qemu/block/plug.c

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/* SPDX-License-Identifier: GPL-2.0-or-later */
/*
* Block I/O plugging
*
* Copyright Red Hat.
*
* This API defers a function call within a blk_io_plug()/blk_io_unplug()
* section, allowing multiple calls to batch up. This is a performance
* optimization that is used in the block layer to submit several I/O requests
* at once instead of individually:
*
* blk_io_plug(); <-- start of plugged region
* ...
* blk_io_plug_call(my_func, my_obj); <-- deferred my_func(my_obj) call
* blk_io_plug_call(my_func, my_obj); <-- another
* blk_io_plug_call(my_func, my_obj); <-- another
* ...
* blk_io_unplug(); <-- end of plugged region, my_func(my_obj) is called once
*
* This code is actually generic and not tied to the block layer. If another
* subsystem needs this functionality, it could be renamed.
*/
#include "qemu/osdep.h"
#include "qemu/coroutine-tls.h"
#include "qemu/notify.h"
#include "qemu/thread.h"
#include "sysemu/block-backend.h"
/* A function call that has been deferred until unplug() */
typedef struct {
void (*fn)(void *);
void *opaque;
} UnplugFn;
/* Per-thread state */
typedef struct {
unsigned count; /* how many times has plug() been called? */
GArray *unplug_fns; /* functions to call at unplug time */
} Plug;
/* Use get_ptr_plug() to fetch this thread-local value */
QEMU_DEFINE_STATIC_CO_TLS(Plug, plug);
/* Called at thread cleanup time */
static void blk_io_plug_atexit(Notifier *n, void *value)
{
Plug *plug = get_ptr_plug();
g_array_free(plug->unplug_fns, TRUE);
}
/* This won't involve coroutines, so use __thread */
static __thread Notifier blk_io_plug_atexit_notifier;
/**
* blk_io_plug_call:
* @fn: a function pointer to be invoked
* @opaque: a user-defined argument to @fn()
*
* Call @fn(@opaque) immediately if not within a blk_io_plug()/blk_io_unplug()
* section.
*
* Otherwise defer the call until the end of the outermost
* blk_io_plug()/blk_io_unplug() section in this thread. If the same
* @fn/@opaque pair has already been deferred, it will only be called once upon
* blk_io_unplug() so that accumulated calls are batched into a single call.
*
* The caller must ensure that @opaque is not freed before @fn() is invoked.
*/
void blk_io_plug_call(void (*fn)(void *), void *opaque)
{
Plug *plug = get_ptr_plug();
/* Call immediately if we're not plugged */
if (plug->count == 0) {
fn(opaque);
return;
}
GArray *array = plug->unplug_fns;
if (!array) {
array = g_array_new(FALSE, FALSE, sizeof(UnplugFn));
plug->unplug_fns = array;
blk_io_plug_atexit_notifier.notify = blk_io_plug_atexit;
qemu_thread_atexit_add(&blk_io_plug_atexit_notifier);
}
UnplugFn *fns = (UnplugFn *)array->data;
UnplugFn new_fn = {
.fn = fn,
.opaque = opaque,
};
/*
* There won't be many, so do a linear search. If this becomes a bottleneck
* then a binary search (glib 2.62+) or different data structure could be
* used.
*/
for (guint i = 0; i < array->len; i++) {
if (memcmp(&fns[i], &new_fn, sizeof(new_fn)) == 0) {
return; /* already exists */
}
}
g_array_append_val(array, new_fn);
}
/**
* blk_io_plug: Defer blk_io_plug_call() functions until blk_io_unplug()
*
* blk_io_plug/unplug are thread-local operations. This means that multiple
* threads can simultaneously call plug/unplug, but the caller must ensure that
* each unplug() is called in the same thread of the matching plug().
*
* Nesting is supported. blk_io_plug_call() functions are only called at the
* outermost blk_io_unplug().
*/
void blk_io_plug(void)
{
Plug *plug = get_ptr_plug();
assert(plug->count < UINT32_MAX);
plug->count++;
}
/**
* blk_io_unplug: Run any pending blk_io_plug_call() functions
*
* There must have been a matching blk_io_plug() call in the same thread prior
* to this blk_io_unplug() call.
*/
void blk_io_unplug(void)
{
Plug *plug = get_ptr_plug();
assert(plug->count > 0);
if (--plug->count > 0) {
return;
}
GArray *array = plug->unplug_fns;
if (!array) {
return;
}
UnplugFn *fns = (UnplugFn *)array->data;
for (guint i = 0; i < array->len; i++) {
fns[i].fn(fns[i].opaque);
}
/*
* This resets the array without freeing memory so that appending is cheap
* in the future.
*/
g_array_set_size(array, 0);
}