2 * buffered writeback throttling. loosely based on CoDel. We can't drop
3 * packets for IO scheduling, so the logic is something like this:
5 * - Monitor latencies in a defined window of time.
6 * - If the minimum latency in the above window exceeds some target, increment
7 * scaling step and scale down queue depth by a factor of 2x. The monitoring
8 * window is then shrunk to 100 / sqrt(scaling step + 1).
9 * - For any window where we don't have solid data on what the latencies
10 * look like, retain status quo.
11 * - If latencies look good, decrement scaling step.
12 * - If we're only doing writes, allow the scaling step to go negative. This
13 * will temporarily boost write performance, snapping back to a stable
14 * scaling step of 0 if reads show up or the heavy writers finish. Unlike
15 * positive scaling steps where we shrink the monitoring window, a negative
16 * scaling step retains the default step==0 window size.
18 * Copyright (C) 2016 Jens Axboe
21 #include <linux/kernel.h>
22 #include <linux/blk_types.h>
23 #include <linux/slab.h>
24 #include <linux/backing-dev.h>
25 #include <linux/swap.h>
28 #include "blk-rq-qos.h"
30 #define CREATE_TRACE_POINTS
31 #include <trace/events/wbt.h>
33 static inline void wbt_clear_state(struct request
*rq
)
38 static inline enum wbt_flags
wbt_flags(struct request
*rq
)
43 static inline bool wbt_is_tracked(struct request
*rq
)
45 return rq
->wbt_flags
& WBT_TRACKED
;
48 static inline bool wbt_is_read(struct request
*rq
)
50 return rq
->wbt_flags
& WBT_READ
;
55 * Default setting, we'll scale up (to 75% of QD max) or down (min 1)
56 * from here depending on device stats
63 RWB_WINDOW_NSEC
= 100 * 1000 * 1000ULL,
66 * Disregard stats, if we don't meet this minimum
68 RWB_MIN_WRITE_SAMPLES
= 3,
71 * If we have this number of consecutive windows with not enough
72 * information to scale up or down, scale up.
77 static inline bool rwb_enabled(struct rq_wb
*rwb
)
79 return rwb
&& rwb
->wb_normal
!= 0;
82 static void wb_timestamp(struct rq_wb
*rwb
, unsigned long *var
)
84 if (rwb_enabled(rwb
)) {
85 const unsigned long cur
= jiffies
;
93 * If a task was rate throttled in balance_dirty_pages() within the last
94 * second or so, use that to indicate a higher cleaning rate.
96 static bool wb_recent_wait(struct rq_wb
*rwb
)
98 struct bdi_writeback
*wb
= &rwb
->rqos
.q
->backing_dev_info
->wb
;
100 return time_before(jiffies
, wb
->dirty_sleep
+ HZ
);
103 static inline struct rq_wait
*get_rq_wait(struct rq_wb
*rwb
,
104 enum wbt_flags wb_acct
)
106 if (wb_acct
& WBT_KSWAPD
)
107 return &rwb
->rq_wait
[WBT_RWQ_KSWAPD
];
108 else if (wb_acct
& WBT_DISCARD
)
109 return &rwb
->rq_wait
[WBT_RWQ_DISCARD
];
111 return &rwb
->rq_wait
[WBT_RWQ_BG
];
114 static void rwb_wake_all(struct rq_wb
*rwb
)
118 for (i
= 0; i
< WBT_NUM_RWQ
; i
++) {
119 struct rq_wait
*rqw
= &rwb
->rq_wait
[i
];
121 if (wq_has_sleeper(&rqw
->wait
))
122 wake_up_all(&rqw
->wait
);
126 static void wbt_rqw_done(struct rq_wb
*rwb
, struct rq_wait
*rqw
,
127 enum wbt_flags wb_acct
)
131 inflight
= atomic_dec_return(&rqw
->inflight
);
134 * wbt got disabled with IO in flight. Wake up any potential
135 * waiters, we don't have to do more than that.
137 if (unlikely(!rwb_enabled(rwb
))) {
143 * For discards, our limit is always the background. For writes, if
144 * the device does write back caching, drop further down before we
147 if (wb_acct
& WBT_DISCARD
)
148 limit
= rwb
->wb_background
;
149 else if (rwb
->wc
&& !wb_recent_wait(rwb
))
152 limit
= rwb
->wb_normal
;
155 * Don't wake anyone up if we are above the normal limit.
157 if (inflight
&& inflight
>= limit
)
160 if (wq_has_sleeper(&rqw
->wait
)) {
161 int diff
= limit
- inflight
;
163 if (!inflight
|| diff
>= rwb
->wb_background
/ 2)
164 wake_up_all(&rqw
->wait
);
168 static void __wbt_done(struct rq_qos
*rqos
, enum wbt_flags wb_acct
)
170 struct rq_wb
*rwb
= RQWB(rqos
);
173 if (!(wb_acct
& WBT_TRACKED
))
176 rqw
= get_rq_wait(rwb
, wb_acct
);
177 wbt_rqw_done(rwb
, rqw
, wb_acct
);
181 * Called on completion of a request. Note that it's also called when
182 * a request is merged, when the request gets freed.
184 static void wbt_done(struct rq_qos
*rqos
, struct request
*rq
)
186 struct rq_wb
*rwb
= RQWB(rqos
);
188 if (!wbt_is_tracked(rq
)) {
189 if (rwb
->sync_cookie
== rq
) {
191 rwb
->sync_cookie
= NULL
;
195 wb_timestamp(rwb
, &rwb
->last_comp
);
197 WARN_ON_ONCE(rq
== rwb
->sync_cookie
);
198 __wbt_done(rqos
, wbt_flags(rq
));
203 static inline bool stat_sample_valid(struct blk_rq_stat
*stat
)
206 * We need at least one read sample, and a minimum of
207 * RWB_MIN_WRITE_SAMPLES. We require some write samples to know
208 * that it's writes impacting us, and not just some sole read on
209 * a device that is in a lower power state.
211 return (stat
[READ
].nr_samples
>= 1 &&
212 stat
[WRITE
].nr_samples
>= RWB_MIN_WRITE_SAMPLES
);
215 static u64
rwb_sync_issue_lat(struct rq_wb
*rwb
)
217 u64 now
, issue
= READ_ONCE(rwb
->sync_issue
);
219 if (!issue
|| !rwb
->sync_cookie
)
222 now
= ktime_to_ns(ktime_get());
233 static int latency_exceeded(struct rq_wb
*rwb
, struct blk_rq_stat
*stat
)
235 struct backing_dev_info
*bdi
= rwb
->rqos
.q
->backing_dev_info
;
236 struct rq_depth
*rqd
= &rwb
->rq_depth
;
240 * If our stored sync issue exceeds the window size, or it
241 * exceeds our min target AND we haven't logged any entries,
242 * flag the latency as exceeded. wbt works off completion latencies,
243 * but for a flooded device, a single sync IO can take a long time
244 * to complete after being issued. If this time exceeds our
245 * monitoring window AND we didn't see any other completions in that
246 * window, then count that sync IO as a violation of the latency.
248 thislat
= rwb_sync_issue_lat(rwb
);
249 if (thislat
> rwb
->cur_win_nsec
||
250 (thislat
> rwb
->min_lat_nsec
&& !stat
[READ
].nr_samples
)) {
251 trace_wbt_lat(bdi
, thislat
);
256 * No read/write mix, if stat isn't valid
258 if (!stat_sample_valid(stat
)) {
260 * If we had writes in this stat window and the window is
261 * current, we're only doing writes. If a task recently
262 * waited or still has writes in flights, consider us doing
263 * just writes as well.
265 if (stat
[WRITE
].nr_samples
|| wb_recent_wait(rwb
) ||
267 return LAT_UNKNOWN_WRITES
;
272 * If the 'min' latency exceeds our target, step down.
274 if (stat
[READ
].min
> rwb
->min_lat_nsec
) {
275 trace_wbt_lat(bdi
, stat
[READ
].min
);
276 trace_wbt_stat(bdi
, stat
);
281 trace_wbt_stat(bdi
, stat
);
286 static void rwb_trace_step(struct rq_wb
*rwb
, const char *msg
)
288 struct backing_dev_info
*bdi
= rwb
->rqos
.q
->backing_dev_info
;
289 struct rq_depth
*rqd
= &rwb
->rq_depth
;
291 trace_wbt_step(bdi
, msg
, rqd
->scale_step
, rwb
->cur_win_nsec
,
292 rwb
->wb_background
, rwb
->wb_normal
, rqd
->max_depth
);
295 static void calc_wb_limits(struct rq_wb
*rwb
)
297 if (rwb
->min_lat_nsec
== 0) {
298 rwb
->wb_normal
= rwb
->wb_background
= 0;
299 } else if (rwb
->rq_depth
.max_depth
<= 2) {
300 rwb
->wb_normal
= rwb
->rq_depth
.max_depth
;
301 rwb
->wb_background
= 1;
303 rwb
->wb_normal
= (rwb
->rq_depth
.max_depth
+ 1) / 2;
304 rwb
->wb_background
= (rwb
->rq_depth
.max_depth
+ 3) / 4;
308 static void scale_up(struct rq_wb
*rwb
)
310 rq_depth_scale_up(&rwb
->rq_depth
);
312 rwb
->unknown_cnt
= 0;
314 rwb_trace_step(rwb
, "scale up");
317 static void scale_down(struct rq_wb
*rwb
, bool hard_throttle
)
319 rq_depth_scale_down(&rwb
->rq_depth
, hard_throttle
);
321 rwb
->unknown_cnt
= 0;
322 rwb_trace_step(rwb
, "scale down");
325 static void rwb_arm_timer(struct rq_wb
*rwb
)
327 struct rq_depth
*rqd
= &rwb
->rq_depth
;
329 if (rqd
->scale_step
> 0) {
331 * We should speed this up, using some variant of a fast
332 * integer inverse square root calculation. Since we only do
333 * this for every window expiration, it's not a huge deal,
336 rwb
->cur_win_nsec
= div_u64(rwb
->win_nsec
<< 4,
337 int_sqrt((rqd
->scale_step
+ 1) << 8));
340 * For step < 0, we don't want to increase/decrease the
343 rwb
->cur_win_nsec
= rwb
->win_nsec
;
346 blk_stat_activate_nsecs(rwb
->cb
, rwb
->cur_win_nsec
);
349 static void wb_timer_fn(struct blk_stat_callback
*cb
)
351 struct rq_wb
*rwb
= cb
->data
;
352 struct rq_depth
*rqd
= &rwb
->rq_depth
;
353 unsigned int inflight
= wbt_inflight(rwb
);
356 status
= latency_exceeded(rwb
, cb
->stat
);
358 trace_wbt_timer(rwb
->rqos
.q
->backing_dev_info
, status
, rqd
->scale_step
,
362 * If we exceeded the latency target, step down. If we did not,
363 * step one level up. If we don't know enough to say either exceeded
364 * or ok, then don't do anything.
368 scale_down(rwb
, true);
373 case LAT_UNKNOWN_WRITES
:
375 * We started a the center step, but don't have a valid
376 * read/write sample, but we do have writes going on.
377 * Allow step to go negative, to increase write perf.
382 if (++rwb
->unknown_cnt
< RWB_UNKNOWN_BUMP
)
385 * We get here when previously scaled reduced depth, and we
386 * currently don't have a valid read/write sample. For that
387 * case, slowly return to center state (step == 0).
389 if (rqd
->scale_step
> 0)
391 else if (rqd
->scale_step
< 0)
392 scale_down(rwb
, false);
399 * Re-arm timer, if we have IO in flight
401 if (rqd
->scale_step
|| inflight
)
405 static void __wbt_update_limits(struct rq_wb
*rwb
)
407 struct rq_depth
*rqd
= &rwb
->rq_depth
;
410 rqd
->scaled_max
= false;
412 rq_depth_calc_max_depth(rqd
);
418 void wbt_update_limits(struct request_queue
*q
)
420 struct rq_qos
*rqos
= wbt_rq_qos(q
);
423 __wbt_update_limits(RQWB(rqos
));
426 u64
wbt_get_min_lat(struct request_queue
*q
)
428 struct rq_qos
*rqos
= wbt_rq_qos(q
);
431 return RQWB(rqos
)->min_lat_nsec
;
434 void wbt_set_min_lat(struct request_queue
*q
, u64 val
)
436 struct rq_qos
*rqos
= wbt_rq_qos(q
);
439 RQWB(rqos
)->min_lat_nsec
= val
;
440 RQWB(rqos
)->enable_state
= WBT_STATE_ON_MANUAL
;
441 __wbt_update_limits(RQWB(rqos
));
445 static bool close_io(struct rq_wb
*rwb
)
447 const unsigned long now
= jiffies
;
449 return time_before(now
, rwb
->last_issue
+ HZ
/ 10) ||
450 time_before(now
, rwb
->last_comp
+ HZ
/ 10);
453 #define REQ_HIPRIO (REQ_SYNC | REQ_META | REQ_PRIO)
455 static inline unsigned int get_limit(struct rq_wb
*rwb
, unsigned long rw
)
460 * If we got disabled, just return UINT_MAX. This ensures that
461 * we'll properly inc a new IO, and dec+wakeup at the end.
463 if (!rwb_enabled(rwb
))
466 if ((rw
& REQ_OP_MASK
) == REQ_OP_DISCARD
)
467 return rwb
->wb_background
;
470 * At this point we know it's a buffered write. If this is
471 * kswapd trying to free memory, or REQ_SYNC is set, then
472 * it's WB_SYNC_ALL writeback, and we'll use the max limit for
473 * that. If the write is marked as a background write, then use
474 * the idle limit, or go to normal if we haven't had competing
477 if ((rw
& REQ_HIPRIO
) || wb_recent_wait(rwb
) || current_is_kswapd())
478 limit
= rwb
->rq_depth
.max_depth
;
479 else if ((rw
& REQ_BACKGROUND
) || close_io(rwb
)) {
481 * If less than 100ms since we completed unrelated IO,
482 * limit us to half the depth for background writeback.
484 limit
= rwb
->wb_background
;
486 limit
= rwb
->wb_normal
;
491 struct wbt_wait_data
{
493 enum wbt_flags wb_acct
;
497 static bool wbt_inflight_cb(struct rq_wait
*rqw
, void *private_data
)
499 struct wbt_wait_data
*data
= private_data
;
500 return rq_wait_inc_below(rqw
, get_limit(data
->rwb
, data
->rw
));
503 static void wbt_cleanup_cb(struct rq_wait
*rqw
, void *private_data
)
505 struct wbt_wait_data
*data
= private_data
;
506 wbt_rqw_done(data
->rwb
, rqw
, data
->wb_acct
);
510 * Block if we will exceed our limit, or if we are currently waiting for
511 * the timer to kick off queuing again.
513 static void __wbt_wait(struct rq_wb
*rwb
, enum wbt_flags wb_acct
,
516 struct rq_wait
*rqw
= get_rq_wait(rwb
, wb_acct
);
517 struct wbt_wait_data data
= {
523 rq_qos_wait(rqw
, &data
, wbt_inflight_cb
, wbt_cleanup_cb
);
526 static inline bool wbt_should_throttle(struct rq_wb
*rwb
, struct bio
*bio
)
528 switch (bio_op(bio
)) {
531 * Don't throttle WRITE_ODIRECT
533 if ((bio
->bi_opf
& (REQ_SYNC
| REQ_IDLE
)) ==
534 (REQ_SYNC
| REQ_IDLE
))
544 static enum wbt_flags
bio_to_wbt_flags(struct rq_wb
*rwb
, struct bio
*bio
)
546 enum wbt_flags flags
= 0;
548 if (!rwb_enabled(rwb
))
551 if (bio_op(bio
) == REQ_OP_READ
) {
553 } else if (wbt_should_throttle(rwb
, bio
)) {
554 if (current_is_kswapd())
556 if (bio_op(bio
) == REQ_OP_DISCARD
)
557 flags
|= WBT_DISCARD
;
558 flags
|= WBT_TRACKED
;
563 static void wbt_cleanup(struct rq_qos
*rqos
, struct bio
*bio
)
565 struct rq_wb
*rwb
= RQWB(rqos
);
566 enum wbt_flags flags
= bio_to_wbt_flags(rwb
, bio
);
567 __wbt_done(rqos
, flags
);
571 * Returns true if the IO request should be accounted, false if not.
572 * May sleep, if we have exceeded the writeback limits. Caller can pass
573 * in an irq held spinlock, if it holds one when calling this function.
574 * If we do sleep, we'll release and re-grab it.
576 static void wbt_wait(struct rq_qos
*rqos
, struct bio
*bio
)
578 struct rq_wb
*rwb
= RQWB(rqos
);
579 enum wbt_flags flags
;
581 flags
= bio_to_wbt_flags(rwb
, bio
);
582 if (!(flags
& WBT_TRACKED
)) {
583 if (flags
& WBT_READ
)
584 wb_timestamp(rwb
, &rwb
->last_issue
);
588 __wbt_wait(rwb
, flags
, bio
->bi_opf
);
590 if (!blk_stat_is_active(rwb
->cb
))
594 static void wbt_track(struct rq_qos
*rqos
, struct request
*rq
, struct bio
*bio
)
596 struct rq_wb
*rwb
= RQWB(rqos
);
597 rq
->wbt_flags
|= bio_to_wbt_flags(rwb
, bio
);
600 void wbt_issue(struct rq_qos
*rqos
, struct request
*rq
)
602 struct rq_wb
*rwb
= RQWB(rqos
);
604 if (!rwb_enabled(rwb
))
608 * Track sync issue, in case it takes a long time to complete. Allows us
609 * to react quicker, if a sync IO takes a long time to complete. Note
610 * that this is just a hint. The request can go away when it completes,
611 * so it's important we never dereference it. We only use the address to
612 * compare with, which is why we store the sync_issue time locally.
614 if (wbt_is_read(rq
) && !rwb
->sync_issue
) {
615 rwb
->sync_cookie
= rq
;
616 rwb
->sync_issue
= rq
->io_start_time_ns
;
620 void wbt_requeue(struct rq_qos
*rqos
, struct request
*rq
)
622 struct rq_wb
*rwb
= RQWB(rqos
);
623 if (!rwb_enabled(rwb
))
625 if (rq
== rwb
->sync_cookie
) {
627 rwb
->sync_cookie
= NULL
;
631 void wbt_set_queue_depth(struct request_queue
*q
, unsigned int depth
)
633 struct rq_qos
*rqos
= wbt_rq_qos(q
);
635 RQWB(rqos
)->rq_depth
.queue_depth
= depth
;
636 __wbt_update_limits(RQWB(rqos
));
640 void wbt_set_write_cache(struct request_queue
*q
, bool write_cache_on
)
642 struct rq_qos
*rqos
= wbt_rq_qos(q
);
644 RQWB(rqos
)->wc
= write_cache_on
;
648 * Enable wbt if defaults are configured that way
650 void wbt_enable_default(struct request_queue
*q
)
652 struct rq_qos
*rqos
= wbt_rq_qos(q
);
653 /* Throttling already enabled? */
657 /* Queue not registered? Maybe shutting down... */
658 if (!test_bit(QUEUE_FLAG_REGISTERED
, &q
->queue_flags
))
661 if (queue_is_mq(q
) && IS_ENABLED(CONFIG_BLK_WBT_MQ
))
664 EXPORT_SYMBOL_GPL(wbt_enable_default
);
666 u64
wbt_default_latency_nsec(struct request_queue
*q
)
669 * We default to 2msec for non-rotational storage, and 75msec
670 * for rotational storage.
672 if (blk_queue_nonrot(q
))
678 static int wbt_data_dir(const struct request
*rq
)
680 const int op
= req_op(rq
);
682 if (op
== REQ_OP_READ
)
684 else if (op_is_write(op
))
691 static void wbt_exit(struct rq_qos
*rqos
)
693 struct rq_wb
*rwb
= RQWB(rqos
);
694 struct request_queue
*q
= rqos
->q
;
696 blk_stat_remove_callback(q
, rwb
->cb
);
697 blk_stat_free_callback(rwb
->cb
);
702 * Disable wbt, if enabled by default.
704 void wbt_disable_default(struct request_queue
*q
)
706 struct rq_qos
*rqos
= wbt_rq_qos(q
);
711 if (rwb
->enable_state
== WBT_STATE_ON_DEFAULT
) {
712 blk_stat_deactivate(rwb
->cb
);
716 EXPORT_SYMBOL_GPL(wbt_disable_default
);
718 #ifdef CONFIG_BLK_DEBUG_FS
719 static int wbt_curr_win_nsec_show(void *data
, struct seq_file
*m
)
721 struct rq_qos
*rqos
= data
;
722 struct rq_wb
*rwb
= RQWB(rqos
);
724 seq_printf(m
, "%llu\n", rwb
->cur_win_nsec
);
728 static int wbt_enabled_show(void *data
, struct seq_file
*m
)
730 struct rq_qos
*rqos
= data
;
731 struct rq_wb
*rwb
= RQWB(rqos
);
733 seq_printf(m
, "%d\n", rwb
->enable_state
);
737 static int wbt_id_show(void *data
, struct seq_file
*m
)
739 struct rq_qos
*rqos
= data
;
741 seq_printf(m
, "%u\n", rqos
->id
);
745 static int wbt_inflight_show(void *data
, struct seq_file
*m
)
747 struct rq_qos
*rqos
= data
;
748 struct rq_wb
*rwb
= RQWB(rqos
);
751 for (i
= 0; i
< WBT_NUM_RWQ
; i
++)
752 seq_printf(m
, "%d: inflight %d\n", i
,
753 atomic_read(&rwb
->rq_wait
[i
].inflight
));
757 static int wbt_min_lat_nsec_show(void *data
, struct seq_file
*m
)
759 struct rq_qos
*rqos
= data
;
760 struct rq_wb
*rwb
= RQWB(rqos
);
762 seq_printf(m
, "%lu\n", rwb
->min_lat_nsec
);
766 static int wbt_unknown_cnt_show(void *data
, struct seq_file
*m
)
768 struct rq_qos
*rqos
= data
;
769 struct rq_wb
*rwb
= RQWB(rqos
);
771 seq_printf(m
, "%u\n", rwb
->unknown_cnt
);
775 static int wbt_normal_show(void *data
, struct seq_file
*m
)
777 struct rq_qos
*rqos
= data
;
778 struct rq_wb
*rwb
= RQWB(rqos
);
780 seq_printf(m
, "%u\n", rwb
->wb_normal
);
784 static int wbt_background_show(void *data
, struct seq_file
*m
)
786 struct rq_qos
*rqos
= data
;
787 struct rq_wb
*rwb
= RQWB(rqos
);
789 seq_printf(m
, "%u\n", rwb
->wb_background
);
793 static const struct blk_mq_debugfs_attr wbt_debugfs_attrs
[] = {
794 {"curr_win_nsec", 0400, wbt_curr_win_nsec_show
},
795 {"enabled", 0400, wbt_enabled_show
},
796 {"id", 0400, wbt_id_show
},
797 {"inflight", 0400, wbt_inflight_show
},
798 {"min_lat_nsec", 0400, wbt_min_lat_nsec_show
},
799 {"unknown_cnt", 0400, wbt_unknown_cnt_show
},
800 {"wb_normal", 0400, wbt_normal_show
},
801 {"wb_background", 0400, wbt_background_show
},
806 static struct rq_qos_ops wbt_rqos_ops
= {
807 .throttle
= wbt_wait
,
810 .requeue
= wbt_requeue
,
812 .cleanup
= wbt_cleanup
,
814 #ifdef CONFIG_BLK_DEBUG_FS
815 .debugfs_attrs
= wbt_debugfs_attrs
,
819 int wbt_init(struct request_queue
*q
)
824 rwb
= kzalloc(sizeof(*rwb
), GFP_KERNEL
);
828 rwb
->cb
= blk_stat_alloc_callback(wb_timer_fn
, wbt_data_dir
, 2, rwb
);
834 for (i
= 0; i
< WBT_NUM_RWQ
; i
++)
835 rq_wait_init(&rwb
->rq_wait
[i
]);
837 rwb
->rqos
.id
= RQ_QOS_WBT
;
838 rwb
->rqos
.ops
= &wbt_rqos_ops
;
840 rwb
->last_comp
= rwb
->last_issue
= jiffies
;
841 rwb
->win_nsec
= RWB_WINDOW_NSEC
;
842 rwb
->enable_state
= WBT_STATE_ON_DEFAULT
;
844 rwb
->rq_depth
.default_depth
= RWB_DEF_DEPTH
;
845 __wbt_update_limits(rwb
);
848 * Assign rwb and add the stats callback.
850 rq_qos_add(q
, &rwb
->rqos
);
851 blk_stat_add_callback(q
, rwb
->cb
);
853 rwb
->min_lat_nsec
= wbt_default_latency_nsec(q
);
855 wbt_set_queue_depth(q
, blk_queue_depth(q
));
856 wbt_set_write_cache(q
, test_bit(QUEUE_FLAG_WC
, &q
->queue_flags
));