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 23 #define CREATE_TRACE_POINTS 24 #include <trace/events/context_tracking.h> 25 26 struct static_key context_tracking_enabled = STATIC_KEY_INIT_FALSE; 27 EXPORT_SYMBOL_GPL(context_tracking_enabled); 28 29 DEFINE_PER_CPU(struct context_tracking, context_tracking); 30 EXPORT_SYMBOL_GPL(context_tracking); 31 32 void context_tracking_cpu_set(int cpu) 33 { 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 } 38 } 39 40 /** 41 * context_tracking_user_enter - Inform the context tracking that the CPU is going to 42 * enter userspace mode. 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 */ 49 void context_tracking_user_enter(void) 50 { 51 unsigned long flags; 52 53 /* 54 * 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 56 * go further. Repeat the check here until they support the inline static 57 * key check. 58 */ 59 if (!context_tracking_is_enabled()) 60 return; 61 62 /* 63 * 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 73 /* Kernel threads aren't supposed to go to userspace */ 74 WARN_ON_ONCE(!current->mm); 75 76 local_irq_save(flags); 77 if ( __this_cpu_read(context_tracking.state) != IN_USER) { 78 if (__this_cpu_read(context_tracking.active)) { 79 trace_user_enter(0); 80 /* 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 } 90 /* 91 * 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. 102 */ 103 __this_cpu_write(context_tracking.state, IN_USER); 104 } 105 local_irq_restore(flags); 106 } 107 108 #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 */ 123 asmlinkage __visible void __sched notrace preempt_schedule_context(void) 124 { 125 enum ctx_state prev_ctx; 126 127 if (likely(!preemptible())) 128 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 } 145 EXPORT_SYMBOL_GPL(preempt_schedule_context); 146 #endif /* CONFIG_PREEMPT */ 147 148 /** 149 * context_tracking_user_exit - Inform the context tracking that the CPU is 150 * exiting userspace mode and entering the kernel. 151 * 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 */ 159 void context_tracking_user_exit(void) 160 { 161 unsigned long flags; 162 163 if (!context_tracking_is_enabled()) 164 return; 165 166 if (in_interrupt()) 167 return; 168 169 local_irq_save(flags); 170 if (__this_cpu_read(context_tracking.state) == IN_USER) { 171 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); 178 trace_user_exit(0); 179 } 180 __this_cpu_write(context_tracking.state, IN_KERNEL); 181 } 182 local_irq_restore(flags); 183 } 184 185 /** 186 * __context_tracking_task_switch - context switch the syscall callbacks 187 * @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 */ 198 void __context_tracking_task_switch(struct task_struct *prev, 199 struct task_struct *next) 200 { 201 clear_tsk_thread_flag(prev, TIF_NOHZ); 202 set_tsk_thread_flag(next, TIF_NOHZ); 203 } 204 205 #ifdef CONFIG_CONTEXT_TRACKING_FORCE 206 void __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 214