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

   1// SPDX-License-Identifier: GPL-2.0
   2/*
   3 * Emulate a local clock event device via a pseudo clock device.
   4 */
   5#include <linux/cpu.h>
   6#include <linux/err.h>
   7#include <linux/hrtimer.h>
   8#include <linux/interrupt.h>
   9#include <linux/percpu.h>
  10#include <linux/profile.h>
  11#include <linux/clockchips.h>
  12#include <linux/sched.h>
  13#include <linux/smp.h>
  14#include <linux/module.h>
  15
  16#include "tick-internal.h"
  17
  18static struct hrtimer bctimer;
  19
  20static int bc_shutdown(struct clock_event_device *evt)
  21{
  22        /*
  23         * Note, we cannot cancel the timer here as we might
  24         * run into the following live lock scenario:
  25         *
  26         * cpu 0                cpu1
  27         * lock(broadcast_lock);
  28         *                      hrtimer_interrupt()
  29         *                      bc_handler()
  30         *                         tick_handle_oneshot_broadcast();
  31         *                          lock(broadcast_lock);
  32         * hrtimer_cancel()
  33         *  wait_for_callback()
  34         */
  35        hrtimer_try_to_cancel(&bctimer);
  36        return 0;
  37}
  38
  39/*
  40 * This is called from the guts of the broadcast code when the cpu
  41 * which is about to enter idle has the earliest broadcast timer event.
  42 */
  43static int bc_set_next(ktime_t expires, struct clock_event_device *bc)
  44{
  45        /*
  46         * This is called either from enter/exit idle code or from the
  47         * broadcast handler. In all cases tick_broadcast_lock is held.
  48         *
  49         * hrtimer_cancel() cannot be called here neither from the
  50         * broadcast handler nor from the enter/exit idle code. The idle
  51         * code can run into the problem described in bc_shutdown() and the
  52         * broadcast handler cannot wait for itself to complete for obvious
  53         * reasons.
  54         *
  55         * Each caller tries to arm the hrtimer on its own CPU, but if the
  56         * hrtimer callback function is currently running, then
  57         * hrtimer_start() cannot move it and the timer stays on the CPU on
  58         * which it is assigned at the moment.
  59         *
  60         * As this can be called from idle code, the hrtimer_start()
  61         * invocation has to be wrapped with RCU_NONIDLE() as
  62         * hrtimer_start() can call into tracing.
  63         */
  64        RCU_NONIDLE( {
  65                hrtimer_start(&bctimer, expires, HRTIMER_MODE_ABS_PINNED_HARD);
  66                /*
  67                 * The core tick broadcast mode expects bc->bound_on to be set
  68                 * correctly to prevent a CPU which has the broadcast hrtimer
  69                 * armed from going deep idle.
  70                 *
  71                 * As tick_broadcast_lock is held, nothing can change the cpu
  72                 * base which was just established in hrtimer_start() above. So
  73                 * the below access is safe even without holding the hrtimer
  74                 * base lock.
  75                 */
  76                bc->bound_on = bctimer.base->cpu_base->cpu;
  77        } );
  78        return 0;
  79}
  80
  81static struct clock_event_device ce_broadcast_hrtimer = {
  82        .name                   = "bc_hrtimer",
  83        .set_state_shutdown     = bc_shutdown,
  84        .set_next_ktime         = bc_set_next,
  85        .features               = CLOCK_EVT_FEAT_ONESHOT |
  86                                  CLOCK_EVT_FEAT_KTIME |
  87                                  CLOCK_EVT_FEAT_HRTIMER,
  88        .rating                 = 0,
  89        .bound_on               = -1,
  90        .min_delta_ns           = 1,
  91        .max_delta_ns           = KTIME_MAX,
  92        .min_delta_ticks        = 1,
  93        .max_delta_ticks        = ULONG_MAX,
  94        .mult                   = 1,
  95        .shift                  = 0,
  96        .cpumask                = cpu_possible_mask,
  97};
  98
  99static enum hrtimer_restart bc_handler(struct hrtimer *t)
 100{
 101        ce_broadcast_hrtimer.event_handler(&ce_broadcast_hrtimer);
 102
 103        return HRTIMER_NORESTART;
 104}
 105
 106void tick_setup_hrtimer_broadcast(void)
 107{
 108        hrtimer_init(&bctimer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_HARD);
 109        bctimer.function = bc_handler;
 110        clockevents_register_device(&ce_broadcast_hrtimer);
 111}
 112