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 * 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 64 /* Kernel threads aren't supposed to go to userspace */ 65 WARN_ON_ONCE(!current->mm); 66 67 local_irq_save(flags); 68 if ( __this_cpu_read(context_tracking.state) != IN_USER) { 69 if (__this_cpu_read(context_tracking.active)) { 70 trace_user_enter(0); 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 } 81 /* 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. 93 */ 94 __this_cpu_write(context_tracking.state, IN_USER); 95 } 96 local_irq_restore(flags); 97 } 98 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 { 116 enum ctx_state prev_ctx; 117 118 if (likely(!preemptible())) 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 */ 138 139 /** 140 * context_tracking_user_exit - Inform the context tracking that the CPU is 141 * exiting userspace mode and entering the kernel. 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 */ 150 void context_tracking_user_exit(void) 151 { 152 unsigned long flags; 153 154 if (in_interrupt()) 155 return; 156 157 local_irq_save(flags); 158 if (__this_cpu_read(context_tracking.state) == IN_USER) { 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); 166 trace_user_exit(0); 167 } 168 __this_cpu_write(context_tracking.state, IN_KERNEL); 169 } 170 local_irq_restore(flags); 171 } 172 173 /** 174 * __context_tracking_task_switch - context switch the syscall callbacks 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 */ 186 void __context_tracking_task_switch(struct task_struct *prev, 187 struct task_struct *next) 188 { 189 clear_tsk_thread_flag(prev, TIF_NOHZ); 190 set_tsk_thread_flag(next, TIF_NOHZ); 191 } 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 202