kernel/time/tick-broadcast-hrtimer.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * linux/kernel/time/tick-broadcast-hrtimer.c
   4  * This file emulates a local clock event device
   5  * via a pseudo clock device.
   6  */
   7 #include <linux/cpu.h>
   8 #include <linux/err.h>
   9 #include <linux/hrtimer.h>
  10 #include <linux/interrupt.h>
  11 #include <linux/percpu.h>
  12 #include <linux/profile.h>
  13 #include <linux/clockchips.h>
  14 #include <linux/sched.h>
  15 #include <linux/smp.h>
  16 #include <linux/module.h>
  17
  18 #include "tick-internal.h"
  19
  20 static struct hrtimer bctimer;
  21
  22 static int bc_shutdown(struct clock_event_device *evt)
  23 {
  24         /*
  25          * Note, we cannot cancel the timer here as we might
  26          * run into the following live lock scenario:
  27          *
  28          * cpu 0                cpu1
  29          * lock(broadcast_lock);
  30          *                      hrtimer_interrupt()
  31          *                      bc_handler()
  32          *                         tick_handle_oneshot_broadcast();
  33          *                          lock(broadcast_lock);
  34          * hrtimer_cancel()
  35          *  wait_for_callback()
  36          */
  37         hrtimer_try_to_cancel(&bctimer);
  38         return 0;
  39 }
  40
  41 /*
  42  * This is called from the guts of the broadcast code when the cpu
  43  * which is about to enter idle has the earliest broadcast timer event.
  44  */
  45 static int bc_set_next(ktime_t expires, struct clock_event_device *bc)
  46 {
  47         /*
  48          * This is called either from enter/exit idle code or from the
  49          * broadcast handler. In all cases tick_broadcast_lock is held.
  50          *
  51          * hrtimer_cancel() cannot be called here neither from the
  52          * broadcast handler nor from the enter/exit idle code. The idle
  53          * code can run into the problem described in bc_shutdown() and the
  54          * broadcast handler cannot wait for itself to complete for obvious
  55          * reasons.
  56          *
  57          * Each caller tries to arm the hrtimer on its own CPU, but if the
  58          * hrtimer callbback function is currently running, then
  59          * hrtimer_start() cannot move it and the timer stays on the CPU on
  60          * which it is assigned at the moment.
  61          *
  62          * As this can be called from idle code, the hrtimer_start()
  63          * invocation has to be wrapped with RCU_NONIDLE() as
  64          * hrtimer_start() can call into tracing.
  65          */
  66         RCU_NONIDLE( {
  67                 hrtimer_start(&bctimer, expires, HRTIMER_MODE_ABS_PINNED);
  68                 /*
  69                  * The core tick broadcast mode expects bc->bound_on to be set
  70                  * correctly to prevent a CPU which has the broadcast hrtimer
  71                  * armed from going deep idle.
  72                  *
  73                  * As tick_broadcast_lock is held, nothing can change the cpu
  74                  * base which was just established in hrtimer_start() above. So
  75                  * the below access is safe even without holding the hrtimer
  76                  * base lock.
  77                  */
  78                 bc->bound_on = bctimer.base->cpu_base->cpu;
  79         } );
  80         return 0;
  81 }
  82
  83 static struct clock_event_device ce_broadcast_hrtimer = {
  84         .name                   = "bc_hrtimer",
  85         .set_state_shutdown     = bc_shutdown,
  86         .set_next_ktime         = bc_set_next,
  87         .features               = CLOCK_EVT_FEAT_ONESHOT |
  88                                   CLOCK_EVT_FEAT_KTIME |
  89                                   CLOCK_EVT_FEAT_HRTIMER,
  90         .rating                 = 0,
  91         .bound_on               = -1,
  92         .min_delta_ns           = 1,
  93         .max_delta_ns           = KTIME_MAX,
  94         .min_delta_ticks        = 1,
  95         .max_delta_ticks        = ULONG_MAX,
  96         .mult                   = 1,
  97         .shift                  = 0,
  98         .cpumask                = cpu_all_mask,
  99 };
 100
 101 static enum hrtimer_restart bc_handler(struct hrtimer *t)
 102 {
 103         ce_broadcast_hrtimer.event_handler(&ce_broadcast_hrtimer);
 104
 105         return HRTIMER_NORESTART;
 106 }
 107
 108 void tick_setup_hrtimer_broadcast(void)
 109 {
 110         hrtimer_init(&bctimer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
 111         bctimer.function = bc_handler;
 112         clockevents_register_device(&ce_broadcast_hrtimer);
 113 }