* [PATCH 0/2] ublk: complete requests via blk_mq_complete_request() so rq_affinity applies
@ 2026-10-06 8:55 Yoav Cohen
2026-10-06 8:55 ` [PATCH 1/2] " Yoav Cohen
2026-10-06 8:55 ` [PATCH 2/2] selftests: ublk: add test for rq_affinity completion redirection Yoav Cohen
0 siblings, 2 replies; 4+ messages in thread
From: Yoav Cohen @ 2026-10-06 8:55 UTC (permalink / raw)
To: Ming Lei, Jens Axboe, linux-block, csander; +Cc: jholzman, omril
ublk ends successfully completed requests inline on the server thread,
so rq_affinity never redirects completion work back to the submitting
CPU the way it does for NVMe, SCSI, virtio-blk, loop, nbd and rnbd.
Patch 1 fixes that and adds a feature flag so servers can detect
support. Patch 2 adds a selftest for it.
With an SPDK-style polling user-space block server at queue depth 96,
rq_affinity=2 gives 7-8% higher IOPS than today's always-inline
completion by offloading a saturated server CPU; rq_affinity=0 keeps
the old behavior for anyone who wants to opt out.
Yoav Cohen (2):
ublk: complete requests via blk_mq_complete_request() so rq_affinity
applies
selftests: ublk: add test for rq_affinity completion redirection
drivers/block/ublk_drv.c | 72 +++++++++++++++++--------
include/uapi/linux/ublk_cmd.h | 8 +++
tools/testing/selftests/ublk/Makefile | 1 +
tools/testing/selftests/ublk/kublk.c | 1 +
tools/testing/selftests/ublk/test_generic_18.sh | 42 +++++++++++++++
5 files changed, 103 insertions(+), 21 deletions(-)
create mode 100755 tools/testing/selftests/ublk/test_generic_18.sh
--
2.50.1 (Apple Git-155)
^ permalink raw reply [flat|nested] 4+ messages in thread* [PATCH 1/2] ublk: complete requests via blk_mq_complete_request() so rq_affinity applies 2026-10-06 8:55 [PATCH 0/2] ublk: complete requests via blk_mq_complete_request() so rq_affinity applies Yoav Cohen @ 2026-10-06 8:55 ` Yoav Cohen 2026-10-06 14:56 ` Ming Lei 2026-10-06 8:55 ` [PATCH 2/2] selftests: ublk: add test for rq_affinity completion redirection Yoav Cohen 1 sibling, 1 reply; 4+ messages in thread From: Yoav Cohen @ 2026-10-06 8:55 UTC (permalink / raw) To: Ming Lei, Jens Axboe, linux-block, csander; +Cc: jholzman, omril, Yoav Cohen ublk ends successfully completed requests inline on the server thread that issued UBLK_IO_COMMIT_AND_FETCH_REQ, so rq_affinity never redirects completion back to the submitting CPU, unlike NVMe, SCSI, virtio-blk, loop, nbd and rnbd. Route completion through blk_mq_complete_request_remote(), with a new ->complete() callback, ublk_end_rq(). Add UBLK_F_SUPPORT_RQ_AFFINITY so servers can detect support via UBLK_CMD_GET_FEATURES. Signed-off-by: Yoav Cohen <yoav@nvidia.com> --- drivers/block/ublk_drv.c | 72 +++++++++++++++++++++++++---------- include/uapi/linux/ublk_cmd.h | 8 ++++ 2 files changed, 59 insertions(+), 21 deletions(-) diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c index 66eb55e7162e..48bdb5d782ee 100644 --- a/drivers/block/ublk_drv.c +++ b/drivers/block/ublk_drv.c @@ -90,7 +90,8 @@ | UBLK_F_BATCH_IO \ | UBLK_F_NO_AUTO_PART_SCAN \ | UBLK_F_SHMEM_ZC \ - | UBLK_F_IO_DESC_SIZE) + | UBLK_F_IO_DESC_SIZE \ + | UBLK_F_SUPPORT_RQ_AFFINITY) #define UBLK_F_ALL_RECOVERY_FLAGS (UBLK_F_USER_RECOVERY \ | UBLK_F_USER_RECOVERY_REISSUE \ @@ -1549,13 +1550,54 @@ static void ublk_end_request(struct request *req, blk_status_t error) local_bh_enable(); } +/* + * Update @req with its result and requeue it if it was only partially + * completed. Returns true if @req was requeued, in which case the caller + * must not touch it any further. + * + * Run bio->bi_end_io() with softirqs disabled. If the final fput happens + * off this path, then that will prevent ublk's blkdev_release() from + * being called on current's task work, see fput() implementation. + * + * This matters for the caller completing @req locally on the ublk + * server's own thread: it may already be holding disk->open_mutex, e.g. + * reading the partition table from bdev_open(), and an fput() running + * inline there could deadlock on it. Preferably we would not be doing + * IO with a mutex held that is also used for release, but this + * work-around will suffice for now. A caller reached instead via + * blk_mq_complete_request_remote()'s softirq/IPI redirect never runs on + * that thread, so it isn't exposed to this hazard, but disabling + * softirqs here is harmless for it too. + */ +static inline bool ublk_update_and_requeue(struct request *req, + struct ublk_io *io) +{ + bool requeue; + + local_bh_disable(); + requeue = blk_update_request(req, BLK_STS_OK, io->res); + local_bh_enable(); + if (requeue) + blk_mq_requeue_request(req, true); + return requeue; +} + +static void ublk_end_rq(struct request *req) +{ + struct ublk_queue *ubq = req->mq_hctx->driver_data; + struct ublk_io *io = &ubq->ios[req->tag]; + + if (!ublk_update_and_requeue(req, io) && + likely(!blk_should_fake_timeout(req->q))) + __blk_mq_end_request(req, BLK_STS_OK); +} + /* todo: handle partial completion */ static inline void __ublk_complete_rq(struct request *req, struct ublk_io *io, bool need_map, struct io_comp_batch *iob) { unsigned int unmapped_bytes; blk_status_t res = BLK_STS_OK; - bool requeue; /* failed read IO if nothing is read */ if (!io->res && req_op(req) == REQ_OP_READ) @@ -1588,25 +1630,11 @@ static inline void __ublk_complete_rq(struct request *req, struct ublk_io *io, io->res = unmapped_bytes; } - /* - * Run bio->bi_end_io() with softirqs disabled. If the final fput - * happens off this path, then that will prevent ublk's blkdev_release() - * from being called on current's task work, see fput() implementation. - * - * Otherwise, ublk server may not provide forward progress in case of - * reading the partition table from bdev_open() with disk->open_mutex - * held, and causes dead lock as we could already be holding - * disk->open_mutex here. - * - * Preferably we would not be doing IO with a mutex held that is also - * used for release, but this work-around will suffice for now. - */ - local_bh_disable(); - requeue = blk_update_request(req, BLK_STS_OK, io->res); - local_bh_enable(); - if (requeue) - blk_mq_requeue_request(req, true); - else if (likely(!blk_should_fake_timeout(req->q))) { + if (blk_mq_complete_request_remote(req)) + return; + + if (!ublk_update_and_requeue(req, io) && + likely(!blk_should_fake_timeout(req->q))) { if (blk_mq_add_to_batch(req, iob, false, blk_mq_end_request_batch)) return; __blk_mq_end_request(req, BLK_STS_OK); @@ -2347,6 +2375,7 @@ static const struct blk_mq_ops ublk_mq_ops = { .queue_rqs = ublk_queue_rqs, .init_hctx = ublk_init_hctx, .timeout = ublk_timeout, + .complete = ublk_end_rq, }; static const struct blk_mq_ops ublk_batch_mq_ops = { @@ -2355,6 +2384,7 @@ static const struct blk_mq_ops ublk_batch_mq_ops = { .queue_rqs = ublk_batch_queue_rqs, .init_hctx = ublk_init_hctx, .timeout = ublk_timeout, + .complete = ublk_end_rq, }; static void ublk_queue_reinit(struct ublk_device *ub, struct ublk_queue *ubq) diff --git a/include/uapi/linux/ublk_cmd.h b/include/uapi/linux/ublk_cmd.h index 33b25dd13965..47b5fba9519c 100644 --- a/include/uapi/linux/ublk_cmd.h +++ b/include/uapi/linux/ublk_cmd.h @@ -420,6 +420,14 @@ struct ublk_shmem_buf_reg { /* ublksrv_io_desc size is specified by ublksrv_ctrl_dev_info's io_desc_size */ #define UBLK_F_IO_DESC_SIZE (1ULL << 20) +/* + * Request completion honors the block device's rq_affinity setting + * (/sys/block/ublkbN/queue/rq_affinity): the submitting CPU's completion + * work can run there instead of always on the ublk server's CPU. Without + * this feature, rq_affinity has no effect on ublk devices. + */ +#define UBLK_F_SUPPORT_RQ_AFFINITY (1ULL << 21) + /* device state */ #define UBLK_S_DEV_DEAD 0 #define UBLK_S_DEV_LIVE 1 -- 2.50.1 (Apple Git-155) ^ permalink raw reply related [flat|nested] 4+ messages in thread
* Re: [PATCH 1/2] ublk: complete requests via blk_mq_complete_request() so rq_affinity applies 2026-10-06 8:55 ` [PATCH 1/2] " Yoav Cohen @ 2026-10-06 14:56 ` Ming Lei 0 siblings, 0 replies; 4+ messages in thread From: Ming Lei @ 2026-10-06 14:56 UTC (permalink / raw) To: Yoav Cohen; +Cc: Jens Axboe, linux-block, csander, jholzman, omril On Tue, Oct 06, 2026 at 11:55:16AM +0300, Yoav Cohen wrote: > ublk ends successfully completed requests inline on the server thread > that issued UBLK_IO_COMMIT_AND_FETCH_REQ, so rq_affinity never > redirects completion back to the submitting CPU, unlike NVMe, SCSI, > virtio-blk, loop, nbd and rnbd. > > Route completion through blk_mq_complete_request_remote(), with a new > ->complete() callback, ublk_end_rq(). Add UBLK_F_SUPPORT_RQ_AFFINITY > so servers can detect support via UBLK_CMD_GET_FEATURES. > > Signed-off-by: Yoav Cohen <yoav@nvidia.com> > --- > drivers/block/ublk_drv.c | 72 +++++++++++++++++++++++++---------- > include/uapi/linux/ublk_cmd.h | 8 ++++ > 2 files changed, 59 insertions(+), 21 deletions(-) > > diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c > index 66eb55e7162e..48bdb5d782ee 100644 > --- a/drivers/block/ublk_drv.c > +++ b/drivers/block/ublk_drv.c > @@ -90,7 +90,8 @@ > | UBLK_F_BATCH_IO \ > | UBLK_F_NO_AUTO_PART_SCAN \ > | UBLK_F_SHMEM_ZC \ > - | UBLK_F_IO_DESC_SIZE) > + | UBLK_F_IO_DESC_SIZE \ > + | UBLK_F_SUPPORT_RQ_AFFINITY) > > #define UBLK_F_ALL_RECOVERY_FLAGS (UBLK_F_USER_RECOVERY \ > | UBLK_F_USER_RECOVERY_REISSUE \ > @@ -1549,13 +1550,54 @@ static void ublk_end_request(struct request *req, blk_status_t error) > local_bh_enable(); > } > > +/* > + * Update @req with its result and requeue it if it was only partially > + * completed. Returns true if @req was requeued, in which case the caller > + * must not touch it any further. > + * > + * Run bio->bi_end_io() with softirqs disabled. If the final fput happens > + * off this path, then that will prevent ublk's blkdev_release() from > + * being called on current's task work, see fput() implementation. > + * > + * This matters for the caller completing @req locally on the ublk > + * server's own thread: it may already be holding disk->open_mutex, e.g. > + * reading the partition table from bdev_open(), and an fput() running > + * inline there could deadlock on it. Preferably we would not be doing > + * IO with a mutex held that is also used for release, but this > + * work-around will suffice for now. A caller reached instead via > + * blk_mq_complete_request_remote()'s softirq/IPI redirect never runs on > + * that thread, so it isn't exposed to this hazard, but disabling > + * softirqs here is harmless for it too. > + */ > +static inline bool ublk_update_and_requeue(struct request *req, > + struct ublk_io *io) > +{ > + bool requeue; > + > + local_bh_disable(); > + requeue = blk_update_request(req, BLK_STS_OK, io->res); > + local_bh_enable(); > + if (requeue) > + blk_mq_requeue_request(req, true); > + return requeue; > +} > + > +static void ublk_end_rq(struct request *req) > +{ > + struct ublk_queue *ubq = req->mq_hctx->driver_data; > + struct ublk_io *io = &ubq->ios[req->tag]; > + > + if (!ublk_update_and_requeue(req, io) && > + likely(!blk_should_fake_timeout(req->q))) > + __blk_mq_end_request(req, BLK_STS_OK); > +} > + > /* todo: handle partial completion */ > static inline void __ublk_complete_rq(struct request *req, struct ublk_io *io, > bool need_map, struct io_comp_batch *iob) > { > unsigned int unmapped_bytes; > blk_status_t res = BLK_STS_OK; > - bool requeue; > > /* failed read IO if nothing is read */ > if (!io->res && req_op(req) == REQ_OP_READ) > @@ -1588,25 +1630,11 @@ static inline void __ublk_complete_rq(struct request *req, struct ublk_io *io, > io->res = unmapped_bytes; > } > > - /* > - * Run bio->bi_end_io() with softirqs disabled. If the final fput > - * happens off this path, then that will prevent ublk's blkdev_release() > - * from being called on current's task work, see fput() implementation. > - * > - * Otherwise, ublk server may not provide forward progress in case of > - * reading the partition table from bdev_open() with disk->open_mutex > - * held, and causes dead lock as we could already be holding > - * disk->open_mutex here. > - * > - * Preferably we would not be doing IO with a mutex held that is also > - * used for release, but this work-around will suffice for now. > - */ > - local_bh_disable(); > - requeue = blk_update_request(req, BLK_STS_OK, io->res); > - local_bh_enable(); > - if (requeue) > - blk_mq_requeue_request(req, true); > - else if (likely(!blk_should_fake_timeout(req->q))) { > + if (blk_mq_complete_request_remote(req)) > + return; Only copy-mode READs get the new completion path. In __ublk_complete_rq(), anything that needs no copy back to the request jumps to exit: ublk_end_request(), which completes inline as before: - all WRITEs, flush and discard; - all USER_COPY, ZERO_COPY, AUTO_BUF_REG and shmem zero-copy I/O; - every error. Only a successful READ in copy mode reaches the new blk_mq_complete_request_remote() call. So the uapi comment "Request completion honors … rq_affinity" is false for most I/O. > + > + if (!ublk_update_and_requeue(req, io) && > + likely(!blk_should_fake_timeout(req->q))) { > if (blk_mq_add_to_batch(req, iob, false, blk_mq_end_request_batch)) > return; > __blk_mq_end_request(req, BLK_STS_OK); > @@ -2347,6 +2375,7 @@ static const struct blk_mq_ops ublk_mq_ops = { > .queue_rqs = ublk_queue_rqs, > .init_hctx = ublk_init_hctx, > .timeout = ublk_timeout, > + .complete = ublk_end_rq, > }; > > static const struct blk_mq_ops ublk_batch_mq_ops = { > @@ -2355,6 +2384,7 @@ static const struct blk_mq_ops ublk_batch_mq_ops = { > .queue_rqs = ublk_batch_queue_rqs, > .init_hctx = ublk_init_hctx, > .timeout = ublk_timeout, > + .complete = ublk_end_rq, > }; > > static void ublk_queue_reinit(struct ublk_device *ub, struct ublk_queue *ubq) > diff --git a/include/uapi/linux/ublk_cmd.h b/include/uapi/linux/ublk_cmd.h > index 33b25dd13965..47b5fba9519c 100644 > --- a/include/uapi/linux/ublk_cmd.h > +++ b/include/uapi/linux/ublk_cmd.h > @@ -420,6 +420,14 @@ struct ublk_shmem_buf_reg { > /* ublksrv_io_desc size is specified by ublksrv_ctrl_dev_info's io_desc_size */ > #define UBLK_F_IO_DESC_SIZE (1ULL << 20) > > +/* > + * Request completion honors the block device's rq_affinity setting > + * (/sys/block/ublkbN/queue/rq_affinity): the submitting CPU's completion > + * work can run there instead of always on the ublk server's CPU. Without > + * this feature, rq_affinity has no effect on ublk devices. > + */ > +#define UBLK_F_SUPPORT_RQ_AFFINITY (1ULL << 21) This flag should be checked somewhere given it is passed from userspace, otherwise it can be nop. Thanks, Ming ^ permalink raw reply [flat|nested] 4+ messages in thread
* [PATCH 2/2] selftests: ublk: add test for rq_affinity completion redirection 2026-10-06 8:55 [PATCH 0/2] ublk: complete requests via blk_mq_complete_request() so rq_affinity applies Yoav Cohen 2026-10-06 8:55 ` [PATCH 1/2] " Yoav Cohen @ 2026-10-06 8:55 ` Yoav Cohen 1 sibling, 0 replies; 4+ messages in thread From: Yoav Cohen @ 2026-10-06 8:55 UTC (permalink / raw) To: Ming Lei, Jens Axboe, linux-block, csander; +Cc: jholzman, omril, Yoav Cohen Add UBLK_F_SUPPORT_RQ_AFFINITY to kublk's feature table, and add test_generic_18.sh: writes and verifies data on a loop device with I/O pinned to one CPU while cycling rq_affinity through 0, 1 and 2. Skipped on kernels without UBLK_F_SUPPORT_RQ_AFFINITY. Signed-off-by: Yoav Cohen <yoav@nvidia.com> --- tools/testing/selftests/ublk/Makefile | 1 + tools/testing/selftests/ublk/kublk.c | 1 + .../testing/selftests/ublk/test_generic_18.sh | 42 +++++++++++++++++++ 3 files changed, 44 insertions(+) create mode 100755 tools/testing/selftests/ublk/test_generic_18.sh diff --git a/tools/testing/selftests/ublk/Makefile b/tools/testing/selftests/ublk/Makefile index 37883e9d50ec..d12e9879b89a 100644 --- a/tools/testing/selftests/ublk/Makefile +++ b/tools/testing/selftests/ublk/Makefile @@ -19,6 +19,7 @@ TEST_PROGS += test_generic_12.sh TEST_PROGS += test_generic_13.sh TEST_PROGS += test_generic_16.sh TEST_PROGS += test_generic_17.sh +TEST_PROGS += test_generic_18.sh TEST_PROGS += test_batch_01.sh TEST_PROGS += test_batch_02.sh diff --git a/tools/testing/selftests/ublk/kublk.c b/tools/testing/selftests/ublk/kublk.c index 2400b4615766..c750e10a69bf 100644 --- a/tools/testing/selftests/ublk/kublk.c +++ b/tools/testing/selftests/ublk/kublk.c @@ -2083,6 +2083,7 @@ static int cmd_dev_get_features(void) FEAT_NAME(UBLK_F_NO_AUTO_PART_SCAN), FEAT_NAME(UBLK_F_SHMEM_ZC), FEAT_NAME(UBLK_F_IO_DESC_SIZE), + FEAT_NAME(UBLK_F_SUPPORT_RQ_AFFINITY), }; struct ublk_dev *dev; __u64 features = 0; diff --git a/tools/testing/selftests/ublk/test_generic_18.sh b/tools/testing/selftests/ublk/test_generic_18.sh new file mode 100755 index 000000000000..5afb202f1ef5 --- /dev/null +++ b/tools/testing/selftests/ublk/test_generic_18.sh @@ -0,0 +1,42 @@ +#!/bin/bash +# SPDX-License-Identifier: GPL-2.0 + +. "$(cd "$(dirname "$0")" && pwd)"/test_common.sh + +ERR_CODE=0 + +if ! _have_program fio; then + exit "$UBLK_SKIP_CODE" +fi + +if ! _have_feature "SUPPORT_RQ_AFFINITY"; then + exit "$UBLK_SKIP_CODE" +fi + +_prep_test "loop" "write and verify I/O across rq_affinity settings" + +_create_backfile 0 256M + +dev_id=$(_add_ublk_dev -t loop "${UBLK_BACKFILES[0]}") +_check_add_dev $TID $? + +rq_affinity_path=/sys/block/ublkb"${dev_id}"/queue/rq_affinity + +for affinity in 0 1 2; do + echo "${affinity}" > "${rq_affinity_path}" + + # Pin submission to a single CPU so rq_affinity has somewhere to + # redirect completion to. + taskset -c 0 fio --name=verify --rw=randwrite --direct=1 \ + --ioengine=libaio --bs=8k --iodepth=32 --verify=crc32c \ + --do_verify=1 --verify_state_save=0 \ + --filename=/dev/ublkb"${dev_id}" --size=256M > /dev/null + if [ $? -ne 0 ]; then + echo "fio verify failed with rq_affinity=${affinity}" + ERR_CODE=255 + break + fi +done + +_cleanup_test +_show_result $TID $ERR_CODE -- 2.50.1 (Apple Git-155) ^ permalink raw reply related [flat|nested] 4+ messages in thread
end of thread, other threads:[~2026-10-06 14:56 UTC | newest] Thread overview: 4+ messages (download: mbox.gz follow: Atom feed -- links below jump to the message on this page -- 2026-10-06 8:55 [PATCH 0/2] ublk: complete requests via blk_mq_complete_request() so rq_affinity applies Yoav Cohen 2026-10-06 8:55 ` [PATCH 1/2] " Yoav Cohen 2026-10-06 14:56 ` Ming Lei 2026-10-06 8:55 ` [PATCH 2/2] selftests: ublk: add test for rq_affinity completion redirection Yoav Cohen
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