linux/kernel/context_tracking.c
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   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
  26struct static_key context_tracking_enabled = STATIC_KEY_INIT_FALSE;
  27EXPORT_SYMBOL_GPL(context_tracking_enabled);
  28
  29DEFINE_PER_CPU(struct context_tracking, context_tracking);
  30EXPORT_SYMBOL_GPL(context_tracking);
  31
  32void 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 */
  49void 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 */
 123void __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}
 145EXPORT_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 */
 159void 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 */
 198void __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
 206void __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
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