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 void context_tracking_cpu_set(int cpu) 34 { 35 if (!per_cpu(context_tracking.active, cpu)) { 36 per_cpu(context_tracking.active, cpu) = true; 37 static_key_slow_inc(&context_tracking_enabled); 38 } 39 } 40 41 /** 42 * context_tracking_user_enter - Inform the context tracking that the CPU is going to 43 * enter userspace mode. 44 * 45 * This function must be called right before we switch from the kernel 46 * to userspace, when it's guaranteed the remaining kernel instructions 47 * to execute won't use any RCU read side critical section because this 48 * function sets RCU in extended quiescent state. 49 */ 50 void context_tracking_user_enter(void) 51 { 52 unsigned long flags; 53 54 /* 55 * Repeat the user_enter() check here because some archs may be calling 56 * this from asm and if no CPU needs context tracking, they shouldn't 57 * go further. Repeat the check here until they support the inline static 58 * key check. 59 */ 60 if (!context_tracking_is_enabled()) 61 return; 62 63 /* 64 * Some contexts may involve an exception occuring in an irq, 65 * leading to that nesting: 66 * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit() 67 * This would mess up the dyntick_nesting count though. And rcu_irq_*() 68 * helpers are enough to protect RCU uses inside the exception. So 69 * just return immediately if we detect we are in an IRQ. 70 */ 71 if (in_interrupt()) 72 return; 73 74 /* Kernel threads aren't supposed to go to userspace */ 75 WARN_ON_ONCE(!current->mm); 76 77 local_irq_save(flags); 78 if ( __this_cpu_read(context_tracking.state) != IN_USER) { 79 if (__this_cpu_read(context_tracking.active)) { 80 trace_user_enter(0); 81 /* 82 * At this stage, only low level arch entry code remains and 83 * then we'll run in userspace. We can assume there won't be 84 * any RCU read-side critical section until the next call to 85 * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency 86 * on the tick. 87 */ 88 vtime_user_enter(current); 89 rcu_user_enter(); 90 } 91 /* 92 * Even if context tracking is disabled on this CPU, because it's outside 93 * the full dynticks mask for example, we still have to keep track of the 94 * context transitions and states to prevent inconsistency on those of 95 * other CPUs. 96 * If a task triggers an exception in userspace, sleep on the exception 97 * handler and then migrate to another CPU, that new CPU must know where 98 * the exception returns by the time we call exception_exit(). 99 * This information can only be provided by the previous CPU when it called 100 * exception_enter(). 101 * OTOH we can spare the calls to vtime and RCU when context_tracking.active 102 * is false because we know that CPU is not tickless. 103 */ 104 __this_cpu_write(context_tracking.state, IN_USER); 105 } 106 local_irq_restore(flags); 107 } 108 NOKPROBE_SYMBOL(context_tracking_user_enter); 109 110 /** 111 * context_tracking_user_exit - Inform the context tracking that the CPU is 112 * exiting userspace mode and entering the kernel. 113 * 114 * This function must be called after we entered the kernel from userspace 115 * before any use of RCU read side critical section. This potentially include 116 * any high level kernel code like syscalls, exceptions, signal handling, etc... 117 * 118 * This call supports re-entrancy. This way it can be called from any exception 119 * handler without needing to know if we came from userspace or not. 120 */ 121 void context_tracking_user_exit(void) 122 { 123 unsigned long flags; 124 125 if (!context_tracking_is_enabled()) 126 return; 127 128 if (in_interrupt()) 129 return; 130 131 local_irq_save(flags); 132 if (__this_cpu_read(context_tracking.state) == IN_USER) { 133 if (__this_cpu_read(context_tracking.active)) { 134 /* 135 * We are going to run code that may use RCU. Inform 136 * RCU core about that (ie: we may need the tick again). 137 */ 138 rcu_user_exit(); 139 vtime_user_exit(current); 140 trace_user_exit(0); 141 } 142 __this_cpu_write(context_tracking.state, IN_KERNEL); 143 } 144 local_irq_restore(flags); 145 } 146 NOKPROBE_SYMBOL(context_tracking_user_exit); 147 148 /** 149 * __context_tracking_task_switch - context switch the syscall callbacks 150 * @prev: the task that is being switched out 151 * @next: the task that is being switched in 152 * 153 * The context tracking uses the syscall slow path to implement its user-kernel 154 * boundaries probes on syscalls. This way it doesn't impact the syscall fast 155 * path on CPUs that don't do context tracking. 156 * 157 * But we need to clear the flag on the previous task because it may later 158 * migrate to some CPU that doesn't do the context tracking. As such the TIF 159 * flag may not be desired there. 160 */ 161 void __context_tracking_task_switch(struct task_struct *prev, 162 struct task_struct *next) 163 { 164 clear_tsk_thread_flag(prev, TIF_NOHZ); 165 set_tsk_thread_flag(next, TIF_NOHZ); 166 } 167 168 #ifdef CONFIG_CONTEXT_TRACKING_FORCE 169 void __init context_tracking_init(void) 170 { 171 int cpu; 172 173 for_each_possible_cpu(cpu) 174 context_tracking_cpu_set(cpu); 175 } 176 #endif 177