* Re: [PATCH] blk-mq: reinsert cached request to the list
2026-05-25 16:07 [PATCH] blk-mq: reinsert cached request to the list Keith Busch
@ 2026-05-26 1:44 ` Ming Lei
2026-05-26 14:02 ` Keith Busch
2026-05-26 13:50 ` kernel test robot
2026-05-26 15:02 ` kernel test robot
2 siblings, 1 reply; 5+ messages in thread
From: Ming Lei @ 2026-05-26 1:44 UTC (permalink / raw)
To: Keith Busch; +Cc: linux-block, axboe, Keith Busch, Christoph Hellwig
On Mon, May 25, 2026 at 09:07:44AM -0700, Keith Busch wrote:
> From: Keith Busch <kbusch@kernel.org>
>
> A previous commit removed an optimization out of caution for a scenario
> that turns out not to be real: all the "queue_exit" goto's are safe to
> reinsert the request into the cached_rq's plug list as they are either
> from a non-blocking path, or a successful merge that already holds the
> queue reference. This optimization is most needed for small sequential
> workloads that successfully merge into larger requests.
>
> Fixes: dc278e9bf2b9 ("blk-mq: pop cached request if it is usable")
> Suggested-by: Ming Lei <tom.leiming@gmail.com>
> Suggested-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Keith Busch <kbusch@kernel.org>
> ---
> block/blk-mq.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 28c2d931e75ea..72c8ac805882c 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -3246,7 +3246,7 @@ void blk_mq_submit_bio(struct bio *bio)
> if (!rq)
> blk_queue_exit(q);
> else
> - blk_mq_free_request(rq);
> + rq_list_push(&plug->cached_rqs, rq);
rq_list_add_head()?
Thanks,
Ming
^ permalink raw reply [flat|nested] 5+ messages in thread* Re: [PATCH] blk-mq: reinsert cached request to the list
2026-05-26 1:44 ` Ming Lei
@ 2026-05-26 14:02 ` Keith Busch
0 siblings, 0 replies; 5+ messages in thread
From: Keith Busch @ 2026-05-26 14:02 UTC (permalink / raw)
To: Ming Lei; +Cc: Keith Busch, linux-block, axboe, Christoph Hellwig
On Mon, May 25, 2026 at 08:44:07PM -0500, Ming Lei wrote:
> On Mon, May 25, 2026 at 09:07:44AM -0700, Keith Busch wrote:
> > + rq_list_push(&plug->cached_rqs, rq);
>
> rq_list_add_head()?
Yes indeed. Serves me right for trying to squeeze this in over a
holiday. Thanks.
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] blk-mq: reinsert cached request to the list
2026-05-25 16:07 [PATCH] blk-mq: reinsert cached request to the list Keith Busch
2026-05-26 1:44 ` Ming Lei
@ 2026-05-26 13:50 ` kernel test robot
2026-05-26 15:02 ` kernel test robot
2 siblings, 0 replies; 5+ messages in thread
From: kernel test robot @ 2026-05-26 13:50 UTC (permalink / raw)
To: Keith Busch, linux-block, axboe
Cc: oe-kbuild-all, Keith Busch, Ming Lei, Christoph Hellwig
Hi Keith,
kernel test robot noticed the following build errors:
[auto build test ERROR on axboe/for-next]
[also build test ERROR on next-20260525]
[cannot apply to linus/master v6.16-rc1]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]
url: https://github.com/intel-lab-lkp/linux/commits/Keith-Busch/blk-mq-reinsert-cached-request-to-the-list/20260526-000916
base: https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux.git for-next
patch link: https://lore.kernel.org/r/20260525160744.896047-1-kbusch%40meta.com
patch subject: [PATCH] blk-mq: reinsert cached request to the list
config: i386-allnoconfig-bpf (https://download.01.org/0day-ci/archive/20260526/202605261526.40AHANmH-lkp@intel.com/config)
compiler: gcc-14 (Debian 14.2.0-19) 14.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260526/202605261526.40AHANmH-lkp@intel.com/reproduce)
If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202605261526.40AHANmH-lkp@intel.com/
All errors (new ones prefixed by >>):
block/blk-mq.c: In function 'blk_mq_submit_bio':
>> block/blk-mq.c:3249:17: error: implicit declaration of function 'rq_list_push'; did you mean 'rq_list_peek'? [-Wimplicit-function-declaration]
3249 | rq_list_push(&plug->cached_rqs, rq);
| ^~~~~~~~~~~~
| rq_list_peek
vim +3249 block/blk-mq.c
3110
3111 /**
3112 * blk_mq_submit_bio - Create and send a request to block device.
3113 * @bio: Bio pointer.
3114 *
3115 * Builds up a request structure from @q and @bio and send to the device. The
3116 * request may not be queued directly to hardware if:
3117 * * This request can be merged with another one
3118 * * We want to place request at plug queue for possible future merging
3119 * * There is an IO scheduler active at this queue
3120 *
3121 * It will not queue the request if there is an error with the bio, or at the
3122 * request creation.
3123 */
3124 void blk_mq_submit_bio(struct bio *bio)
3125 {
3126 struct request_queue *q = bdev_get_queue(bio->bi_bdev);
3127 struct blk_plug *plug = current->plug;
3128 const int is_sync = op_is_sync(bio->bi_opf);
3129 unsigned int integrity_action;
3130 struct blk_mq_hw_ctx *hctx;
3131 unsigned int nr_segs;
3132 struct request *rq;
3133 blk_status_t ret;
3134
3135 /*
3136 * If the plug has a cached request for this queue, try to use it.
3137 */
3138 rq = blk_mq_get_cached_request(plug, q, bio->bi_opf);
3139
3140 /*
3141 * A BIO that was released from a zone write plug has already been
3142 * through the preparation in this function, already holds a reference
3143 * on the queue usage counter, and is the only write BIO in-flight for
3144 * the target zone. Go straight to preparing a request for it.
3145 */
3146 if (bio_zone_write_plugging(bio)) {
3147 nr_segs = bio->__bi_nr_segments;
3148 if (rq)
3149 blk_queue_exit(q);
3150 goto new_request;
3151 }
3152
3153 /*
3154 * The cached request already holds a q_usage_counter reference and we
3155 * don't have to acquire a new one if we use it.
3156 */
3157 if (!rq) {
3158 if (unlikely(bio_queue_enter(bio)))
3159 return;
3160 }
3161
3162 /*
3163 * Device reconfiguration may change logical block size or reduce the
3164 * number of poll queues, so the checks for alignment and poll support
3165 * have to be done with queue usage counter held.
3166 */
3167 if (unlikely(bio_unaligned(bio, q))) {
3168 bio_io_error(bio);
3169 goto queue_exit;
3170 }
3171
3172 if ((bio->bi_opf & REQ_POLLED) && !blk_mq_can_poll(q)) {
3173 bio->bi_status = BLK_STS_NOTSUPP;
3174 bio_endio(bio);
3175 goto queue_exit;
3176 }
3177
3178 bio = __bio_split_to_limits(bio, &q->limits, &nr_segs);
3179 if (!bio)
3180 goto queue_exit;
3181
3182 integrity_action = bio_integrity_action(bio);
3183 if (integrity_action)
3184 bio_integrity_prep(bio, integrity_action);
3185
3186 blk_mq_bio_issue_init(q, bio);
3187 if (blk_mq_attempt_bio_merge(q, bio, nr_segs))
3188 goto queue_exit;
3189
3190 if (bio_needs_zone_write_plugging(bio)) {
3191 if (blk_zone_plug_bio(bio, nr_segs))
3192 goto queue_exit;
3193 }
3194
3195 new_request:
3196 if (rq) {
3197 rq_qos_throttle(rq->q, bio);
3198 blk_mq_rq_time_init(rq, blk_time_get_ns());
3199 rq->cmd_flags = bio->bi_opf;
3200 INIT_LIST_HEAD(&rq->queuelist);
3201 } else {
3202 rq = blk_mq_get_new_requests(q, plug, bio);
3203 if (unlikely(!rq)) {
3204 if (bio->bi_opf & REQ_NOWAIT)
3205 bio_wouldblock_error(bio);
3206 goto queue_exit;
3207 }
3208 }
3209
3210 trace_block_getrq(bio);
3211
3212 rq_qos_track(q, rq, bio);
3213
3214 blk_mq_bio_to_request(rq, bio, nr_segs);
3215
3216 ret = blk_crypto_rq_get_keyslot(rq);
3217 if (ret != BLK_STS_OK) {
3218 bio->bi_status = ret;
3219 bio_endio(bio);
3220 blk_mq_free_request(rq);
3221 return;
3222 }
3223
3224 if (bio_zone_write_plugging(bio))
3225 blk_zone_write_plug_init_request(rq);
3226
3227 if (op_is_flush(bio->bi_opf) && blk_insert_flush(rq))
3228 return;
3229
3230 if (plug) {
3231 blk_add_rq_to_plug(plug, rq);
3232 return;
3233 }
3234
3235 hctx = rq->mq_hctx;
3236 if ((rq->rq_flags & RQF_USE_SCHED) ||
3237 (hctx->dispatch_busy && (q->nr_hw_queues == 1 || !is_sync))) {
3238 blk_mq_insert_request(rq, 0);
3239 blk_mq_run_hw_queue(hctx, true);
3240 } else {
3241 blk_mq_run_dispatch_ops(q, blk_mq_try_issue_directly(hctx, rq));
3242 }
3243 return;
3244
3245 queue_exit:
3246 if (!rq)
3247 blk_queue_exit(q);
3248 else
> 3249 rq_list_push(&plug->cached_rqs, rq);
3250 }
3251
--
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki
^ permalink raw reply [flat|nested] 5+ messages in thread* Re: [PATCH] blk-mq: reinsert cached request to the list
2026-05-25 16:07 [PATCH] blk-mq: reinsert cached request to the list Keith Busch
2026-05-26 1:44 ` Ming Lei
2026-05-26 13:50 ` kernel test robot
@ 2026-05-26 15:02 ` kernel test robot
2 siblings, 0 replies; 5+ messages in thread
From: kernel test robot @ 2026-05-26 15:02 UTC (permalink / raw)
To: Keith Busch, linux-block, axboe
Cc: llvm, oe-kbuild-all, Keith Busch, Ming Lei, Christoph Hellwig
Hi Keith,
kernel test robot noticed the following build errors:
[auto build test ERROR on axboe/for-next]
[also build test ERROR on next-20260525]
[cannot apply to linus/master v6.16-rc1]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]
url: https://github.com/intel-lab-lkp/linux/commits/Keith-Busch/blk-mq-reinsert-cached-request-to-the-list/20260526-000916
base: https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux.git for-next
patch link: https://lore.kernel.org/r/20260525160744.896047-1-kbusch%40meta.com
patch subject: [PATCH] blk-mq: reinsert cached request to the list
config: x86_64-rhel-9.4-rust (https://download.01.org/0day-ci/archive/20260526/202605261716.TITwjvlB-lkp@intel.com/config)
compiler: clang version 20.1.8 (https://github.com/llvm/llvm-project 87f0227cb60147a26a1eeb4fb06e3b505e9c7261)
rustc: rustc 1.88.0 (6b00bc388 2025-06-23)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260526/202605261716.TITwjvlB-lkp@intel.com/reproduce)
If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202605261716.TITwjvlB-lkp@intel.com/
All errors (new ones prefixed by >>):
>> block/blk-mq.c:3249:3: error: call to undeclared function 'rq_list_push'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
3249 | rq_list_push(&plug->cached_rqs, rq);
| ^
1 error generated.
vim +/rq_list_push +3249 block/blk-mq.c
3110
3111 /**
3112 * blk_mq_submit_bio - Create and send a request to block device.
3113 * @bio: Bio pointer.
3114 *
3115 * Builds up a request structure from @q and @bio and send to the device. The
3116 * request may not be queued directly to hardware if:
3117 * * This request can be merged with another one
3118 * * We want to place request at plug queue for possible future merging
3119 * * There is an IO scheduler active at this queue
3120 *
3121 * It will not queue the request if there is an error with the bio, or at the
3122 * request creation.
3123 */
3124 void blk_mq_submit_bio(struct bio *bio)
3125 {
3126 struct request_queue *q = bdev_get_queue(bio->bi_bdev);
3127 struct blk_plug *plug = current->plug;
3128 const int is_sync = op_is_sync(bio->bi_opf);
3129 unsigned int integrity_action;
3130 struct blk_mq_hw_ctx *hctx;
3131 unsigned int nr_segs;
3132 struct request *rq;
3133 blk_status_t ret;
3134
3135 /*
3136 * If the plug has a cached request for this queue, try to use it.
3137 */
3138 rq = blk_mq_get_cached_request(plug, q, bio->bi_opf);
3139
3140 /*
3141 * A BIO that was released from a zone write plug has already been
3142 * through the preparation in this function, already holds a reference
3143 * on the queue usage counter, and is the only write BIO in-flight for
3144 * the target zone. Go straight to preparing a request for it.
3145 */
3146 if (bio_zone_write_plugging(bio)) {
3147 nr_segs = bio->__bi_nr_segments;
3148 if (rq)
3149 blk_queue_exit(q);
3150 goto new_request;
3151 }
3152
3153 /*
3154 * The cached request already holds a q_usage_counter reference and we
3155 * don't have to acquire a new one if we use it.
3156 */
3157 if (!rq) {
3158 if (unlikely(bio_queue_enter(bio)))
3159 return;
3160 }
3161
3162 /*
3163 * Device reconfiguration may change logical block size or reduce the
3164 * number of poll queues, so the checks for alignment and poll support
3165 * have to be done with queue usage counter held.
3166 */
3167 if (unlikely(bio_unaligned(bio, q))) {
3168 bio_io_error(bio);
3169 goto queue_exit;
3170 }
3171
3172 if ((bio->bi_opf & REQ_POLLED) && !blk_mq_can_poll(q)) {
3173 bio->bi_status = BLK_STS_NOTSUPP;
3174 bio_endio(bio);
3175 goto queue_exit;
3176 }
3177
3178 bio = __bio_split_to_limits(bio, &q->limits, &nr_segs);
3179 if (!bio)
3180 goto queue_exit;
3181
3182 integrity_action = bio_integrity_action(bio);
3183 if (integrity_action)
3184 bio_integrity_prep(bio, integrity_action);
3185
3186 blk_mq_bio_issue_init(q, bio);
3187 if (blk_mq_attempt_bio_merge(q, bio, nr_segs))
3188 goto queue_exit;
3189
3190 if (bio_needs_zone_write_plugging(bio)) {
3191 if (blk_zone_plug_bio(bio, nr_segs))
3192 goto queue_exit;
3193 }
3194
3195 new_request:
3196 if (rq) {
3197 rq_qos_throttle(rq->q, bio);
3198 blk_mq_rq_time_init(rq, blk_time_get_ns());
3199 rq->cmd_flags = bio->bi_opf;
3200 INIT_LIST_HEAD(&rq->queuelist);
3201 } else {
3202 rq = blk_mq_get_new_requests(q, plug, bio);
3203 if (unlikely(!rq)) {
3204 if (bio->bi_opf & REQ_NOWAIT)
3205 bio_wouldblock_error(bio);
3206 goto queue_exit;
3207 }
3208 }
3209
3210 trace_block_getrq(bio);
3211
3212 rq_qos_track(q, rq, bio);
3213
3214 blk_mq_bio_to_request(rq, bio, nr_segs);
3215
3216 ret = blk_crypto_rq_get_keyslot(rq);
3217 if (ret != BLK_STS_OK) {
3218 bio->bi_status = ret;
3219 bio_endio(bio);
3220 blk_mq_free_request(rq);
3221 return;
3222 }
3223
3224 if (bio_zone_write_plugging(bio))
3225 blk_zone_write_plug_init_request(rq);
3226
3227 if (op_is_flush(bio->bi_opf) && blk_insert_flush(rq))
3228 return;
3229
3230 if (plug) {
3231 blk_add_rq_to_plug(plug, rq);
3232 return;
3233 }
3234
3235 hctx = rq->mq_hctx;
3236 if ((rq->rq_flags & RQF_USE_SCHED) ||
3237 (hctx->dispatch_busy && (q->nr_hw_queues == 1 || !is_sync))) {
3238 blk_mq_insert_request(rq, 0);
3239 blk_mq_run_hw_queue(hctx, true);
3240 } else {
3241 blk_mq_run_dispatch_ops(q, blk_mq_try_issue_directly(hctx, rq));
3242 }
3243 return;
3244
3245 queue_exit:
3246 if (!rq)
3247 blk_queue_exit(q);
3248 else
> 3249 rq_list_push(&plug->cached_rqs, rq);
3250 }
3251
--
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki
^ permalink raw reply [flat|nested] 5+ messages in thread