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 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> 23 24 #define CREATE_TRACE_POINTS 25 #include <trace/events/context_tracking.h> 26 27 struct static_key context_tracking_enabled = STATIC_KEY_INIT_FALSE; 28 EXPORT_SYMBOL_GPL(context_tracking_enabled); 29 30 DEFINE_PER_CPU(struct context_tracking, context_tracking); 31 EXPORT_SYMBOL_GPL(context_tracking); 32 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 52 /** 53 * context_tracking_enter - Inform the context tracking that the CPU is going 54 * enter user or guest space mode. 55 * 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. 60 */ 61 void __context_tracking_enter(enum ctx_state state) 62 { 63 /* Kernel threads aren't supposed to go to userspace */ 64 WARN_ON_ONCE(!current->mm); 65 66 if (!context_tracking_recursion_enter()) 67 return; 68 69 if ( __this_cpu_read(context_tracking.state) != state) { 70 if (__this_cpu_read(context_tracking.active)) { 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 if (state == CONTEXT_USER) { 79 trace_user_enter(0); 80 vtime_user_enter(current); 81 } 82 rcu_user_enter(); 83 } 84 /* 85 * Even if context tracking is disabled on this CPU, because it's outside 86 * the full dynticks mask for example, we still have to keep track of the 87 * context transitions and states to prevent inconsistency on those of 88 * other CPUs. 89 * If a task triggers an exception in userspace, sleep on the exception 90 * handler and then migrate to another CPU, that new CPU must know where 91 * the exception returns by the time we call exception_exit(). 92 * This information can only be provided by the previous CPU when it called 93 * exception_enter(). 94 * OTOH we can spare the calls to vtime and RCU when context_tracking.active 95 * is false because we know that CPU is not tickless. 96 */ 97 __this_cpu_write(context_tracking.state, state); 98 } 99 context_tracking_recursion_exit(); 100 } 101 NOKPROBE_SYMBOL(__context_tracking_enter); 102 EXPORT_SYMBOL_GPL(__context_tracking_enter); 103 104 void context_tracking_enter(enum ctx_state state) 105 { 106 unsigned long flags; 107 108 /* 109 * Some contexts may involve an exception occuring in an irq, 110 * leading to that nesting: 111 * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit() 112 * This would mess up the dyntick_nesting count though. And rcu_irq_*() 113 * helpers are enough to protect RCU uses inside the exception. So 114 * just return immediately if we detect we are in an IRQ. 115 */ 116 if (in_interrupt()) 117 return; 118 119 local_irq_save(flags); 120 __context_tracking_enter(state); 121 local_irq_restore(flags); 122 } 123 NOKPROBE_SYMBOL(context_tracking_enter); 124 EXPORT_SYMBOL_GPL(context_tracking_enter); 125 126 void context_tracking_user_enter(void) 127 { 128 user_enter(); 129 } 130 NOKPROBE_SYMBOL(context_tracking_user_enter); 131 132 /** 133 * context_tracking_exit - Inform the context tracking that the CPU is 134 * exiting user or guest mode and entering the kernel. 135 * 136 * This function must be called after we entered the kernel from user or 137 * guest space before any use of RCU read side critical section. This 138 * potentially include any high level kernel code like syscalls, exceptions, 139 * signal handling, etc... 140 * 141 * This call supports re-entrancy. This way it can be called from any exception 142 * handler without needing to know if we came from userspace or not. 143 */ 144 void __context_tracking_exit(enum ctx_state state) 145 { 146 if (!context_tracking_recursion_enter()) 147 return; 148 149 if (__this_cpu_read(context_tracking.state) == state) { 150 if (__this_cpu_read(context_tracking.active)) { 151 /* 152 * We are going to run code that may use RCU. Inform 153 * RCU core about that (ie: we may need the tick again). 154 */ 155 rcu_user_exit(); 156 if (state == CONTEXT_USER) { 157 vtime_user_exit(current); 158 trace_user_exit(0); 159 } 160 } 161 __this_cpu_write(context_tracking.state, CONTEXT_KERNEL); 162 } 163 context_tracking_recursion_exit(); 164 } 165 NOKPROBE_SYMBOL(__context_tracking_exit); 166 EXPORT_SYMBOL_GPL(__context_tracking_exit); 167 168 void context_tracking_exit(enum ctx_state state) 169 { 170 unsigned long flags; 171 172 if (in_interrupt()) 173 return; 174 175 local_irq_save(flags); 176 __context_tracking_exit(state); 177 local_irq_restore(flags); 178 } 179 NOKPROBE_SYMBOL(context_tracking_exit); 180 EXPORT_SYMBOL_GPL(context_tracking_exit); 181 182 void context_tracking_user_exit(void) 183 { 184 user_exit(); 185 } 186 NOKPROBE_SYMBOL(context_tracking_user_exit); 187 188 void __init context_tracking_cpu_set(int cpu) 189 { 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; 208 } 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 219