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
  18 static struct hrtimer bctimer;
  19
  20 static 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  */
  43 static 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 callbback 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
  81 static 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
  99 static 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
 106 void 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 }