path: root/kernel/context_tracking.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * Context tracking: Probe on high level context boundaries such as kernel,
 * userspace, guest or idle.
 *
 * This is used by RCU to remove its dependency on the timer tick while a CPU
 * runs in idle, userspace or guest mode.
 *
 * User/guest tracking started by Frederic Weisbecker:
 *
 * Copyright (C) 2012 Red Hat, Inc., Frederic Weisbecker
 *
 * Many thanks to Gilad Ben-Yossef, Paul McKenney, Ingo Molnar, Andrew Morton,
 * Steven Rostedt, Peter Zijlstra for suggestions and improvements.
 *
 * RCU extended quiescent state bits imported from kernel/rcu/tree.c
 * where the relevant authorship may be found.
 */

#include <linux/context_tracking.h>
#include <linux/rcupdate.h>
#include <linux/sched.h>
#include <linux/hardirq.h>
#include <linux/export.h>
#include <linux/kprobes.h>
#include <trace/events/rcu.h>


DEFINE_PER_CPU(struct context_tracking, context_tracking) = {
#ifdef CONFIG_CONTEXT_TRACKING_IDLE
	.dynticks_nesting = 1,
	.dynticks_nmi_nesting = DYNTICK_IRQ_NONIDLE,
#endif
	.state = ATOMIC_INIT(RCU_DYNTICKS_IDX),
};
EXPORT_SYMBOL_GPL(context_tracking);

#ifdef CONFIG_CONTEXT_TRACKING_IDLE
#define TPS(x)  tracepoint_string(x)

/* Record the current task on dyntick-idle entry. */
static __always_inline void rcu_dynticks_task_enter(void)
{
#if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL)
	WRITE_ONCE(current->rcu_tasks_idle_cpu, smp_processor_id());
#endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
}

/* Record no current task on dyntick-idle exit. */
static __always_inline void rcu_dynticks_task_exit(void)
{
#if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL)
	WRITE_ONCE(current->rcu_tasks_idle_cpu, -1);
#endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
}

/* Turn on heavyweight RCU tasks trace readers on idle/user entry. */
static __always_inline void rcu_dynticks_task_trace_enter(void)
{
#ifdef CONFIG_TASKS_TRACE_RCU
	if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
		current->trc_reader_special.b.need_mb = true;
#endif /* #ifdef CONFIG_TASKS_TRACE_RCU */
}

/* Turn off heavyweight RCU tasks trace readers on idle/user exit. */
static __always_inline void rcu_dynticks_task_trace_exit(void)
{
#ifdef CONFIG_TASKS_TRACE_RCU
	if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB))
		current->trc_reader_special.b.need_mb = false;
#endif /* #ifdef CONFIG_TASKS_TRACE_RCU */
}

/*
 * Record entry into an extended quiescent state.  This is only to be
 * called when not already in an extended quiescent state, that is,
 * RCU is watching prior to the call to this function and is no longer
 * watching upon return.
 */
static noinstr void ct_kernel_exit_state(int offset)
{
	int seq;

	/*
	 * CPUs seeing atomic_add_return() must see prior RCU read-side
	 * critical sections, and we also must force ordering with the
	 * next idle sojourn.
	 */
	rcu_dynticks_task_trace_enter();  // Before ->dynticks update!
	seq = ct_state_inc(offset);
	// RCU is no longer watching.  Better be in extended quiescent state!
	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && (seq & RCU_DYNTICKS_IDX));
}

/*
 * Record exit from an extended quiescent state.  This is only to be
 * called from an extended quiescent state, that is, RCU is not watching
 * prior to the call to this function and is watching upon return.
 */
static noinstr void ct_kernel_enter_state(int offset)
{
	int seq;

	/*
	 * CPUs seeing atomic_add_return() must see prior idle sojourns,
	 * and we also must force ordering with the next RCU read-side
	 * critical section.
	 */
	seq = ct_state_inc(offset);
	// RCU is now watching.  Better not be in an extended quiescent state!
	rcu_dynticks_task_trace_exit();  // After ->dynticks update!
	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !(seq & RCU_DYNTICKS_IDX));
}

/*
 * Enter an RCU extended quiescent state, which can be either the
 * idle loop or adaptive-tickless usermode execution.
 *
 * We crowbar the ->dynticks_nmi_nesting field to zero to allow for
 * the possibility of usermode upcalls having messed up our count
 * of interrupt nesting level during the prior busy period.
 */
static void noinstr ct_kernel_exit(bool user, int offset)
{
	struct context_tracking *ct = this_cpu_ptr(&context_tracking);

	WARN_ON_ONCE(ct_dynticks_nmi_nesting() != DYNTICK_IRQ_NONIDLE);
	WRITE_ONCE(ct->dynticks_nmi_nesting, 0);
	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
		     ct_dynticks_nesting() == 0);
	if (ct_dynticks_nesting() != 1) {
		// RCU will still be watching, so just do accounting and leave.
		ct->dynticks_nesting--;
		return;
	}

	instrumentation_begin();
	lockdep_assert_irqs_disabled();
	trace_rcu_dyntick(TPS("Start"), ct_dynticks_nesting(), 0, ct_dynticks());
	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
	rcu_preempt_deferred_qs(current);

	// instrumentation for the noinstr ct_kernel_exit_state()
	instrument_atomic_write(&ct->state, sizeof(ct->state));

	instrumentation_end();
	WRITE_ONCE(ct->dynticks_nesting, 0); /* Avoid irq-access tearing. */
	// RCU is watching here ...
	ct_kernel_exit_state(offset);
	// ... but is no longer watching here.
	rcu_dynticks_task_enter();
}

/*
 * Exit an RCU extended quiescent state, which can be either the
 * idle loop or adaptive-tickless usermode execution.
 *
 * We crowbar the ->dynticks_nmi_nesting field to DYNTICK_IRQ_NONIDLE to
 * allow for the possibility of usermode upcalls messing up our count of
 * interrupt nesting level during the busy period that is just now starting.
 */
static void noinstr ct_kernel_enter(bool user, int offset)
{
	struct context_tracking *ct = this_cpu_ptr(&context_tracking);
	long oldval;

	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !raw_irqs_disabled());
	oldval = ct_dynticks_nesting();
	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && oldval < 0);
	if (oldval) {
		// RCU was already watching, so just do accounting and leave.
		ct->dynticks_nesting++;
		return;
	}
	rcu_dynticks_task_exit();
	// RCU is not watching here ...
	ct_kernel_enter_state(offset);
	// ... but is watching here.
	instrumentation_begin();

	// instrumentation for the noinstr ct_kernel_enter_state()
	instrument_atomic_write(&ct->state, sizeof(ct->state));

	trace_rcu_dyntick(TPS("End"), ct_dynticks_nesting(), 1, ct_dynticks());
	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current));
	WRITE_ONCE(ct->dynticks_nesting, 1);
	WARN_ON_ONCE(ct_dynticks_nmi_nesting());
	WRITE_ONCE(ct->dynticks_nmi_nesting, DYNTICK_IRQ_NONIDLE);
	instrumentation_end();
}

/**
 * ct_nmi_exit - inform RCU of exit from NMI context
 *
 * If we are returning from the outermost NMI handler that interrupted an
 * RCU-idle period, update ct->state and ct->dynticks_nmi_nesting
 * to let the RCU grace-period handling know that the CPU is back to
 * being RCU-idle.
 *
 * If you add or remove a call to ct_nmi_exit(), be sure to test
 * with CONFIG_RCU_EQS_DEBUG=y.
 */
void noinstr ct_nmi_exit(void)
{
	struct context_tracking *ct = this_cpu_ptr(&context_tracking);

	instrumentation_begin();
	/*
	 * Check for ->dynticks_nmi_nesting underflow and bad ->dynticks.
	 * (We are exiting an NMI handler, so RCU better be paying attention
	 * to us!)
	 */
	WARN_ON_ONCE(ct_dynticks_nmi_nesting() <= 0);
	WARN_ON_ONCE(rcu_dynticks_curr_cpu_in_eqs());

	/*
	 * If the nesting level is not 1, the CPU wasn't RCU-idle, so
	 * leave it in non-RCU-idle state.
	 */
	if (ct_dynticks_nmi_nesting() != 1) {
		trace_rcu_dyntick(TPS("--="), ct_dynticks_nmi_nesting(), ct_dynticks_nmi_nesting() - 2,
				  ct_dynticks());
		WRITE_ONCE(ct->dynticks_nmi_nesting, /* No store tearing. */
			   ct_dynticks_nmi_nesting() - 2);
		instrumentation_end();
		return;
	}

	/* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */
	trace_rcu_dyntick(TPS("Startirq"), ct_dynticks_nmi_nesting(), 0, ct_dynticks());
	WRITE_ONCE(ct->dynticks_nmi_nesting, 0); /* Avoid store tearing. */

	// instrumentation for the noinstr ct_kernel_exit_state()
	instrument_atomic_write(&ct->state, sizeof(ct->state));
	instrumentation_end();

	// RCU is watching here ...
	ct_kernel_exit_state(RCU_DYNTICKS_IDX);
	// ... but is no longer watching here.

	if (!in_nmi())
		rcu_dynticks_task_enter();
}

/**
 * ct_nmi_enter - inform RCU of entry to NMI context
 *
 * If the CPU was idle from RCU's viewpoint, update ct->state and
 * ct->dynticks_nmi_nesting to let the RCU grace-period handling know
 * that the CPU is active.  This implementation permits nested NMIs, as
 * long as the nesting level does not overflow an int.  (You will probably
 * run out of stack space first.)
 *
 * If you add or remove a call to ct_nmi_enter(), be sure to test
 * with CONFIG_RCU_EQS_DEBUG=y.
 */
void noinstr ct_nmi_enter(void)
{
	long incby = 2;
	struct context_tracking *ct = this_cpu_ptr(&context_tracking);

	/* Complain about underflow. */
	WARN_ON_ONCE(ct_dynticks_nmi_nesting() < 0);

	/*
	 * If idle from RCU viewpoint, atomically increment ->dynticks
	 * to mark non-idle and increment ->dynticks_nmi_nesting by one.
	 * Otherwise, increment ->dynticks_nmi_nesting by two.  This means
	 * if ->dynticks_nmi_nesting is equal to one, we are guaranteed
	 * to be in the outermost NMI handler that interrupted an RCU-idle
	 * period (observation due to Andy Lutomirski).
	 */
	if (rcu_dynticks_curr_cpu_in_eqs()) {

		if (!in_nmi())
			rcu_dynticks_task_exit();

		// RCU is not watching here ...
		ct_kernel_enter_state(RCU_DYNTICKS_IDX);
		// ... but is watching here.

		instrumentation_begin();
		// instrumentation for the noinstr rcu_dynticks_curr_cpu_in_eqs()
		instrument_atomic_read(&ct->state, sizeof(ct->state));
		// instrumentation for the noinstr ct_kernel_enter_state()
		instrument_atomic_write(&ct->state, sizeof(ct->state));

		incby = 1;
	} else if (!in_nmi()) {
		instrumentation_begin();
		rcu_irq_enter_check_tick();
	} else  {
		instrumentation_begin();
	}

	trace_rcu_dyntick(incby == 1 ? TPS("Endirq") : TPS("++="),
			  ct_dynticks_nmi_nesting(),
			  ct_dynticks_nmi_nesting() + incby, ct_dynticks());
	instrumentation_end();
	WRITE_ONCE(ct->dynticks_nmi_nesting, /* Prevent store tearing. */
		   ct_dynticks_nmi_nesting() + incby);
	barrier();
}

/**
 * ct_idle_enter - inform RCU that current CPU is entering idle
 *
 * Enter idle mode, in other words, -leave- the mode in which RCU
 * read-side critical sections can occur.  (Though RCU read-side
 * critical sections can occur in irq handlers in idle, a possibility
 * handled by irq_enter() and irq_exit().)
 *
 * If you add or remove a call to ct_idle_enter(), be sure to test with
 * CONFIG_RCU_EQS_DEBUG=y.
 */
void noinstr ct_idle_enter(void)
{
	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !raw_irqs_disabled());
	ct_kernel_exit(false