linux-next/kernel/time/tick-broadcast-hrtimer.c

// SPDX-License-Identifier: GPL-2.0
/*
 * Emulate a local clock event device via a pseudo clock device.
 */
#include <linux/cpu.h>
#include <linux/err.h>
#include <linux/hrtimer.h>
#include <linux/interrupt.h>
#include <linux/percpu.h>
#include <linux/profile.h>
#include <linux/clockchips.h>
#include <linux/sched.h>
#include <linux/smp.h>
#include <linux/module.h>

#include "tick-internal.h"

static struct hrtimer bctimer;

static int bc_shutdown(struct clock_event_device *evt)
{
	/*
	 * Note, we cannot cancel the timer here as we might
	 * run into the following live lock scenario:
	 *
	 * cpu 0		cpu1
	 * lock(broadcast_lock);
	 *			hrtimer_interrupt()
	 *			bc_handler()
	 *			   tick_handle_oneshot_broadcast();
	 *			    lock(broadcast_lock);
	 * hrtimer_cancel()
	 *  wait_for_callback()
	 */
	hrtimer_try_to_cancel(&bctimer);
	return 0;
}

/*
 * This is called from the guts of the broadcast code when the cpu
 * which is about to enter idle has the earliest broadcast timer event.
 */
static int bc_set_next(ktime_t expires, struct clock_event_device *bc)
{
	/*
	 * This is called either from enter/exit idle code or from the
	 * broadcast handler. In all cases tick_broadcast_lock is held.
	 *
	 * hrtimer_cancel() cannot be called here neither from the
	 * broadcast handler nor from the enter/exit idle code. The idle
	 * code can run into the problem described in bc_shutdown() and the
	 * broadcast handler cannot wait for itself to complete for obvious
	 * reasons.
	 *
	 * Each caller tries to arm the hrtimer on its own CPU, but if the
	 * hrtimer callback function is currently running, then
	 * hrtimer_start() cannot move it and the timer stays on the CPU on
	 * which it is assigned at the moment.
	 *
	 * As this can be called from idle code, the hrtimer_start()
	 * invocation has to be wrapped with RCU_NONIDLE() as
	 * hrtimer_start() can call into tracing.
	 */
	RCU_NONIDLE( {
		hrtimer_start(&bctimer, expires, HRTIMER_MODE_ABS_PINNED_HARD);
		/*
		 * The core tick broadcast mode expects bc->bound_on to be set
		 * correctly to prevent a CPU which has the broadcast hrtimer
		 * armed from going deep idle.
		 *
		 * As tick_broadcast_lock is held, nothing can change the cpu
		 * base which was just established in hrtimer_start() above. So
		 * the below access is safe even without holding the hrtimer
		 * base lock.
		 */
		bc->bound_on = bctimer.base->cpu_base->cpu;
	} );
	return 0;
}

static struct clock_event_device ce_broadcast_hrtimer = {
	.name			= "bc_hrtimer",
	.set_state_shutdown	= bc_shutdown,
	.set_next_ktime		= bc_set_next,
	.features		= CLOCK_EVT_FEAT_ONESHOT |
				  CLOCK_EVT_FEAT_KTIME |
				  CLOCK_EVT_FEAT_HRTIMER,
	.rating			= 0,
	.bound_on		= -1,
	.min_delta_ns		= 1,
	.max_delta_ns		= KTIME_MAX,
	.min_delta_ticks	= 1,
	.max_delta_ticks	= ULONG_MAX,
	.mult			= 1,
	.shift			= 0,
	.cpumask		= cpu_possible_mask,
};

static enum hrtimer_restart bc_handler(struct hrtimer *t)
{
	ce_broadcast_hrtimer.event_handler(&ce_broadcast_hrtimer);

	return HRTIMER_NORESTART;
}

void tick_setup_hrtimer_broadcast(void)
{
	hrtimer_init(&bctimer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_HARD);
	bctimer.function = bc_handler;
	clockevents_register_device(&ce_broadcast_hrtimer);
}