIF_NOHZ); |
190 | set_tsk_thread_flag(next, TIF_NOHZ); | |
91d1aa43 | 191 | } |
65f382fd FW |
192 | |
193 | #ifdef CONFIG_CONTEXT_TRACKING_FORCE | |
194 | void __init context_tracking_init(void) | |
195 | { | |
196 | int cpu; | |
197 | ||
198 | for_each_possible_cpu(cpu) | |
199 | context_tracking_cpu_set(cpu); | |
200 | } | |
201 | #endif |