kernel/context_tracking.c

   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 static key
  57          * check.
  58          */
  59         if (!static_key_false(&context_tracking_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 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 (!static_key_false(&context_tracking_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