block/throttle-groups.c - qemu - Git at Google

 /*
  * QEMU block throttling group infrastructure
  *
  * Copyright (C) Nodalink, EURL. 2014
  * Copyright (C) Igalia, S.L. 2015
  *
  * Authors:
  *   Benoît Canet <benoit.canet@nodalink.com>
  *   Alberto Garcia <berto@igalia.com>
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License as
  * published by the Free Software Foundation; either version 2 or
  * (at your option) version 3 of the License.
  *
  * This program is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
  * along with this program; if not, see <http://www.gnu.org/licenses/>.
  */

 #include "qemu/osdep.h"
 #include "block/throttle-groups.h"
 #include "qemu/queue.h"
 #include "qemu/thread.h"
 #include "sysemu/qtest.h"

 /* The ThrottleGroup structure (with its ThrottleState) is shared
  * among different BlockDriverState and it's independent from
  * AioContext, so in order to use it from different threads it needs
  * its own locking.
  *
  * This locking is however handled internally in this file, so it's
  * transparent to outside users.
  *
  * The whole ThrottleGroup structure is private and invisible to
  * outside users, that only use it through its ThrottleState.
  *
  * In addition to the ThrottleGroup structure, BlockDriverState has
  * fields that need to be accessed by other members of the group and
  * therefore also need to be protected by this lock. Once a BDS is
  * registered in a group those fields can be accessed by other threads
  * any time.
  *
  * Again, all this is handled internally and is mostly transparent to
  * the outside. The 'throttle_timers' field however has an additional
  * constraint because it may be temporarily invalid (see for example
  * bdrv_set_aio_context()). Therefore in this file a thread will
  * access some other BDS's timers only after verifying that that BDS
  * has throttled requests in the queue.
  */
 typedef struct ThrottleGroup {
     char *name; /* This is constant during the lifetime of the group */

     QemuMutex lock; /* This lock protects the following four fields */
     ThrottleState ts;
     QLIST_HEAD(, BlockDriverState) head;
     BlockDriverState *tokens[2];
     bool any_timer_armed[2];

     /* These two are protected by the global throttle_groups_lock */
     unsigned refcount;
     QTAILQ_ENTRY(ThrottleGroup) list;
 } ThrottleGroup;

 static QemuMutex throttle_groups_lock;
 static QTAILQ_HEAD(, ThrottleGroup) throttle_groups =
     QTAILQ_HEAD_INITIALIZER(throttle_groups);

 /* Increments the reference count of a ThrottleGroup given its name.
  *
  * If no ThrottleGroup is found with the given name a new one is
  * created.
  *
  * @name: the name of the ThrottleGroup
  * @ret:  the ThrottleState member of the ThrottleGroup
  */
 ThrottleState *throttle_group_incref(const char *name)
 {
     ThrottleGroup *tg = NULL;
     ThrottleGroup *iter;

     qemu_mutex_lock(&throttle_groups_lock);

     /* Look for an existing group with that name */
     QTAILQ_FOREACH(iter, &throttle_groups, list) {
         if (!strcmp(name, iter->name)) {
             tg = iter;
             break;
         }
     }

     /* Create a new one if not found */
     if (!tg) {
         tg = g_new0(ThrottleGroup, 1);
         tg->name = g_strdup(name);
         qemu_mutex_init(&tg->lock);
         throttle_init(&tg->ts);
         QLIST_INIT(&tg->head);

         QTAILQ_INSERT_TAIL(&throttle_groups, tg, list);
     }

     tg->refcount++;

     qemu_mutex_unlock(&throttle_groups_lock);

     return &tg->ts;
 }

 /* Decrease the reference count of a ThrottleGroup.
  *
  * When the reference count reaches zero the ThrottleGroup is
  * destroyed.
  *
  * @ts:  The ThrottleGroup to unref, given by its ThrottleState member
  */
 void throttle_group_unref(ThrottleState *ts)
 {
     ThrottleGroup *tg = container_of(ts, ThrottleGroup, ts);

     qemu_mutex_lock(&throttle_groups_lock);
     if (--tg->refcount == 0) {
         QTAILQ_REMOVE(&throttle_groups, tg, list);
         qemu_mutex_destroy(&tg->lock);
         g_free(tg->name);
         g_free(tg);
     }
     qemu_mutex_unlock(&throttle_groups_lock);
 }

 /* Get the name from a BlockDriverState's ThrottleGroup. The name (and
  * the pointer) is guaranteed to remain constant during the lifetime
  * of the group.
  *
  * @bs:   a BlockDriverState that is member of a throttling group
  * @ret:  the name of the group.
  */
 const char *throttle_group_get_name(BlockDriverState *bs)
 {
     ThrottleGroup *tg = container_of(bs->throttle_state, ThrottleGroup, ts);
     return tg->name;
 }

 /* Return the next BlockDriverState in the round-robin sequence,
  * simulating a circular list.
  *
  * This assumes that tg->lock is held.
  *
  * @bs:  the current BlockDriverState
  * @ret: the next BlockDriverState in the sequence
  */
 static BlockDriverState *throttle_group_next_bs(BlockDriverState *bs)
 {
     ThrottleState *ts = bs->throttle_state;
     ThrottleGroup *tg = container_of(ts, ThrottleGroup, ts);
     BlockDriverState *next = QLIST_NEXT(bs, round_robin);

     if (!next) {
         return QLIST_FIRST(&tg->head);
     }

     return next;
 }

 /* Return the next BlockDriverState in the round-robin sequence with
  * pending I/O requests.
  *
  * This assumes that tg->lock is held.
  *
  * @bs:        the current BlockDriverState
  * @is_write:  the type of operation (read/write)
  * @ret:       the next BlockDriverState with pending requests, or bs
  *             if there is none.
  */
 static BlockDriverState *next_throttle_token(BlockDriverState *bs,
                                              bool is_write)
 {
     ThrottleGroup *tg = container_of(bs->throttle_state, ThrottleGroup, ts);
     BlockDriverState *token, *start;

     start = token = tg->tokens[is_write];

     /* get next bs round in round robin style */
     token = throttle_group_next_bs(token);
     while (token != start && !token->pending_reqs[is_write]) {
         token = throttle_group_next_bs(token);
     }

     /* If no IO are queued for scheduling on the next round robin token
      * then decide the token is the current bs because chances are
      * the current bs get the current request queued.
      */
     if (token == start && !token->pending_reqs[is_write]) {
         token = bs;
     }

     return token;
 }

 /* Check if the next I/O request for a BlockDriverState needs to be
  * throttled or not. If there's no timer set in this group, set one
  * and update the token accordingly.
  *
  * This assumes that tg->lock is held.
  *
  * @bs:         the current BlockDriverState
  * @is_write:   the type of operation (read/write)
  * @ret:        whether the I/O request needs to be throttled or not
  */
 static bool throttle_group_schedule_timer(BlockDriverState *bs,
                                           bool is_write)
 {
     ThrottleState *ts = bs->throttle_state;
     ThrottleTimers *tt = &bs->throttle_timers;
     ThrottleGroup *tg = container_of(ts, ThrottleGroup, ts);
     bool must_wait;

     /* Check if any of the timers in this group is already armed */
     if (tg->any_timer_armed[is_write]) {
         return true;
     }

     must_wait = throttle_schedule_timer(ts, tt, is_write);

     /* If a timer just got armed, set bs as the current token */
     if (must_wait) {
         tg->tokens[is_write] = bs;
         tg->any_timer_armed[is_write] = true;
     }

     return must_wait;
 }

 /* Look for the next pending I/O request and schedule it.
  *
  * This assumes that tg->lock is held.
  *
  * @bs:        the current BlockDriverState
  * @is_write:  the type of operation (read/write)
  */
 static void schedule_next_request(BlockDriverState *bs, bool is_write)
 {
     ThrottleGroup *tg = container_of(bs->throttle_state, ThrottleGroup, ts);
     bool must_wait;
     BlockDriverState *token;

     /* Check if there's any pending request to schedule next */
     token = next_throttle_token(bs, is_write);
     if (!token->pending_reqs[is_write]) {
         return;
     }

     /* Set a timer for the request if it needs to be throttled */
     must_wait = throttle_group_schedule_timer(token, is_write);

     /* If it doesn't have to wait, queue it for immediate execution */
     if (!must_wait) {
         /* Give preference to requests from the current bs */
         if (qemu_in_coroutine() &&
             qemu_co_queue_next(&bs->throttled_reqs[is_write])) {
             token = bs;
         } else {
             ThrottleTimers *tt = &token->throttle_timers;
             int64_t now = qemu_clock_get_ns(tt->clock_type);
             timer_mod(tt->timers[is_write], now + 1);
             tg->any_timer_armed[is_write] = true;
         }
         tg->tokens[is_write] = token;
     }
 }

 /* Check if an I/O request needs to be throttled, wait and set a timer
  * if necessary, and schedule the next request using a round robin
  * algorithm.
  *
  * @bs:        the current BlockDriverState
  * @bytes:     the number of bytes for this I/O
  * @is_write:  the type of operation (read/write)
  */
 void coroutine_fn throttle_group_co_io_limits_intercept(BlockDriverState *bs,
                                                         unsigned int bytes,
                                                         bool is_write)
 {
     bool must_wait;
     BlockDriverState *token;

     ThrottleGroup *tg = container_of(bs->throttle_state, ThrottleGroup, ts);
     qemu_mutex_lock(&tg->lock);

     /* First we check if this I/O has to be throttled. */
     token = next_throttle_token(bs, is_write);
     must_wait = throttle_group_schedule_timer(token, is_write);

     /* Wait if there's a timer set or queued requests of this type */
     if (must_wait || bs->pending_reqs[is_write]) {
         bs->pending_reqs[is_write]++;
         qemu_mutex_unlock(&tg->lock);
         qemu_co_queue_wait(&bs->throttled_reqs[is_write]);
         qemu_mutex_lock(&tg->lock);
         bs->pending_reqs[is_write]--;
     }

     /* The I/O will be executed, so do the accounting */
     throttle_account(bs->throttle_state, is_write, bytes);

     /* Schedule the next request */
     schedule_next_request(bs, is_write);

     qemu_mutex_unlock(&tg->lock);
 }

 /* Update the throttle configuration for a particular group. Similar
  * to throttle_config(), but guarantees atomicity within the
  * throttling group.
  *
  * @bs:  a BlockDriverState that is member of the group
  * @cfg: the configuration to set
  */
 void throttle_group_config(BlockDriverState *bs, ThrottleConfig *cfg)
 {
     ThrottleTimers *tt = &bs->throttle_timers;
     ThrottleState *ts = bs->throttle_state;
     ThrottleGroup *tg = container_of(ts, ThrottleGroup, ts);
     qemu_mutex_lock(&tg->lock);
     /* throttle_config() cancels the timers */
     if (timer_pending(tt->timers[0])) {
         tg->any_timer_armed[0] = false;
     }
     if (timer_pending(tt->timers[1])) {
         tg->any_timer_armed[1] = false;
     }
     throttle_config(ts, tt, cfg);
     qemu_mutex_unlock(&tg->lock);
 }

 /* Get the throttle configuration from a particular group. Similar to
  * throttle_get_config(), but guarantees atomicity within the
  * throttling group.
  *
  * @bs:  a BlockDriverState that is member of the group
  * @cfg: the configuration will be written here
  */
 void throttle_group_get_config(BlockDriverState *bs, ThrottleConfig *cfg)
 {
     ThrottleState *ts = bs->throttle_state;
     ThrottleGroup *tg = container_of(ts, ThrottleGroup, ts);
     qemu_mutex_lock(&tg->lock);
     throttle_get_config(ts, cfg);
     qemu_mutex_unlock(&tg->lock);
 }

 /* ThrottleTimers callback. This wakes up a request that was waiting
  * because it had been throttled.
  *
  * @bs:        the BlockDriverState whose request had been throttled
  * @is_write:  the type of operation (read/write)
  */
 static void timer_cb(BlockDriverState *bs, bool is_write)
 {
     ThrottleState *ts = bs->throttle_state;
     ThrottleGroup *tg = container_of(ts, ThrottleGroup, ts);
     bool empty_queue;

     /* The timer has just been fired, so we can update the flag */
     qemu_mutex_lock(&tg->lock);
     tg->any_timer_armed[is_write] = false;
     qemu_mutex_unlock(&tg->lock);

     /* Run the request that was waiting for this timer */
     empty_queue = !qemu_co_enter_next(&bs->throttled_reqs[is_write]);

     /* If the request queue was empty then we have to take care of
      * scheduling the next one */
     if (empty_queue) {
         qemu_mutex_lock(&tg->lock);
         schedule_next_request(bs, is_write);
         qemu_mutex_unlock(&tg->lock);
     }
 }

 static void read_timer_cb(void *opaque)
 {
     timer_cb(opaque, false);
 }

 static void write_timer_cb(void *opaque)
 {
     timer_cb(opaque, true);
 }

 /* Register a BlockDriverState in the throttling group, also
  * initializing its timers and updating its throttle_state pointer to
  * point to it. If a throttling group with that name does not exist
  * yet, it will be created.
  *
  * @bs:        the BlockDriverState to insert
  * @groupname: the name of the group
  */
 void throttle_group_register_bs(BlockDriverState *bs, const char *groupname)
 {
     int i;
     ThrottleState *ts = throttle_group_incref(groupname);
     ThrottleGroup *tg = container_of(ts, ThrottleGroup, ts);
     int clock_type = QEMU_CLOCK_REALTIME;

     if (qtest_enabled()) {
         /* For testing block IO throttling only */
         clock_type = QEMU_CLOCK_VIRTUAL;
     }

     bs->throttle_state = ts;

     qemu_mutex_lock(&tg->lock);
     /* If the ThrottleGroup is new set this BlockDriverState as the token */
     for (i = 0; i < 2; i++) {
         if (!tg->tokens[i]) {
             tg->tokens[i] = bs;
         }
     }

     QLIST_INSERT_HEAD(&tg->head, bs, round_robin);

     throttle_timers_init(&bs->throttle_timers,
                          bdrv_get_aio_context(bs),
                          clock_type,
                          read_timer_cb,
                          write_timer_cb,
                          bs);

     qemu_mutex_unlock(&tg->lock);
 }

 /* Unregister a BlockDriverState from its group, removing it from the
  * list, destroying the timers and setting the throttle_state pointer
  * to NULL.
  *
  * The BlockDriverState must not have pending throttled requests, so
  * the caller has to drain them first.
  *
  * The group will be destroyed if it's empty after this operation.
  *
  * @bs: the BlockDriverState to remove
  */
 void throttle_group_unregister_bs(BlockDriverState *bs)
 {
     ThrottleGroup *tg = container_of(bs->throttle_state, ThrottleGroup, ts);
     int i;

     assert(bs->pending_reqs[0] == 0 && bs->pending_reqs[1] == 0);
     assert(qemu_co_queue_empty(&bs->throttled_reqs[0]));
     assert(qemu_co_queue_empty(&bs->throttled_reqs[1]));

     qemu_mutex_lock(&tg->lock);
     for (i = 0; i < 2; i++) {
         if (tg->tokens[i] == bs) {
             BlockDriverState *token = throttle_group_next_bs(bs);
             /* Take care of the case where this is the last bs in the group */
             if (token == bs) {
                 token = NULL;
             }
             tg->tokens[i] = token;
         }
     }

     /* remove the current bs from the list */
     QLIST_REMOVE(bs, round_robin);
     throttle_timers_destroy(&bs->throttle_timers);
     qemu_mutex_unlock(&tg->lock);

     throttle_group_unref(&tg->ts);
     bs->throttle_state = NULL;
 }

 static void throttle_groups_init(void)
 {
     qemu_mutex_init(&throttle_groups_lock);
 }

 block_init(throttle_groups_init);
	/*
	* QEMU block throttling group infrastructure
	*
	* Copyright (C) Nodalink, EURL. 2014
	* Copyright (C) Igalia, S.L. 2015
	*
	* Authors:
	* Benoît Canet <benoit.canet@nodalink.com>
	* Alberto Garcia <berto@igalia.com>
	*
	* This program is free software; you can redistribute it and/or
	* modify it under the terms of the GNU General Public License as
	* published by the Free Software Foundation; either version 2 or
	* (at your option) version 3 of the License.
	*
	* This program is distributed in the hope that it will be useful,
	* but WITHOUT ANY WARRANTY; without even the implied warranty of
	* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
	* GNU General Public License for more details.
	*
	* You should have received a copy of the GNU General Public License
	* along with this program; if not, see <http://www.gnu.org/licenses/>.
	*/

	#include "qemu/osdep.h"
	#include "block/throttle-groups.h"
	#include "qemu/queue.h"
	#include "qemu/thread.h"
	#include "sysemu/qtest.h"

	/* The ThrottleGroup structure (with its ThrottleState) is shared
	* among different BlockDriverState and it's independent from
	* AioContext, so in order to use it from different threads it needs
	* its own locking.
	*
	* This locking is however handled internally in this file, so it's
	* transparent to outside users.
	*
	* The whole ThrottleGroup structure is private and invisible to
	* outside users, that only use it through its ThrottleState.
	*
	* In addition to the ThrottleGroup structure, BlockDriverState has
	* fields that need to be accessed by other members of the group and
	* therefore also need to be protected by this lock. Once a BDS is
	* registered in a group those fields can be accessed by other threads
	* any time.
	*
	* Again, all this is handled internally and is mostly transparent to
	* the outside. The 'throttle_timers' field however has an additional
	* constraint because it may be temporarily invalid (see for example
	* bdrv_set_aio_context()). Therefore in this file a thread will
	* access some other BDS's timers only after verifying that that BDS
	* has throttled requests in the queue.
	*/
	typedef struct ThrottleGroup {
	char name; / This is constant during the lifetime of the group */

	QemuMutex lock; /* This lock protects the following four fields */
	ThrottleState ts;
	QLIST_HEAD(, BlockDriverState) head;
	BlockDriverState *tokens[2];
	bool any_timer_armed[2];

	/* These two are protected by the global throttle_groups_lock */
	unsigned refcount;
	QTAILQ_ENTRY(ThrottleGroup) list;
	} ThrottleGroup;

	static QemuMutex throttle_groups_lock;
	static QTAILQ_HEAD(, ThrottleGroup) throttle_groups =
	QTAILQ_HEAD_INITIALIZER(throttle_groups);

	/* Increments the reference count of a ThrottleGroup given its name.
	*
	* If no ThrottleGroup is found with the given name a new one is
	* created.
	*
	* @name: the name of the ThrottleGroup
	* @ret: the ThrottleState member of the ThrottleGroup
	*/
	ThrottleState throttle_group_incref(const char name)
	{
	ThrottleGroup *tg = NULL;
	ThrottleGroup *iter;

	qemu_mutex_lock(&throttle_groups_lock);

	/* Look for an existing group with that name */
	QTAILQ_FOREACH(iter, &throttle_groups, list) {
	if (!strcmp(name, iter->name)) {
	tg = iter;
	break;
	}
	}

	/* Create a new one if not found */
	if (!tg) {
	tg = g_new0(ThrottleGroup, 1);
	tg->name = g_strdup(name);
	qemu_mutex_init(&tg->lock);
	throttle_init(&tg->ts);
	QLIST_INIT(&tg->head);

	QTAILQ_INSERT_TAIL(&throttle_groups, tg, list);
	}

	tg->refcount++;

	qemu_mutex_unlock(&throttle_groups_lock);

	return &tg->ts;
	}

	/* Decrease the reference count of a ThrottleGroup.
	*
	* When the reference count reaches zero the ThrottleGroup is
	* destroyed.
	*
	* @ts: The ThrottleGroup to unref, given by its ThrottleState member
	*/
	void throttle_group_unref(ThrottleState *ts)
	{
	ThrottleGroup *tg = container_of(ts, ThrottleGroup, ts);

	qemu_mutex_lock(&throttle_groups_lock);
	if (--tg->refcount == 0) {
	QTAILQ_REMOVE(&throttle_groups, tg, list);
	qemu_mutex_destroy(&tg->lock);
	g_free(tg->name);
	g_free(tg);
	}
	qemu_mutex_unlock(&throttle_groups_lock);
	}

	/* Get the name from a BlockDriverState's ThrottleGroup. The name (and
	* the pointer) is guaranteed to remain constant during the lifetime
	* of the group.
	*
	* @bs: a BlockDriverState that is member of a throttling group
	* @ret: the name of the group.
	*/
	const char throttle_group_get_name(BlockDriverState bs)
	{
	ThrottleGroup *tg = container_of(bs->throttle_state, ThrottleGroup, ts);
	return tg->name;
	}

	/* Return the next BlockDriverState in the round-robin sequence,
	* simulating a circular list.
	*
	* This assumes that tg->lock is held.
	*
	* @bs: the current BlockDriverState
	* @ret: the next BlockDriverState in the sequence
	*/
	static BlockDriverState throttle_group_next_bs(BlockDriverState bs)
	{
	ThrottleState *ts = bs->throttle_state;
	ThrottleGroup *tg = container_of(ts, ThrottleGroup, ts);
	BlockDriverState *next = QLIST_NEXT(bs, round_robin);

	if (!next) {
	return QLIST_FIRST(&tg->head);
	}

	return next;
	}

	/* Return the next BlockDriverState in the round-robin sequence with
	* pending I/O requests.
	*
	* This assumes that tg->lock is held.
	*
	* @bs: the current BlockDriverState
	* @is_write: the type of operation (read/write)
	* @ret: the next BlockDriverState with pending requests, or bs
	* if there is none.
	*/
	static BlockDriverState next_throttle_token(BlockDriverState bs,
	bool is_write)
	{
	ThrottleGroup *tg = container_of(bs->throttle_state, ThrottleGroup, ts);
	BlockDriverState token, start;

	start = token = tg->tokens[is_write];

	/* get next bs round in round robin style */
	token = throttle_group_next_bs(token);
	while (token != start && !token->pending_reqs[is_write]) {
	token = throttle_group_next_bs(token);
	}

	/* If no IO are queued for scheduling on the next round robin token
	* then decide the token is the current bs because chances are
	* the current bs get the current request queued.
	*/
	if (token == start && !token->pending_reqs[is_write]) {
	token = bs;
	}

	return token;
	}

	/* Check if the next I/O request for a BlockDriverState needs to be
	* throttled or not. If there's no timer set in this group, set one
	* and update the token accordingly.
	*
	* This assumes that tg->lock is held.
	*
	* @bs: the current BlockDriverState
	* @is_write: the type of operation (read/write)
	* @ret: whether the I/O request needs to be throttled or not
	*/
	static bool throttle_group_schedule_timer(BlockDriverState *bs,
	bool is_write)
	{
	ThrottleState *ts = bs->throttle_state;
	ThrottleTimers *tt = &bs->throttle_timers;
	ThrottleGroup *tg = container_of(ts, ThrottleGroup, ts);
	bool must_wait;

	/* Check if any of the timers in this group is already armed */
	if (tg->any_timer_armed[is_write]) {
	return true;
	}

	must_wait = throttle_schedule_timer(ts, tt, is_write);

	/* If a timer just got armed, set bs as the current token */
	if (must_wait) {
	tg->tokens[is_write] = bs;
	tg->any_timer_armed[is_write] = true;
	}

	return must_wait;
	}

	/* Look for the next pending I/O request and schedule it.
	*
	* This assumes that tg->lock is held.
	*
	* @bs: the current BlockDriverState
	* @is_write: the type of operation (read/write)
	*/
	static void schedule_next_request(BlockDriverState *bs, bool is_write)
	{
	ThrottleGroup *tg = container_of(bs->throttle_state, ThrottleGroup, ts);
	bool must_wait;
	BlockDriverState *token;

	/* Check if there's any pending request to schedule next */
	token = next_throttle_token(bs, is_write);
	if (!token->pending_reqs[is_write]) {
	return;
	}

	/* Set a timer for the request if it needs to be throttled */
	must_wait = throttle_group_schedule_timer(token, is_write);

	/* If it doesn't have to wait, queue it for immediate execution */
	if (!must_wait) {
	/* Give preference to requests from the current bs */
	if (qemu_in_coroutine() &&
	qemu_co_queue_next(&bs->throttled_reqs[is_write])) {
	token = bs;
	} else {
	ThrottleTimers *tt = &token->throttle_timers;
	int64_t now = qemu_clock_get_ns(tt->clock_type);
	timer_mod(tt->timers[is_write], now + 1);
	tg->any_timer_armed[is_write] = true;
	}
	tg->tokens[is_write] = token;
	}
	}

	/* Check if an I/O request needs to be throttled, wait and set a timer
	* if necessary, and schedule the next request using a round robin
	* algorithm.
	*
	* @bs: the current BlockDriverState
	* @bytes: the number of bytes for this I/O
	* @is_write: the type of operation (read/write)
	*/
	void coroutine_fn throttle_group_co_io_limits_intercept(BlockDriverState *bs,
	unsigned int bytes,
	bool is_write)
	{
	bool must_wait;
	BlockDriverState *token;

	ThrottleGroup *tg = container_of(bs->throttle_state, ThrottleGroup, ts);
	qemu_mutex_lock(&tg->lock);

	/* First we check if this I/O has to be throttled. */
	token = next_throttle_token(bs, is_write);
	must_wait = throttle_group_schedule_timer(token, is_write);

	/* Wait if there's a timer set or queued requests of this type */
	if (must_wait \|\| bs->pending_reqs[is_write]) {
	bs->pending_reqs[is_write]++;
	qemu_mutex_unlock(&tg->lock);
	qemu_co_queue_wait(&bs->throttled_reqs[is_write]);
	qemu_mutex_lock(&tg->lock);
	bs->pending_reqs[is_write]--;
	}

	/* The I/O will be executed, so do the accounting */
	throttle_account(bs->throttle_state, is_write, bytes);

	/* Schedule the next request */
	schedule_next_request(bs, is_write);

	qemu_mutex_unlock(&tg->lock);
	}

	/* Update the throttle configuration for a particular group. Similar
	* to throttle_config(), but guarantees atomicity within the
	* throttling group.
	*
	* @bs: a BlockDriverState that is member of the group
	* @cfg: the configuration to set
	*/
	void throttle_group_config(BlockDriverState bs, ThrottleConfig cfg)
	{
	ThrottleTimers *tt = &bs->throttle_timers;
	ThrottleState *ts = bs->throttle_state;
	ThrottleGroup *tg = container_of(ts, ThrottleGroup, ts);
	qemu_mutex_lock(&tg->lock);
	/* throttle_config() cancels the timers */
	if (timer_pending(tt->timers[0])) {
	tg->any_timer_armed[0] = false;
	}
	if (timer_pending(tt->timers[1])) {
	tg->any_timer_armed[1] = false;
	}
	throttle_config(ts, tt, cfg);
	qemu_mutex_unlock(&tg->lock);
	}

	/* Get the throttle configuration from a particular group. Similar to
	* throttle_get_config(), but guarantees atomicity within the
	* throttling group.
	*
	* @bs: a BlockDriverState that is member of the group
	* @cfg: the configuration will be written here
	*/
	void throttle_group_get_config(BlockDriverState bs, ThrottleConfig cfg)
	{
	ThrottleState *ts = bs->throttle_state;
	ThrottleGroup *tg = container_of(ts, ThrottleGroup, ts);
	qemu_mutex_lock(&tg->lock);
	throttle_get_config(ts, cfg);
	qemu_mutex_unlock(&tg->lock);
	}

	/* ThrottleTimers callback. This wakes up a request that was waiting
	* because it had been throttled.
	*
	* @bs: the BlockDriverState whose request had been throttled
	* @is_write: the type of operation (read/write)
	*/
	static void timer_cb(BlockDriverState *bs, bool is_write)
	{
	ThrottleState *ts = bs->throttle_state;
	ThrottleGroup *tg = container_of(ts, ThrottleGroup, ts);
	bool empty_queue;

	/* The timer has just been fired, so we can update the flag */
	qemu_mutex_lock(&tg->lock);
	tg->any_timer_armed[is_write] = false;
	qemu_mutex_unlock(&tg->lock);

	/* Run the request that was waiting for this timer */
	empty_queue = !qemu_co_enter_next(&bs->throttled_reqs[is_write]);

	/* If the request queue was empty then we have to take care of
	* scheduling the next one */
	if (empty_queue) {
	qemu_mutex_lock(&tg->lock);
	schedule_next_request(bs, is_write);
	qemu_mutex_unlock(&tg->lock);
	}
	}

	static void read_timer_cb(void *opaque)
	{
	timer_cb(opaque, false);
	}

	static void write_timer_cb(void *opaque)
	{
	timer_cb(opaque, true);
	}

	/* Register a BlockDriverState in the throttling group, also
	* initializing its timers and updating its throttle_state pointer to
	* point to it. If a throttling group with that name does not exist
	* yet, it will be created.
	*
	* @bs: the BlockDriverState to insert
	* @groupname: the name of the group
	*/
	void throttle_group_register_bs(BlockDriverState bs, const char groupname)
	{
	int i;
	ThrottleState *ts = throttle_group_incref(groupname);
	ThrottleGroup *tg = container_of(ts, ThrottleGroup, ts);
	int clock_type = QEMU_CLOCK_REALTIME;

	if (qtest_enabled()) {
	/* For testing block IO throttling only */
	clock_type = QEMU_CLOCK_VIRTUAL;
	}

	bs->throttle_state = ts;

	qemu_mutex_lock(&tg->lock);
	/* If the ThrottleGroup is new set this BlockDriverState as the token */
	for (i = 0; i < 2; i++) {
	if (!tg->tokens[i]) {
	tg->tokens[i] = bs;
	}
	}

	QLIST_INSERT_HEAD(&tg->head, bs, round_robin);

	throttle_timers_init(&bs->throttle_timers,
	bdrv_get_aio_context(bs),
	clock_type,
	read_timer_cb,
	write_timer_cb,
	bs);

	qemu_mutex_unlock(&tg->lock);
	}

	/* Unregister a BlockDriverState from its group, removing it from the
	* list, destroying the timers and setting the throttle_state pointer
	* to NULL.
	*
	* The BlockDriverState must not have pending throttled requests, so
	* the caller has to drain them first.
	*
	* The group will be destroyed if it's empty after this operation.
	*
	* @bs: the BlockDriverState to remove
	*/
	void throttle_group_unregister_bs(BlockDriverState *bs)
	{
	ThrottleGroup *tg = container_of(bs->throttle_state, ThrottleGroup, ts);
	int i;

	assert(bs->pending_reqs[0] == 0 && bs->pending_reqs[1] == 0);
	assert(qemu_co_queue_empty(&bs->throttled_reqs[0]));
	assert(qemu_co_queue_empty(&bs->throttled_reqs[1]));

	qemu_mutex_lock(&tg->lock);
	for (i = 0; i < 2; i++) {
	if (tg->tokens[i] == bs) {
	BlockDriverState *token = throttle_group_next_bs(bs);
	/* Take care of the case where this is the last bs in the group */
	if (token == bs) {
	token = NULL;
	}
	tg->tokens[i] = token;
	}
	}

	/* remove the current bs from the list */
	QLIST_REMOVE(bs, round_robin);
	throttle_timers_destroy(&bs->throttle_timers);
	qemu_mutex_unlock(&tg->lock);

	throttle_group_unref(&tg->ts);
	bs->throttle_state = NULL;
	}

	static void throttle_groups_init(void)
	{
	qemu_mutex_init(&throttle_groups_lock);
	}

	block_init(throttle_groups_init);