util/defer-call.c

   1 /* SPDX-License-Identifier: GPL-2.0-or-later */
   2 /*
   3  * Deferred calls
   4  *
   5  * Copyright Red Hat.
   6  *
   7  * This API defers a function call within a defer_call_begin()/defer_call_end()
   8  * section, allowing multiple calls to batch up. This is a performance
   9  * optimization that is used in the block layer to submit several I/O requests
  10  * at once instead of individually:
  11  *
  12  *   defer_call_begin(); <-- start of section
  13  *   ...
  14  *   defer_call(my_func, my_obj); <-- deferred my_func(my_obj) call
  15  *   defer_call(my_func, my_obj); <-- another
  16  *   defer_call(my_func, my_obj); <-- another
  17  *   ...
  18  *   defer_call_end(); <-- end of section, my_func(my_obj) is called once
  19  */
  20
  21 #include "qemu/osdep.h"
  22 #include "qemu/coroutine-tls.h"
  23 #include "qemu/notify.h"
  24 #include "qemu/thread.h"
  25 #include "qemu/defer-call.h"
  26
  27 /* A function call that has been deferred until defer_call_end() */
  28 typedef struct {
  29     void (*fn)(void *);
  30     void *opaque;
  31 } DeferredCall;
  32
  33 /* Per-thread state */
  34 typedef struct {
  35     unsigned nesting_level;
  36     GArray *deferred_call_array;
  37 } DeferCallThreadState;
  38
  39 /* Use get_ptr_defer_call_thread_state() to fetch this thread-local value */
  40 QEMU_DEFINE_STATIC_CO_TLS(DeferCallThreadState, defer_call_thread_state);
  41
  42 /* Called at thread cleanup time */
  43 static void defer_call_atexit(Notifier *n, void *value)
  44 {
  45     DeferCallThreadState *thread_state = get_ptr_defer_call_thread_state();
  46     g_array_free(thread_state->deferred_call_array, TRUE);
  47 }
  48
  49 /* This won't involve coroutines, so use __thread */
  50 static __thread Notifier defer_call_atexit_notifier;
  51
  52 /**
  53  * defer_call:
  54  * @fn: a function pointer to be invoked
  55  * @opaque: a user-defined argument to @fn()
  56  *
  57  * Call @fn(@opaque) immediately if not within a
  58  * defer_call_begin()/defer_call_end() section.
  59  *
  60  * Otherwise defer the call until the end of the outermost
  61  * defer_call_begin()/defer_call_end() section in this thread. If the same
  62  * @fn/@opaque pair has already been deferred, it will only be called once upon
  63  * defer_call_end() so that accumulated calls are batched into a single call.
  64  *
  65  * The caller must ensure that @opaque is not freed before @fn() is invoked.
  66  */
  67 void defer_call(void (*fn)(void *), void *opaque)
  68 {
  69     DeferCallThreadState *thread_state = get_ptr_defer_call_thread_state();
  70
  71     /* Call immediately if we're not deferring calls */
  72     if (thread_state->nesting_level == 0) {
  73         fn(opaque);
  74         return;
  75     }
  76
  77     GArray *array = thread_state->deferred_call_array;
  78     if (!array) {
  79         array = g_array_new(FALSE, FALSE, sizeof(DeferredCall));
  80         thread_state->deferred_call_array = array;
  81         defer_call_atexit_notifier.notify = defer_call_atexit;
  82         qemu_thread_atexit_add(&defer_call_atexit_notifier);
  83     }
  84
  85     DeferredCall *fns = (DeferredCall *)array->data;
  86     DeferredCall new_fn = {
  87         .fn = fn,
  88         .opaque = opaque,
  89     };
  90
  91     /*
  92      * There won't be many, so do a linear search. If this becomes a bottleneck
  93      * then a binary search (glib 2.62+) or different data structure could be
  94      * used.
  95      */
  96     for (guint i = 0; i < array->len; i++) {
  97         if (memcmp(&fns[i], &new_fn, sizeof(new_fn)) == 0) {
  98             return; /* already exists */
  99         }
 100     }
 101
 102     g_array_append_val(array, new_fn);
 103 }
 104
 105 /**
 106  * defer_call_begin: Defer defer_call() functions until defer_call_end()
 107  *
 108  * defer_call_begin() and defer_call_end() are thread-local operations. The
 109  * caller must ensure that each defer_call_begin() has a matching
 110  * defer_call_end() in the same thread.
 111  *
 112  * Nesting is supported. defer_call() functions are only called at the
 113  * outermost defer_call_end().
 114  */
 115 void defer_call_begin(void)
 116 {
 117     DeferCallThreadState *thread_state = get_ptr_defer_call_thread_state();
 118
 119     assert(thread_state->nesting_level < UINT32_MAX);
 120
 121     thread_state->nesting_level++;
 122 }
 123
 124 /**
 125  * defer_call_end: Run any pending defer_call() functions
 126  *
 127  * There must have been a matching defer_call_begin() call in the same thread
 128  * prior to this defer_call_end() call.
 129  */
 130 void defer_call_end(void)
 131 {
 132     DeferCallThreadState *thread_state = get_ptr_defer_call_thread_state();
 133
 134     assert(thread_state->nesting_level > 0);
 135
 136     if (--thread_state->nesting_level > 0) {
 137         return;
 138     }
 139
 140     GArray *array = thread_state->deferred_call_array;
 141     if (!array) {
 142         return;
 143     }
 144
 145     DeferredCall *fns = (DeferredCall *)array->data;
 146
 147     for (guint i = 0; i < array->len; i++) {
 148         fns[i].fn(fns[i].opaque);
 149     }
 150
 151     /*
 152      * This resets the array without freeing memory so that appending is cheap
 153      * in the future.
 154      */
 155     g_array_set_size(array, 0);
 156 }