1 // SPDX-License-Identifier: GPL-2.0
3 * buffered writeback throttling. loosely based on CoDel. We can't drop
4 * packets for IO scheduling, so the logic is something like this:
6 * - Monitor latencies in a defined window of time.
7 * - If the minimum latency in the above window exceeds some target, increment
8 * scaling step and scale down queue depth by a factor of 2x. The monitoring
9 * window is then shrunk to 100 / sqrt(scaling step + 1).
10 * - For any window where we don't have solid data on what the latencies
11 * look like, retain status quo.
12 * - If latencies look good, decrement scaling step.
13 * - If we're only doing writes, allow the scaling step to go negative. This
14 * will temporarily boost write performance, snapping back to a stable
15 * scaling step of 0 if reads show up or the heavy writers finish. Unlike
16 * positive scaling steps where we shrink the monitoring window, a negative
17 * scaling step retains the default step==0 window size.
19 * Copyright (C) 2016 Jens Axboe
22 #include <linux/kernel.h>
23 #include <linux/blk_types.h>
24 #include <linux/slab.h>
25 #include <linux/backing-dev.h>
26 #include <linux/swap.h>
29 #include "blk-rq-qos.h"
31 #define CREATE_TRACE_POINTS
32 #include <trace/events/wbt.h>
34 static inline void wbt_clear_state(struct request
*rq
)
39 static inline enum wbt_flags
wbt_flags(struct request
*rq
)
44 static inline bool wbt_is_tracked(struct request
*rq
)
46 return rq
->wbt_flags
& WBT_TRACKED
;
49 static inline bool wbt_is_read(struct request
*rq
)
51 return rq
->wbt_flags
& WBT_READ
;
56 * Default setting, we'll scale up (to 75% of QD max) or down (min 1)
57 * from here depending on device stats
64 RWB_WINDOW_NSEC
= 100 * 1000 * 1000ULL,
67 * Disregard stats, if we don't meet this minimum
69 RWB_MIN_WRITE_SAMPLES
= 3,
72 * If we have this number of consecutive windows with not enough
73 * information to scale up or down, scale up.
78 static inline bool rwb_enabled(struct rq_wb
*rwb
)
80 return rwb
&& rwb
->enable_state
!= WBT_STATE_OFF_DEFAULT
&&
84 static void wb_timestamp(struct rq_wb
*rwb
, unsigned long *var
)
86 if (rwb_enabled(rwb
)) {
87 const unsigned long cur
= jiffies
;
95 * If a task was rate throttled in balance_dirty_pages() within the last
96 * second or so, use that to indicate a higher cleaning rate.
98 static bool wb_recent_wait(struct rq_wb
*rwb
)
100 struct bdi_writeback
*wb
= &rwb
->rqos
.q
->backing_dev_info
->wb
;
102 return time_before(jiffies
, wb
->dirty_sleep
+ HZ
);
105 static inline struct rq_wait
*get_rq_wait(struct rq_wb
*rwb
,
106 enum wbt_flags wb_acct
)
108 if (wb_acct
& WBT_KSWAPD
)
109 return &rwb
->rq_wait
[WBT_RWQ_KSWAPD
];
110 else if (wb_acct
& WBT_DISCARD
)
111 return &rwb
->rq_wait
[WBT_RWQ_DISCARD
];
113 return &rwb
->rq_wait
[WBT_RWQ_BG
];
116 static void rwb_wake_all(struct rq_wb
*rwb
)
120 for (i
= 0; i
< WBT_NUM_RWQ
; i
++) {
121 struct rq_wait
*rqw
= &rwb
->rq_wait
[i
];
123 if (wq_has_sleeper(&rqw
->wait
))
124 wake_up_all(&rqw
->wait
);
128 static void wbt_rqw_done(struct rq_wb
*rwb
, struct rq_wait
*rqw
,
129 enum wbt_flags wb_acct
)
133 inflight
= atomic_dec_return(&rqw
->inflight
);
136 * wbt got disabled with IO in flight. Wake up any potential
137 * waiters, we don't have to do more than that.
139 if (unlikely(!rwb_enabled(rwb
))) {
145 * For discards, our limit is always the background. For writes, if
146 * the device does write back caching, drop further down before we
149 if (wb_acct
& WBT_DISCARD
)
150 limit
= rwb
->wb_background
;
151 else if (rwb
->wc
&& !wb_recent_wait(rwb
))
154 limit
= rwb
->wb_normal
;
157 * Don't wake anyone up if we are above the normal limit.
159 if (inflight
&& inflight
>= limit
)
162 if (wq_has_sleeper(&rqw
->wait
)) {
163 int diff
= limit
- inflight
;
165 if (!inflight
|| diff
>= rwb
->wb_background
/ 2)
166 wake_up_all(&rqw
->wait
);
170 static void __wbt_done(struct rq_qos
*rqos
, enum wbt_flags wb_acct
)
172 struct rq_wb
*rwb
= RQWB(rqos
);
175 if (!(wb_acct
& WBT_TRACKED
))
178 rqw
= get_rq_wait(rwb
, wb_acct
);
179 wbt_rqw_done(rwb
, rqw
, wb_acct
);
183 * Called on completion of a request. Note that it's also called when
184 * a request is merged, when the request gets freed.
186 static void wbt_done(struct rq_qos
*rqos
, struct request
*rq
)
188 struct rq_wb
*rwb
= RQWB(rqos
);
190 if (!wbt_is_tracked(rq
)) {
191 if (rwb
->sync_cookie
== rq
) {
193 rwb
->sync_cookie
= NULL
;
197 wb_timestamp(rwb
, &rwb
->last_comp
);
199 WARN_ON_ONCE(rq
== rwb
->sync_cookie
);
200 __wbt_done(rqos
, wbt_flags(rq
));
205 static inline bool stat_sample_valid(struct blk_rq_stat
*stat
)
208 * We need at least one read sample, and a minimum of
209 * RWB_MIN_WRITE_SAMPLES. We require some write samples to know
210 * that it's writes impacting us, and not just some sole read on
211 * a device that is in a lower power state.
213 return (stat
[READ
].nr_samples
>= 1 &&
214 stat
[WRITE
].nr_samples
>= RWB_MIN_WRITE_SAMPLES
);
217 static u64
rwb_sync_issue_lat(struct rq_wb
*rwb
)
219 u64 now
, issue
= READ_ONCE(rwb
->sync_issue
);
221 if (!issue
|| !rwb
->sync_cookie
)
224 now
= ktime_to_ns(ktime_get());
235 static int latency_exceeded(struct rq_wb
*rwb
, struct blk_rq_stat
*stat
)
237 struct backing_dev_info
*bdi
= rwb
->rqos
.q
->backing_dev_info
;
238 struct rq_depth
*rqd
= &rwb
->rq_depth
;
242 * If our stored sync issue exceeds the window size, or it
243 * exceeds our min target AND we haven't logged any entries,
244 * flag the latency as exceeded. wbt works off completion latencies,
245 * but for a flooded device, a single sync IO can take a long time
246 * to complete after being issued. If this time exceeds our
247 * monitoring window AND we didn't see any other completions in that
248 * window, then count that sync IO as a violation of the latency.
250 thislat
= rwb_sync_issue_lat(rwb
);
251 if (thislat
> rwb
->cur_win_nsec
||
252 (thislat
> rwb
->min_lat_nsec
&& !stat
[READ
].nr_samples
)) {
253 trace_wbt_lat(bdi
, thislat
);
258 * No read/write mix, if stat isn't valid
260 if (!stat_sample_valid(stat
)) {
262 * If we had writes in this stat window and the window is
263 * current, we're only doing writes. If a task recently
264 * waited or still has writes in flights, consider us doing
265 * just writes as well.
267 if (stat
[WRITE
].nr_samples
|| wb_recent_wait(rwb
) ||
269 return LAT_UNKNOWN_WRITES
;
274 * If the 'min' latency exceeds our target, step down.
276 if (stat
[READ
].min
> rwb
->min_lat_nsec
) {
277 trace_wbt_lat(bdi
, stat
[READ
].min
);
278 trace_wbt_stat(bdi
, stat
);
283 trace_wbt_stat(bdi
, stat
);
288 static void rwb_trace_step(struct rq_wb
*rwb
, const char *msg
)
290 struct backing_dev_info
*bdi
= rwb
->rqos
.q
->backing_dev_info
;
291 struct rq_depth
*rqd
= &rwb
->rq_depth
;
293 trace_wbt_step(bdi
, msg
, rqd
->scale_step
, rwb
->cur_win_nsec
,
294 rwb
->wb_background
, rwb
->wb_normal
, rqd
->max_depth
);
297 static void calc_wb_limits(struct rq_wb
*rwb
)
299 if (rwb
->min_lat_nsec
== 0) {
300 rwb
->wb_normal
= rwb
->wb_background
= 0;
301 } else if (rwb
->rq_depth
.max_depth
<= 2) {
302 rwb
->wb_normal
= rwb
->rq_depth
.max_depth
;
303 rwb
->wb_background
= 1;
305 rwb
->wb_normal
= (rwb
->rq_depth
.max_depth
+ 1) / 2;
306 rwb
->wb_background
= (rwb
->rq_depth
.max_depth
+ 3) / 4;
310 static void scale_up(struct rq_wb
*rwb
)
312 if (!rq_depth_scale_up(&rwb
->rq_depth
))
315 rwb
->unknown_cnt
= 0;
317 rwb_trace_step(rwb
, tracepoint_string("scale up"));
320 static void scale_down(struct rq_wb
*rwb
, bool hard_throttle
)
322 if (!rq_depth_scale_down(&rwb
->rq_depth
, hard_throttle
))
325 rwb
->unknown_cnt
= 0;
326 rwb_trace_step(rwb
, tracepoint_string("scale down"));
329 static void rwb_arm_timer(struct rq_wb
*rwb
)
331 struct rq_depth
*rqd
= &rwb
->rq_depth
;
333 if (rqd
->scale_step
> 0) {
335 * We should speed this up, using some variant of a fast
336 * integer inverse square root calculation. Since we only do
337 * this for every window expiration, it's not a huge deal,
340 rwb
->cur_win_nsec
= div_u64(rwb
->win_nsec
<< 4,
341 int_sqrt((rqd
->scale_step
+ 1) << 8));
344 * For step < 0, we don't want to increase/decrease the
347 rwb
->cur_win_nsec
= rwb
->win_nsec
;
350 blk_stat_activate_nsecs(rwb
->cb
, rwb
->cur_win_nsec
);
353 static void wb_timer_fn(struct blk_stat_callback
*cb
)
355 struct rq_wb
*rwb
= cb
->data
;
356 struct rq_depth
*rqd
= &rwb
->rq_depth
;
357 unsigned int inflight
= wbt_inflight(rwb
);
360 status
= latency_exceeded(rwb
, cb
->stat
);
362 trace_wbt_timer(rwb
->rqos
.q
->backing_dev_info
, status
, rqd
->scale_step
,
366 * If we exceeded the latency target, step down. If we did not,
367 * step one level up. If we don't know enough to say either exceeded
368 * or ok, then don't do anything.
372 scale_down(rwb
, true);
377 case LAT_UNKNOWN_WRITES
:
379 * We started a the center step, but don't have a valid
380 * read/write sample, but we do have writes going on.
381 * Allow step to go negative, to increase write perf.
386 if (++rwb
->unknown_cnt
< RWB_UNKNOWN_BUMP
)
389 * We get here when previously scaled reduced depth, and we
390 * currently don't have a valid read/write sample. For that
391 * case, slowly return to center state (step == 0).
393 if (rqd
->scale_step
> 0)
395 else if (rqd
->scale_step
< 0)
396 scale_down(rwb
, false);
403 * Re-arm timer, if we have IO in flight
405 if (rqd
->scale_step
|| inflight
)
409 static void wbt_update_limits(struct rq_wb
*rwb
)
411 struct rq_depth
*rqd
= &rwb
->rq_depth
;
414 rqd
->scaled_max
= false;
416 rq_depth_calc_max_depth(rqd
);
422 u64
wbt_get_min_lat(struct request_queue
*q
)
424 struct rq_qos
*rqos
= wbt_rq_qos(q
);
427 return RQWB(rqos
)->min_lat_nsec
;
430 void wbt_set_min_lat(struct request_queue
*q
, u64 val
)
432 struct rq_qos
*rqos
= wbt_rq_qos(q
);
435 RQWB(rqos
)->min_lat_nsec
= val
;
436 RQWB(rqos
)->enable_state
= WBT_STATE_ON_MANUAL
;
437 wbt_update_limits(RQWB(rqos
));
441 static bool close_io(struct rq_wb
*rwb
)
443 const unsigned long now
= jiffies
;
445 return time_before(now
, rwb
->last_issue
+ HZ
/ 10) ||
446 time_before(now
, rwb
->last_comp
+ HZ
/ 10);
449 #define REQ_HIPRIO (REQ_SYNC | REQ_META | REQ_PRIO)
451 static inline unsigned int get_limit(struct rq_wb
*rwb
, unsigned long rw
)
456 * If we got disabled, just return UINT_MAX. This ensures that
457 * we'll properly inc a new IO, and dec+wakeup at the end.
459 if (!rwb_enabled(rwb
))
462 if ((rw
& REQ_OP_MASK
) == REQ_OP_DISCARD
)
463 return rwb
->wb_background
;
466 * At this point we know it's a buffered write. If this is
467 * kswapd trying to free memory, or REQ_SYNC is set, then
468 * it's WB_SYNC_ALL writeback, and we'll use the max limit for
469 * that. If the write is marked as a background write, then use
470 * the idle limit, or go to normal if we haven't had competing
473 if ((rw
& REQ_HIPRIO
) || wb_recent_wait(rwb
) || current_is_kswapd())
474 limit
= rwb
->rq_depth
.max_depth
;
475 else if ((rw
& REQ_BACKGROUND
) || close_io(rwb
)) {
477 * If less than 100ms since we completed unrelated IO,
478 * limit us to half the depth for background writeback.
480 limit
= rwb
->wb_background
;
482 limit
= rwb
->wb_normal
;
487 struct wbt_wait_data
{
489 enum wbt_flags wb_acct
;
493 static bool wbt_inflight_cb(struct rq_wait
*rqw
, void *private_data
)
495 struct wbt_wait_data
*data
= private_data
;
496 return rq_wait_inc_below(rqw
, get_limit(data
->rwb
, data
->rw
));
499 static void wbt_cleanup_cb(struct rq_wait
*rqw
, void *private_data
)
501 struct wbt_wait_data
*data
= private_data
;
502 wbt_rqw_done(data
->rwb
, rqw
, data
->wb_acct
);
506 * Block if we will exceed our limit, or if we are currently waiting for
507 * the timer to kick off queuing again.
509 static void __wbt_wait(struct rq_wb
*rwb
, enum wbt_flags wb_acct
,
512 struct rq_wait
*rqw
= get_rq_wait(rwb
, wb_acct
);
513 struct wbt_wait_data data
= {
519 rq_qos_wait(rqw
, &data
, wbt_inflight_cb
, wbt_cleanup_cb
);
522 static inline bool wbt_should_throttle(struct bio
*bio
)
524 switch (bio_op(bio
)) {
527 * Don't throttle WRITE_ODIRECT
529 if ((bio
->bi_opf
& (REQ_SYNC
| REQ_IDLE
)) ==
530 (REQ_SYNC
| REQ_IDLE
))
540 static enum wbt_flags
bio_to_wbt_flags(struct rq_wb
*rwb
, struct bio
*bio
)
542 enum wbt_flags flags
= 0;
544 if (!rwb_enabled(rwb
))
547 if (bio_op(bio
) == REQ_OP_READ
) {
549 } else if (wbt_should_throttle(bio
)) {
550 if (current_is_kswapd())
552 if (bio_op(bio
) == REQ_OP_DISCARD
)
553 flags
|= WBT_DISCARD
;
554 flags
|= WBT_TRACKED
;
559 static void wbt_cleanup(struct rq_qos
*rqos
, struct bio
*bio
)
561 struct rq_wb
*rwb
= RQWB(rqos
);
562 enum wbt_flags flags
= bio_to_wbt_flags(rwb
, bio
);
563 __wbt_done(rqos
, flags
);
567 * May sleep, if we have exceeded the writeback limits. Caller can pass
568 * in an irq held spinlock, if it holds one when calling this function.
569 * If we do sleep, we'll release and re-grab it.
571 static void wbt_wait(struct rq_qos
*rqos
, struct bio
*bio
)
573 struct rq_wb
*rwb
= RQWB(rqos
);
574 enum wbt_flags flags
;
576 flags
= bio_to_wbt_flags(rwb
, bio
);
577 if (!(flags
& WBT_TRACKED
)) {
578 if (flags
& WBT_READ
)
579 wb_timestamp(rwb
, &rwb
->last_issue
);
583 __wbt_wait(rwb
, flags
, bio
->bi_opf
);
585 if (!blk_stat_is_active(rwb
->cb
))
589 static void wbt_track(struct rq_qos
*rqos
, struct request
*rq
, struct bio
*bio
)
591 struct rq_wb
*rwb
= RQWB(rqos
);
592 rq
->wbt_flags
|= bio_to_wbt_flags(rwb
, bio
);
595 static void wbt_issue(struct rq_qos
*rqos
, struct request
*rq
)
597 struct rq_wb
*rwb
= RQWB(rqos
);
599 if (!rwb_enabled(rwb
))
603 * Track sync issue, in case it takes a long time to complete. Allows us
604 * to react quicker, if a sync IO takes a long time to complete. Note
605 * that this is just a hint. The request can go away when it completes,
606 * so it's important we never dereference it. We only use the address to
607 * compare with, which is why we store the sync_issue time locally.
609 if (wbt_is_read(rq
) && !rwb
->sync_issue
) {
610 rwb
->sync_cookie
= rq
;
611 rwb
->sync_issue
= rq
->io_start_time_ns
;
615 static void wbt_requeue(struct rq_qos
*rqos
, struct request
*rq
)
617 struct rq_wb
*rwb
= RQWB(rqos
);
618 if (!rwb_enabled(rwb
))
620 if (rq
== rwb
->sync_cookie
) {
622 rwb
->sync_cookie
= NULL
;
626 void wbt_set_write_cache(struct request_queue
*q
, bool write_cache_on
)
628 struct rq_qos
*rqos
= wbt_rq_qos(q
);
630 RQWB(rqos
)->wc
= write_cache_on
;
634 * Enable wbt if defaults are configured that way
636 void wbt_enable_default(struct request_queue
*q
)
638 struct rq_qos
*rqos
= wbt_rq_qos(q
);
640 /* Throttling already enabled? */
642 if (RQWB(rqos
)->enable_state
== WBT_STATE_OFF_DEFAULT
)
643 RQWB(rqos
)->enable_state
= WBT_STATE_ON_DEFAULT
;
647 /* Queue not registered? Maybe shutting down... */
648 if (!blk_queue_registered(q
))
651 if (queue_is_mq(q
) && IS_ENABLED(CONFIG_BLK_WBT_MQ
))
654 EXPORT_SYMBOL_GPL(wbt_enable_default
);
656 u64
wbt_default_latency_nsec(struct request_queue
*q
)
659 * We default to 2msec for non-rotational storage, and 75msec
660 * for rotational storage.
662 if (blk_queue_nonrot(q
))
668 static int wbt_data_dir(const struct request
*rq
)
670 const int op
= req_op(rq
);
672 if (op
== REQ_OP_READ
)
674 else if (op_is_write(op
))
681 static void wbt_queue_depth_changed(struct rq_qos
*rqos
)
683 RQWB(rqos
)->rq_depth
.queue_depth
= blk_queue_depth(rqos
->q
);
684 wbt_update_limits(RQWB(rqos
));
687 static void wbt_exit(struct rq_qos
*rqos
)
689 struct rq_wb
*rwb
= RQWB(rqos
);
690 struct request_queue
*q
= rqos
->q
;
692 blk_stat_remove_callback(q
, rwb
->cb
);
693 blk_stat_free_callback(rwb
->cb
);
698 * Disable wbt, if enabled by default.
700 void wbt_disable_default(struct request_queue
*q
)
702 struct rq_qos
*rqos
= wbt_rq_qos(q
);
707 if (rwb
->enable_state
== WBT_STATE_ON_DEFAULT
) {
708 blk_stat_deactivate(rwb
->cb
);
709 rwb
->enable_state
= WBT_STATE_OFF_DEFAULT
;
712 EXPORT_SYMBOL_GPL(wbt_disable_default
);
714 #ifdef CONFIG_BLK_DEBUG_FS
715 static int wbt_curr_win_nsec_show(void *data
, struct seq_file
*m
)
717 struct rq_qos
*rqos
= data
;
718 struct rq_wb
*rwb
= RQWB(rqos
);
720 seq_printf(m
, "%llu\n", rwb
->cur_win_nsec
);
724 static int wbt_enabled_show(void *data
, struct seq_file
*m
)
726 struct rq_qos
*rqos
= data
;
727 struct rq_wb
*rwb
= RQWB(rqos
);
729 seq_printf(m
, "%d\n", rwb
->enable_state
);
733 static int wbt_id_show(void *data
, struct seq_file
*m
)
735 struct rq_qos
*rqos
= data
;
737 seq_printf(m
, "%u\n", rqos
->id
);
741 static int wbt_inflight_show(void *data
, struct seq_file
*m
)
743 struct rq_qos
*rqos
= data
;
744 struct rq_wb
*rwb
= RQWB(rqos
);
747 for (i
= 0; i
< WBT_NUM_RWQ
; i
++)
748 seq_printf(m
, "%d: inflight %d\n", i
,
749 atomic_read(&rwb
->rq_wait
[i
].inflight
));
753 static int wbt_min_lat_nsec_show(void *data
, struct seq_file
*m
)
755 struct rq_qos
*rqos
= data
;
756 struct rq_wb
*rwb
= RQWB(rqos
);
758 seq_printf(m
, "%lu\n", rwb
->min_lat_nsec
);
762 static int wbt_unknown_cnt_show(void *data
, struct seq_file
*m
)
764 struct rq_qos
*rqos
= data
;
765 struct rq_wb
*rwb
= RQWB(rqos
);
767 seq_printf(m
, "%u\n", rwb
->unknown_cnt
);
771 static int wbt_normal_show(void *data
, struct seq_file
*m
)
773 struct rq_qos
*rqos
= data
;
774 struct rq_wb
*rwb
= RQWB(rqos
);
776 seq_printf(m
, "%u\n", rwb
->wb_normal
);
780 static int wbt_background_show(void *data
, struct seq_file
*m
)
782 struct rq_qos
*rqos
= data
;
783 struct rq_wb
*rwb
= RQWB(rqos
);
785 seq_printf(m
, "%u\n", rwb
->wb_background
);
789 static const struct blk_mq_debugfs_attr wbt_debugfs_attrs
[] = {
790 {"curr_win_nsec", 0400, wbt_curr_win_nsec_show
},
791 {"enabled", 0400, wbt_enabled_show
},
792 {"id", 0400, wbt_id_show
},
793 {"inflight", 0400, wbt_inflight_show
},
794 {"min_lat_nsec", 0400, wbt_min_lat_nsec_show
},
795 {"unknown_cnt", 0400, wbt_unknown_cnt_show
},
796 {"wb_normal", 0400, wbt_normal_show
},
797 {"wb_background", 0400, wbt_background_show
},
802 static struct rq_qos_ops wbt_rqos_ops
= {
803 .throttle
= wbt_wait
,
806 .requeue
= wbt_requeue
,
808 .cleanup
= wbt_cleanup
,
809 .queue_depth_changed
= wbt_queue_depth_changed
,
811 #ifdef CONFIG_BLK_DEBUG_FS
812 .debugfs_attrs
= wbt_debugfs_attrs
,
816 int wbt_init(struct request_queue
*q
)
821 rwb
= kzalloc(sizeof(*rwb
), GFP_KERNEL
);
825 rwb
->cb
= blk_stat_alloc_callback(wb_timer_fn
, wbt_data_dir
, 2, rwb
);
831 for (i
= 0; i
< WBT_NUM_RWQ
; i
++)
832 rq_wait_init(&rwb
->rq_wait
[i
]);
834 rwb
->rqos
.id
= RQ_QOS_WBT
;
835 rwb
->rqos
.ops
= &wbt_rqos_ops
;
837 rwb
->last_comp
= rwb
->last_issue
= jiffies
;
838 rwb
->win_nsec
= RWB_WINDOW_NSEC
;
839 rwb
->enable_state
= WBT_STATE_ON_DEFAULT
;
841 rwb
->rq_depth
.default_depth
= RWB_DEF_DEPTH
;
844 * Assign rwb and add the stats callback.
846 rq_qos_add(q
, &rwb
->rqos
);
847 blk_stat_add_callback(q
, rwb
->cb
);
849 rwb
->min_lat_nsec
= wbt_default_latency_nsec(q
);
851 wbt_queue_depth_changed(&rwb
->rqos
);
852 wbt_set_write_cache(q
, test_bit(QUEUE_FLAG_WC
, &q
->queue_flags
));