Commit | Line | Data |
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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> |
4cdf77a8 | 22 | #include <linux/kprobes.h> |
91d1aa43 | 23 | |
1b6a259a FW |
24 | #define CREATE_TRACE_POINTS |
25 | #include <trace/events/context_tracking.h> | |
26 | ||
65f382fd | 27 | struct static_key context_tracking_enabled = STATIC_KEY_INIT_FALSE; |
48d6a816 | 28 | EXPORT_SYMBOL_GPL(context_tracking_enabled); |
65f382fd FW |
29 | |
30 | DEFINE_PER_CPU(struct context_tracking, context_tracking); | |
48d6a816 | 31 | EXPORT_SYMBOL_GPL(context_tracking); |
91d1aa43 | 32 | |
aed5ed47 FW |
33 | static bool context_tracking_recursion_enter(void) |
34 | { | |
35 | int recursion; | |
36 | ||
37 | recursion = __this_cpu_inc_return(context_tracking.recursion); | |
38 | if (recursion == 1) | |
39 | return true; | |
40 | ||
41 | WARN_ONCE((recursion < 1), "Invalid context tracking recursion value %d\n", recursion); | |
42 | __this_cpu_dec(context_tracking.recursion); | |
43 | ||
44 | return false; | |
45 | } | |
46 | ||
47 | static void context_tracking_recursion_exit(void) | |
48 | { | |
49 | __this_cpu_dec(context_tracking.recursion); | |
50 | } | |
51 | ||
4eacdf18 | 52 | /** |
3aab4f50 RR |
53 | * context_tracking_enter - Inform the context tracking that the CPU is going |
54 | * enter user or guest space mode. | |
4eacdf18 FW |
55 | * |
56 | * This function must be called right before we switch from the kernel | |
3aab4f50 RR |
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. | |
4eacdf18 | 60 | */ |
3aab4f50 | 61 | void context_tracking_enter(enum ctx_state state) |
91d1aa43 FW |
62 | { |
63 | unsigned long flags; | |
64 | ||
0c06a5d4 FW |
65 | /* |
66 | * Repeat the user_enter() check here because some archs may be calling | |
67 | * this from asm and if no CPU needs context tracking, they shouldn't | |
58135f57 FW |
68 | * go further. Repeat the check here until they support the inline static |
69 | * key check. | |
0c06a5d4 | 70 | */ |
58135f57 | 71 | if (!context_tracking_is_enabled()) |
0c06a5d4 FW |
72 | return; |
73 | ||
91d1aa43 FW |
74 | /* |
75 | * Some contexts may involve an exception occuring in an irq, | |
76 | * leading to that nesting: | |
77 | * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit() | |
78 | * This would mess up the dyntick_nesting count though. And rcu_irq_*() | |
79 | * helpers are enough to protect RCU uses inside the exception. So | |
80 | * just return immediately if we detect we are in an IRQ. | |
81 | */ | |
82 | if (in_interrupt()) | |
83 | return; | |
84 | ||
4eacdf18 | 85 | /* Kernel threads aren't supposed to go to userspace */ |
91d1aa43 FW |
86 | WARN_ON_ONCE(!current->mm); |
87 | ||
88 | local_irq_save(flags); | |
aed5ed47 FW |
89 | if (!context_tracking_recursion_enter()) |
90 | goto out_irq_restore; | |
91 | ||
3aab4f50 | 92 | if ( __this_cpu_read(context_tracking.state) != state) { |
d65ec121 FW |
93 | if (__this_cpu_read(context_tracking.active)) { |
94 | /* | |
95 | * At this stage, only low level arch entry code remains and | |
96 | * then we'll run in userspace. We can assume there won't be | |
97 | * any RCU read-side critical section until the next call to | |
98 | * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency | |
99 | * on the tick. | |
100 | */ | |
19fdd98b RR |
101 | if (state == CONTEXT_USER) { |
102 | trace_user_enter(0); | |
103 | vtime_user_enter(current); | |
104 | } | |
d65ec121 FW |
105 | rcu_user_enter(); |
106 | } | |
4eacdf18 | 107 | /* |
d65ec121 FW |
108 | * Even if context tracking is disabled on this CPU, because it's outside |
109 | * the full dynticks mask for example, we still have to keep track of the | |
110 | * context transitions and states to prevent inconsistency on those of | |
111 | * other CPUs. | |
112 | * If a task triggers an exception in userspace, sleep on the exception | |
113 | * handler and then migrate to another CPU, that new CPU must know where | |
114 | * the exception returns by the time we call exception_exit(). | |
115 | * This information can only be provided by the previous CPU when it called | |
116 | * exception_enter(). | |
117 | * OTOH we can spare the calls to vtime and RCU when context_tracking.active | |
118 | * is false because we know that CPU is not tickless. | |
4eacdf18 | 119 | */ |
3aab4f50 | 120 | __this_cpu_write(context_tracking.state, state); |
91d1aa43 | 121 | } |
aed5ed47 FW |
122 | context_tracking_recursion_exit(); |
123 | out_irq_restore: | |
91d1aa43 FW |
124 | local_irq_restore(flags); |
125 | } | |
3aab4f50 | 126 | NOKPROBE_SYMBOL(context_tracking_enter); |
efc1e2c9 | 127 | EXPORT_SYMBOL_GPL(context_tracking_enter); |
3aab4f50 RR |
128 | |
129 | void context_tracking_user_enter(void) | |
130 | { | |
131 | context_tracking_enter(CONTEXT_USER); | |
132 | } | |
4cdf77a8 | 133 | NOKPROBE_SYMBOL(context_tracking_user_enter); |
91d1aa43 | 134 | |
4eacdf18 | 135 | /** |
3aab4f50 RR |
136 | * context_tracking_exit - Inform the context tracking that the CPU is |
137 | * exiting user or guest mode and entering the kernel. | |
4eacdf18 | 138 | * |
3aab4f50 RR |
139 | * This function must be called after we entered the kernel from user or |
140 | * guest space before any use of RCU read side critical section. This | |
141 | * potentially include any high level kernel code like syscalls, exceptions, | |
142 | * signal handling, etc... | |
4eacdf18 FW |
143 | * |
144 | * This call supports re-entrancy. This way it can be called from any exception | |
145 | * handler without needing to know if we came from userspace or not. | |
146 | */ | |
3aab4f50 | 147 | void context_tracking_exit(enum ctx_state state) |
91d1aa43 FW |
148 | { |
149 | unsigned long flags; | |
150 | ||
58135f57 | 151 | if (!context_tracking_is_enabled()) |
0c06a5d4 FW |
152 | return; |
153 | ||
91d1aa43 FW |
154 | if (in_interrupt()) |
155 | return; | |
156 | ||
157 | local_irq_save(flags); | |
aed5ed47 FW |
158 | if (!context_tracking_recursion_enter()) |
159 | goto out_irq_restore; | |
160 | ||
3aab4f50 | 161 | if (__this_cpu_read(context_tracking.state) == state) { |
d65ec121 FW |
162 | if (__this_cpu_read(context_tracking.active)) { |
163 | /* | |
164 | * We are going to run code that may use RCU. Inform | |
165 | * RCU core about that (ie: we may need the tick again). | |
166 | */ | |
167 | rcu_user_exit(); | |
19fdd98b RR |
168 | if (state == CONTEXT_USER) { |
169 | vtime_user_exit(current); | |
170 | trace_user_exit(0); | |
171 | } | |
d65ec121 | 172 | } |
c467ea76 | 173 | __this_cpu_write(context_tracking.state, CONTEXT_KERNEL); |
91d1aa43 | 174 | } |
aed5ed47 FW |
175 | context_tracking_recursion_exit(); |
176 | out_irq_restore: | |
91d1aa43 FW |
177 | local_irq_restore(flags); |
178 | } | |
3aab4f50 | 179 | NOKPROBE_SYMBOL(context_tracking_exit); |
efc1e2c9 | 180 | EXPORT_SYMBOL_GPL(context_tracking_exit); |
3aab4f50 RR |
181 | |
182 | void context_tracking_user_exit(void) | |
183 | { | |
184 | context_tracking_exit(CONTEXT_USER); | |
185 | } | |
4cdf77a8 | 186 | NOKPROBE_SYMBOL(context_tracking_user_exit); |
91d1aa43 | 187 | |
fafe870f | 188 | void __init context_tracking_cpu_set(int cpu) |
91d1aa43 | 189 | { |
fafe870f FW |
190 | static __initdata bool initialized = false; |
191 | ||
192 | if (!per_cpu(context_tracking.active, cpu)) { | |
193 | per_cpu(context_tracking.active, cpu) = true; | |
194 | static_key_slow_inc(&context_tracking_enabled); | |
195 | } | |
196 | ||
197 | if (initialized) | |
198 | return; | |
199 | ||
200 | /* | |
201 | * Set TIF_NOHZ to init/0 and let it propagate to all tasks through fork | |
202 | * This assumes that init is the only task at this early boot stage. | |
203 | */ | |
204 | set_tsk_thread_flag(&init_task, TIF_NOHZ); | |
205 | WARN_ON_ONCE(!tasklist_empty()); | |
206 | ||
207 | initialized = true; | |
91d1aa43 | 208 | } |
65f382fd FW |
209 | |
210 | #ifdef CONFIG_CONTEXT_TRACKING_FORCE | |
211 | void __init context_tracking_init(void) | |
212 | { | |
213 | int cpu; | |
214 | ||
215 | for_each_possible_cpu(cpu) | |
216 | context_tracking_cpu_set(cpu); | |
217 | } | |
218 | #endif |