[RFC v3 0/9] fixed worker

[RFC v3 0/9] fixed worker

[Hao Xu]

This is the third version of fixed worker implementation.
Wrote a nop test program to test it, 3 fixed-workers VS 3 normal workers.
normal workers:
./run_nop_wqe.sh nop_wqe_normal 200000 100 3 1-3
        time spent: 10464397 usecs      IOPS: 1911242
        time spent: 9610976 usecs       IOPS: 2080954
        time spent: 9807361 usecs       IOPS: 2039284

fixed workers:
./run_nop_wqe.sh nop_wqe_fixed 200000 100 3 1-3
        time spent: 17314274 usecs      IOPS: 1155116
        time spent: 17016942 usecs      IOPS: 1175299
        time spent: 17908684 usecs      IOPS: 1116776

About 2x improvement. From perf result, almost no acct->lock contension.
Test program: https://github.com/HowHsu/liburing/tree/fixed_worker
liburing/test/nop_wqe.c

v2->v3:
 - change dispatch work strategy from random to round-robin

things to be done:
 - Still need some thinking about the work cancellation
 - not very sure IO_WORKER_F_EXIT is safe enough on synchronization
 - the iowq hash stuff is not compatible with fixed worker for now

Any comments are welcome. Thanks in advance.

Hao Xu (9):
  io-wq: add a worker flag for individual exit
  io-wq: change argument of create_io_worker() for convienence
  io-wq: add infra data structure for fixed workers
  io-wq: tweak io_get_acct()
  io-wq: fixed worker initialization
  io-wq: fixed worker exit
  io-wq: implement fixed worker logic
  io-wq: batch the handling of fixed worker private works
  io_uring: add register fixed worker interface

 fs/io-wq.c                    | 460 ++++++++++++++++++++++++++++++----
 fs/io-wq.h                    |   8 +
 fs/io_uring.c                 |  71 ++++++
 include/uapi/linux/io_uring.h |  11 +
 4 files changed, 501 insertions(+), 49 deletions(-)

-- 
2.36.0

[PATCH 1/9] io-wq: add a worker flag for individual exit

[Hao Xu]

From: Hao Xu <howeyxu@tencent.com>

Add a worker flag to control exit of an individual worker, this is
needed for fixed worker in the next patches but also as a generic
functionality.

Signed-off-by: Hao Xu <howeyxu@tencent.com>
---
 fs/io-wq.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/fs/io-wq.c b/fs/io-wq.c
index 824623bcf1a5..0c26805ca6de 100644
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -26,6 +26,7 @@ enum {
 	IO_WORKER_F_RUNNING	= 2,	/* account as running */
 	IO_WORKER_F_FREE	= 4,	/* worker on free list */
 	IO_WORKER_F_BOUND	= 8,	/* is doing bounded work */
+	IO_WORKER_F_EXIT	= 32,	/* worker is exiting */
 };
 
 enum {
@@ -639,8 +640,12 @@ static int io_wqe_worker(void *data)
 	while (!test_bit(IO_WQ_BIT_EXIT, &wq->state)) {
 		long ret;
 
+		if (worker->flags & IO_WORKER_F_EXIT)
+			break;
+
 		set_current_state(TASK_INTERRUPTIBLE);
-		while (io_acct_run_queue(acct))
+		while (!(worker->flags & IO_WORKER_F_EXIT) &&
+		       io_acct_run_queue(acct))
 			io_worker_handle_work(worker);
 
 		raw_spin_lock(&wqe->lock);
@@ -656,6 +661,10 @@ static int io_wqe_worker(void *data)
 		raw_spin_unlock(&wqe->lock);
 		if (io_flush_signals())
 			continue;
+		if (worker->flags & IO_WORKER_F_EXIT) {
+			__set_current_state(TASK_RUNNING);
+			break;
+		}
 		ret = schedule_timeout(WORKER_IDLE_TIMEOUT);
 		if (signal_pending(current)) {
 			struct ksignal ksig;
-- 
2.36.0

[PATCH 2/9] io-wq: change argument of create_io_worker() for convienence

[Hao Xu]

From: Hao Xu <howeyxu@tencent.com>

Change index to acct itself for create_io_worker() for convienence in
the next patches.

Signed-off-by: Hao Xu <howeyxu@tencent.com>
---
 fs/io-wq.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/fs/io-wq.c b/fs/io-wq.c
index 0c26805ca6de..35ce622f77ba 100644
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -139,7 +139,8 @@ struct io_cb_cancel_data {
 	bool cancel_all;
 };
 
-static bool create_io_worker(struct io_wq *wq, struct io_wqe *wqe, int index);
+static bool create_io_worker(struct io_wq *wq, struct io_wqe *wqe,
+			     struct io_wqe_acct *acct);
 static void io_wqe_dec_running(struct io_worker *worker);
 static bool io_acct_cancel_pending_work(struct io_wqe *wqe,
 					struct io_wqe_acct *acct,
@@ -306,7 +307,7 @@ static bool io_wqe_create_worker(struct io_wqe *wqe, struct io_wqe_acct *acct)
 	raw_spin_unlock(&wqe->lock);
 	atomic_inc(&acct->nr_running);
 	atomic_inc(&wqe->wq->worker_refs);
-	return create_io_worker(wqe->wq, wqe, acct->index);
+	return create_io_worker(wqe->wq, wqe, acct);
 }
 
 static void io_wqe_inc_running(struct io_worker *worker)
@@ -335,7 +336,7 @@ static void create_worker_cb(struct callback_head *cb)
 	}
 	raw_spin_unlock(&wqe->lock);
 	if (do_create) {
-		create_io_worker(wq, wqe, worker->create_index);
+		create_io_worker(wq, wqe, acct);
 	} else {
 		atomic_dec(&acct->nr_running);
 		io_worker_ref_put(wq);
@@ -812,9 +813,10 @@ static void io_workqueue_create(struct work_struct *work)
 		kfree(worker);
 }
 
-static bool create_io_worker(struct io_wq *wq, struct io_wqe *wqe, int index)
+static bool create_io_worker(struct io_wq *wq, struct io_wqe *wqe,
+			     struct io_wqe_acct *acct)
 {
-	struct io_wqe_acct *acct = &wqe->acct[index];
+	int index = acct->index;
 	struct io_worker *worker;
 	struct task_struct *tsk;
 
-- 
2.36.0

[PATCH 3/9] io-wq: add infra data structure for fixed workers

[Hao Xu]

From: Hao Xu <howeyxu@tencent.com>

Add data sttructure and basic initialization for fixed worker.

Signed-off-by: Hao Xu <howeyxu@tencent.com>
---
 fs/io-wq.c | 98 ++++++++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 87 insertions(+), 11 deletions(-)

diff --git a/fs/io-wq.c b/fs/io-wq.c
index 35ce622f77ba..ac8faf1f7a0a 100644
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -26,6 +26,7 @@ enum {
 	IO_WORKER_F_RUNNING	= 2,	/* account as running */
 	IO_WORKER_F_FREE	= 4,	/* worker on free list */
 	IO_WORKER_F_BOUND	= 8,	/* is doing bounded work */
+	IO_WORKER_F_FIXED	= 16,	/* is a fixed worker */
 	IO_WORKER_F_EXIT	= 32,	/* worker is exiting */
 };
 
@@ -37,6 +38,61 @@ enum {
 	IO_ACCT_STALLED_BIT	= 0,	/* stalled on hash */
 };
 
+struct io_wqe_acct {
+	/*
+	 * union {
+	 *	1) for normal worker
+	 *	struct {
+	 *		unsigned nr_workers;
+	 *		unsigned max_workers;
+	 *		struct io_wq_work_list work_list;
+	 *	};
+	 *	2) for fixed worker
+	 *	struct {
+	 *		unsigned nr_workers; // not meaningful
+	 *		unsigned max_workers; // not meaningful
+	 *		unsigned nr_fixed;
+	 *		unsigned max_works;
+	 *		struct io_worker **fixed_workers;
+	 *	};
+	 *	3) for fixed worker's private acct
+	 *	struct {
+	 *		unsigned nr_works;
+	 *		unsigned max_works;
+	 *		struct io_wq_work_list work_list;
+	 *	};
+	 *};
+	 */
+	union {
+		unsigned nr_workers;
+		unsigned nr_works;
+	};
+	unsigned max_workers;
+	unsigned nr_fixed;
+	unsigned max_works;
+	union {
+		struct io_wq_work_list work_list;
+		struct io_worker **fixed_workers;
+	};
+	/*
+	 * nr_running is not meaningful for fixed worker
+	 * but still keep the same logic for it for the
+	 * convinence for now. So do nr_workers and
+	 * max_workers.
+	 */
+	atomic_t nr_running;
+	/*
+	 * For 1), it protects the work_list, the other two member nr_workers
+	 * and max_workers are protected by wqe->lock.
+	 * For 2), it protects nr_fixed, max_works, fixed_workers
+	 * For 3), it protects nr_works, max_works and work_list.
+	 */
+	raw_spinlock_t lock;
+	int index;
+	unsigned long flags;
+	bool fixed_worker_registered;
+};
+
 /*
  * One for each thread in a wqe pool
  */
@@ -62,6 +118,8 @@ struct io_worker {
 		struct rcu_head rcu;
 		struct work_struct work;
 	};
+	int index;
+	struct io_wqe_acct acct;
 };
 
 #if BITS_PER_LONG == 64
@@ -72,16 +130,6 @@ struct io_worker {
 
 #define IO_WQ_NR_HASH_BUCKETS	(1u << IO_WQ_HASH_ORDER)
 
-struct io_wqe_acct {
-	unsigned nr_workers;
-	unsigned max_workers;
-	int index;
-	atomic_t nr_running;
-	raw_spinlock_t lock;
-	struct io_wq_work_list work_list;
-	unsigned long flags;
-};
-
 enum {
 	IO_WQ_ACCT_BOUND,
 	IO_WQ_ACCT_UNBOUND,
@@ -94,6 +142,7 @@ enum {
 struct io_wqe {
 	raw_spinlock_t lock;
 	struct io_wqe_acct acct[IO_WQ_ACCT_NR];
+	struct io_wqe_acct fixed_acct[IO_WQ_ACCT_NR];
 
 	int node;
 
@@ -1205,6 +1254,31 @@ struct io_wq *io_wq_create(unsigned bounded, struct io_wq_data *data)
 			atomic_set(&acct->nr_running, 0);
 			INIT_WQ_LIST(&acct->work_list);
 			raw_spin_lock_init(&acct->lock);
+
+			acct = &wqe->fixed_acct[i];
+			acct->index = i;
+			INIT_WQ_LIST(&acct->work_list);
+			raw_spin_lock_init(&acct->lock);
+			/*
+			 * nr_running for a fixed worker is meaningless
+			 * for now, init it to 1 to wround around the
+			 * io_wqe_dec_running logic
+			 */
+			atomic_set(&acct->nr_running, 1);
+			/*
+			 * max_workers for a fixed worker is meaningless
+			 * for now, init it so since number of fixed workers
+			 * should be controlled by users.
+			 */
+			acct->max_workers = task_rlimit(current, RLIMIT_NPROC);
+			raw_spin_lock_init(&acct->lock);
+			/*
+			 * For fixed worker, not necessary
+			 * but do it explicitly for clearity
+			 */
+			acct->nr_fixed = 0;
+			acct->max_works = 0;
+			acct->fixed_workers = NULL;
 		}
 		wqe->wq = wq;
 		raw_spin_lock_init(&wqe->lock);
@@ -1287,7 +1361,7 @@ static void io_wq_exit_workers(struct io_wq *wq)
 
 static void io_wq_destroy(struct io_wq *wq)
 {
-	int node;
+	int i, node;
 
 	cpuhp_state_remove_instance_nocalls(io_wq_online, &wq->cpuhp_node);
 
@@ -1299,6 +1373,8 @@ static void io_wq_destroy(struct io_wq *wq)
 		};
 		io_wqe_cancel_pending_work(wqe, &match);
 		free_cpumask_var(wqe->cpu_mask);
+		for (i = 0; i < IO_WQ_ACCT_NR; i++)
+			kfree(wqe->fixed_acct[i].fixed_workers);
 		kfree(wqe);
 	}
 	io_wq_put_hash(wq->hash);
-- 
2.36.0

[PATCH 4/9] io-wq: tweak io_get_acct()

[Hao Xu]

From: Hao Xu <howeyxu@tencent.com>

Add an argument for io_get_acct() to indicate fixed or normal worker

Signed-off-by: Hao Xu <howeyxu@tencent.com>
---
 fs/io-wq.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/fs/io-wq.c b/fs/io-wq.c
index ac8faf1f7a0a..c67bd5e5d117 100644
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -208,20 +208,24 @@ static void io_worker_release(struct io_worker *worker)
 		complete(&worker->ref_done);
 }
 
-static inline struct io_wqe_acct *io_get_acct(struct io_wqe *wqe, bool bound)
+static inline struct io_wqe_acct *io_get_acct(struct io_wqe *wqe, bool bound,
+					      bool fixed)
 {
-	return &wqe->acct[bound ? IO_WQ_ACCT_BOUND : IO_WQ_ACCT_UNBOUND];
+	unsigned index = bound ? IO_WQ_ACCT_BOUND : IO_WQ_ACCT_UNBOUND;
+
+	return fixed ? &wqe->fixed_acct[index] : &wqe->acct[index];
 }
 
 static inline struct io_wqe_acct *io_work_get_acct(struct io_wqe *wqe,
 						   struct io_wq_work *work)
 {
-	return io_get_acct(wqe, !(work->flags & IO_WQ_WORK_UNBOUND));
+	return io_get_acct(wqe, !(work->flags & IO_WQ_WORK_UNBOUND), false);
 }
 
 static inline struct io_wqe_acct *io_wqe_get_acct(struct io_worker *worker)
 {
-	return io_get_acct(worker->wqe, worker->flags & IO_WORKER_F_BOUND);
+	return io_get_acct(worker->wqe, worker->flags & IO_WORKER_F_BOUND,
+			   worker->flags & IO_WORKER_F_FIXED);
 }
 
 static void io_worker_ref_put(struct io_wq *wq)
@@ -1124,7 +1128,7 @@ static void io_wqe_cancel_pending_work(struct io_wqe *wqe,
 	int i;
 retry:
 	for (i = 0; i < IO_WQ_ACCT_NR; i++) {
-		struct io_wqe_acct *acct = io_get_acct(wqe, i == 0);
+		struct io_wqe_acct *acct = io_get_acct(wqe, i == 0, false);
 
 		if (io_acct_cancel_pending_work(wqe, acct, match)) {
 			if (match->cancel_all)
-- 
2.36.0

[PATCH 5/9] io-wq: fixed worker initialization

[Hao Xu]

From: Hao Xu <howeyxu@tencent.com>

Implementation of the fixed worker initialization.

Signed-off-by: Hao Xu <howeyxu@tencent.com>
---
 fs/io-wq.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/fs/io-wq.c b/fs/io-wq.c
index c67bd5e5d117..a1a10fb204a7 100644
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -774,6 +774,26 @@ void io_wq_worker_sleeping(struct task_struct *tsk)
 	io_wqe_dec_running(worker);
 }
 
+static void io_init_new_fixed_worker(struct io_wqe *wqe,
+				     struct io_worker *worker)
+{
+	struct io_wqe_acct *acct = io_wqe_get_acct(worker);
+	struct io_wqe_acct *iw_acct = &worker->acct;
+	unsigned index = acct->index;
+	unsigned *nr_fixed;
+
+	raw_spin_lock(&acct->lock);
+	nr_fixed = &acct->nr_fixed;
+	acct->fixed_workers[*nr_fixed] = worker;
+	worker->index = (*nr_fixed)++;
+	iw_acct->nr_works = 0;
+	iw_acct->max_works = acct->max_works;
+	iw_acct->index = index;
+	INIT_WQ_LIST(&iw_acct->work_list);
+	raw_spin_lock_init(&iw_acct->lock);
+	raw_spin_unlock(&acct->lock);
+}
+
 static void io_init_new_worker(struct io_wqe *wqe, struct io_worker *worker,
 			       struct task_struct *tsk)
 {
@@ -787,6 +807,8 @@ static void io_init_new_worker(struct io_wqe *wqe, struct io_worker *worker,
 	list_add_tail_rcu(&worker->all_list, &wqe->all_list);
 	worker->flags |= IO_WORKER_F_FREE;
 	raw_spin_unlock(&wqe->lock);
+	if (worker->flags & IO_WORKER_F_FIXED)
+		io_init_new_fixed_worker(wqe, worker);
 	wake_up_new_task(tsk);
 }
 
@@ -893,6 +915,8 @@ static bool create_io_worker(struct io_wq *wq, struct io_wqe *wqe,
 
 	if (index == IO_WQ_ACCT_BOUND)
 		worker->flags |= IO_WORKER_F_BOUND;
+	if (&wqe->fixed_acct[index] == acct)
+		worker->flags |= IO_WORKER_F_FIXED;
 
 	tsk = create_io_thread(io_wqe_worker, worker, wqe->node);
 	if (!IS_ERR(tsk)) {
-- 
2.36.0

[PATCH 6/9] io-wq: fixed worker exit

[Hao Xu]

From: Hao Xu <howeyxu@tencent.com>

Implement the fixed worker exit

Signed-off-by: Hao Xu <howeyxu@tencent.com>
---
 fs/io-wq.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/fs/io-wq.c b/fs/io-wq.c
index a1a10fb204a7..2feff19970ca 100644
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -259,6 +259,29 @@ static bool io_task_worker_match(struct callback_head *cb, void *data)
 	return worker == data;
 }
 
+static void io_fixed_worker_exit(struct io_worker *worker)
+{
+	int *nr_fixed;
+	int index = worker->acct.index;
+	struct io_wqe *wqe = worker->wqe;
+	struct io_wqe_acct *acct = io_get_acct(wqe, index == 0, true);
+	struct io_worker **fixed_workers;
+
+	raw_spin_lock(&acct->lock);
+	fixed_workers = acct->fixed_workers;
+	if (!fixed_workers || worker->index == -1) {
+		raw_spin_unlock(&acct->lock);
+		return;
+	}
+	nr_fixed = &acct->nr_fixed;
+	/* reuse variable index to represent fixed worker index in its array */
+	index = worker->index;
+	fixed_workers[index] = fixed_workers[*nr_fixed - 1];
+	(*nr_fixed)--;
+	fixed_workers[index]->index = index;
+	raw_spin_unlock(&acct->lock);
+}
+
 static void io_worker_exit(struct io_worker *worker)
 {
 	struct io_wqe *wqe = worker->wqe;
@@ -682,6 +705,7 @@ static int io_wqe_worker(void *data)
 	struct io_wqe *wqe = worker->wqe;
 	struct io_wq *wq = wqe->wq;
 	bool last_timeout = false;
+	bool fixed = worker->flags & IO_WORKER_F_FIXED;
 	char buf[TASK_COMM_LEN];
 
 	worker->flags |= (IO_WORKER_F_UP | IO_WORKER_F_RUNNING);
@@ -732,6 +756,8 @@ static int io_wqe_worker(void *data)
 
 	if (test_bit(IO_WQ_BIT_EXIT, &wq->state))
 		io_worker_handle_work(worker);
+	if (fixed)
+		io_fixed_worker_exit(worker);
 
 	audit_free(current);
 	io_worker_exit(worker);
-- 
2.36.0

[PATCH 7/9] io-wq: implement fixed worker logic

[Hao Xu]

From: Hao Xu <howeyxu@tencent.com>

The current implementation of io-wq has big spinlock contension. The
main reason is the single work list model. All producers(who insert
works) and consumers(io-workers) have to grap wqe->lock to move ahead.
Set max_worker to 3 or 4, do a fio read test, we can see 40%~50% lock
contension.
Introduce fixed io-workers which sticks there to handle works and have
their own work list.

previous:

  producer0 ---insert---> work_list ---get---> io-worker0,1,2

now:

                     ---> private work_list0 --get--> fixed-worker0
                    /
  producer0 --insert----> private work_list1 --get--> fixed-worker1
      |             \
      |              ---> private work_list2 --get--> fixed-worker2
      |
      |---insert---> public work_list --get--> (normal)io-worker

Since each fixed-worker has a private work list, the contension will be
limited to a smaller range(the private work list).
Logic of fixed-worker: first handle private works then public ones.
Logic of normal io-worker: only handle public works.
Logic of producer: 1) randomly pick a private work list and check if it
                      is full, insert the work if it's not
                   2) insert the work to the public work list if 1)
                      fails.
The get logic of a private list: fixed-worker grab all the works in
its private work list(like what tctx_task_work() does) rather than one
by one.(this code is in the next patches as a optimization)

To achieve this, we need to add an io_wqe_acct for each fixed-worker
struct, and though this we can leverage the old code as much as
possible, which makes the new design clean and compatible.

Good things of this feature:
  1) bound and unbound work lists now have different spinlocks.
  2) much smaller contension between work producers and consumers.
  3) fixed workers are friendly for users to control: binding cpus,
     reset priority etc.

Wrote a nop test program to test it, 3 fixed-workers VS 3 normal workers.
normal workers:
./run_nop_wqe.sh nop_wqe_normal 200000 100 3 1-3
        time spent: 10464397 usecs      IOPS: 1911242
        time spent: 9610976 usecs       IOPS: 2080954
        time spent: 9807361 usecs       IOPS: 2039284

fixed workers:
./run_nop_wqe.sh nop_wqe_fixed 200000 100 3 1-3
        time spent: 17314274 usecs      IOPS: 1155116
        time spent: 17016942 usecs      IOPS: 1175299
        time spent: 17908684 usecs      IOPS: 1116776

About 2x improvement. From perf result, almost no acct->lock contension.
Test program: https://github.com/HowHsu/liburing/tree/fixed_worker

Signed-off-by: Hao Xu <howeyxu@tencent.com>
---
 fs/io-wq.c | 148 +++++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 122 insertions(+), 26 deletions(-)

diff --git a/fs/io-wq.c b/fs/io-wq.c
index 2feff19970ca..aaa9cea7d39a 100644
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -70,6 +70,7 @@ struct io_wqe_acct {
 	unsigned max_workers;
 	unsigned nr_fixed;
 	unsigned max_works;
+	unsigned work_seq;
 	union {
 		struct io_wq_work_list work_list;
 		struct io_worker **fixed_workers;
@@ -624,9 +625,9 @@ static void io_assign_current_work(struct io_worker *worker,
 
 static void io_wqe_enqueue(struct io_wqe *wqe, struct io_wq_work *work);
 
-static void io_worker_handle_work(struct io_worker *worker)
+static void io_worker_handle_work(struct io_worker *worker,
+				  struct io_wqe_acct *acct)
 {
-	struct io_wqe_acct *acct = io_wqe_get_acct(worker);
 	struct io_wqe *wqe = worker->wqe;
 	struct io_wq *wq = wqe->wq;
 	bool do_kill = test_bit(IO_WQ_BIT_EXIT, &wq->state);
@@ -698,19 +699,31 @@ static void io_worker_handle_work(struct io_worker *worker)
 	} while (1);
 }
 
+static inline void io_worker_handle_private_work(struct io_worker *worker)
+{
+	io_worker_handle_work(worker, &worker->acct);
+}
+
+static inline void io_worker_handle_public_work(struct io_worker *worker)
+{
+	io_worker_handle_work(worker, io_wqe_get_acct(worker));
+}
+
 static int io_wqe_worker(void *data)
 {
 	struct io_worker *worker = data;
-	struct io_wqe_acct *acct = io_wqe_get_acct(worker);
 	struct io_wqe *wqe = worker->wqe;
 	struct io_wq *wq = wqe->wq;
-	bool last_timeout = false;
+	struct io_wqe_acct *acct =
+		io_get_acct(wqe, worker->flags & IO_WORKER_F_BOUND, false);
 	bool fixed = worker->flags & IO_WORKER_F_FIXED;
+	bool last_timeout = false;
 	char buf[TASK_COMM_LEN];
 
 	worker->flags |= (IO_WORKER_F_UP | IO_WORKER_F_RUNNING);
 
-	snprintf(buf, sizeof(buf), "iou-wrk-%d", wq->task->pid);
+	snprintf(buf, sizeof(buf), fixed ? "iou-fix-%d" : "iou-wrk-%d",
+		 wq->task->pid);
 	set_task_comm(current, buf);
 
 	audit_alloc_kernel(current);
@@ -722,13 +735,24 @@ static int io_wqe_worker(void *data)
 			break;
 
 		set_current_state(TASK_INTERRUPTIBLE);
-		while (!(worker->flags & IO_WORKER_F_EXIT) &&
-		       io_acct_run_queue(acct))
-			io_worker_handle_work(worker);
-
+		if (fixed) {
+			while (io_acct_run_queue(&worker->acct))
+				io_worker_handle_private_work(worker);
+			if (io_acct_run_queue(acct))
+				io_worker_handle_public_work(worker);
+		} else {
+			while (io_acct_run_queue(acct))
+				io_worker_handle_public_work(worker);
+		}
 		raw_spin_lock(&wqe->lock);
-		/* timed out, exit unless we're the last worker */
-		if (last_timeout && acct->nr_workers > 1) {
+		/* timed out, a worker will exit only if:
+		 * - not a fixed worker
+		 * - not the last non-fixed worker
+		 *
+		 * the second condition is due to we need at least one worker to
+		 * handle the public work list.
+		 */
+		if (last_timeout && !fixed && acct->nr_workers > 1) {
 			acct->nr_workers--;
 			raw_spin_unlock(&wqe->lock);
 			__set_current_state(TASK_RUNNING);
@@ -754,10 +778,18 @@ static int io_wqe_worker(void *data)
 		last_timeout = !ret;
 	}
 
-	if (test_bit(IO_WQ_BIT_EXIT, &wq->state))
-		io_worker_handle_work(worker);
-	if (fixed)
+	if (test_bit(IO_WQ_BIT_EXIT, &wq->state) && !fixed)
+		io_worker_handle_public_work(worker);
+	if (fixed) {
 		io_fixed_worker_exit(worker);
+		/*
+		 * Check and handle private work list again
+		 * to avoid race with private work insertion
+		 * TODO: an alternative way is to deliver
+		 * works to the public work list
+		 */
+		io_worker_handle_private_work(worker);
+	}
 
 	audit_free(current);
 	io_worker_exit(worker);
@@ -1001,9 +1033,9 @@ static void io_run_cancel(struct io_wq_work *work, struct io_wqe *wqe)
 	} while (work);
 }
 
-static void io_wqe_insert_work(struct io_wqe *wqe, struct io_wq_work *work)
+static void io_wqe_insert_work(struct io_wqe *wqe, struct io_wq_work *work,
+			       struct io_wqe_acct *acct)
 {
-	struct io_wqe_acct *acct = io_work_get_acct(wqe, work);
 	unsigned int hash;
 	struct io_wq_work *tail;
 
@@ -1022,6 +1054,45 @@ static void io_wqe_insert_work(struct io_wqe *wqe, struct io_wq_work *work)
 	wq_list_add_after(&work->list, &tail->list, &acct->work_list);
 }
 
+static bool io_wqe_insert_private_work(struct io_wqe *wqe,
+				       struct io_wq_work *work,
+				       struct io_wqe_acct *acct)
+{
+	unsigned int nr_fixed;
+	struct io_worker *fixed_worker;
+	struct io_wqe_acct *iw_acct;
+	unsigned int fixed_worker_index;
+
+	raw_spin_lock(&acct->lock);
+	nr_fixed = acct->nr_fixed;
+	if (!nr_fixed) {
+		raw_spin_unlock(&acct->lock);
+		return false;
+	}
+
+	fixed_worker_index = (acct->work_seq++) % nr_fixed;
+	fixed_worker = acct->fixed_workers[fixed_worker_index];
+	if (!fixed_worker || fixed_worker->flags & IO_WORKER_F_EXIT) {
+		raw_spin_unlock(&acct->lock);
+		return false;
+	}
+	iw_acct = &fixed_worker->acct;
+
+	raw_spin_lock(&iw_acct->lock);
+	if (iw_acct->nr_works < iw_acct->max_works) {
+		io_wqe_insert_work(wqe, work, iw_acct);
+		iw_acct->nr_works++;
+		raw_spin_unlock(&iw_acct->lock);
+		wake_up_process(fixed_worker->task);
+		raw_spin_unlock(&acct->lock);
+		return true;
+	}
+	raw_spin_unlock(&iw_acct->lock);
+	raw_spin_unlock(&acct->lock);
+
+	return false;
+}
+
 static bool io_wq_work_match_item(struct io_wq_work *work, void *data)
 {
 	return work == data;
@@ -1030,6 +1101,7 @@ static bool io_wq_work_match_item(struct io_wq_work *work, void *data)
 static void io_wqe_enqueue(struct io_wqe *wqe, struct io_wq_work *work)
 {
 	struct io_wqe_acct *acct = io_work_get_acct(wqe, work);
+	struct io_wqe_acct *fixed_acct;
 	struct io_cb_cancel_data match;
 	unsigned work_flags = work->flags;
 	bool do_create;
@@ -1044,8 +1116,14 @@ static void io_wqe_enqueue(struct io_wqe *wqe, struct io_wq_work *work)
 		return;
 	}
 
+	fixed_acct = io_get_acct(wqe, !acct->index, true);
+	if (fixed_acct->fixed_worker_registered && !io_wq_is_hashed(work)) {
+		if (io_wqe_insert_private_work(wqe, work, fixed_acct))
+			return;
+	}
+
 	raw_spin_lock(&acct->lock);
-	io_wqe_insert_work(wqe, work);
+	io_wqe_insert_work(wqe, work, acct);
 	clear_bit(IO_ACCT_STALLED_BIT, &acct->flags);
 	raw_spin_unlock(&acct->lock);
 
@@ -1131,9 +1209,9 @@ static bool io_wq_worker_cancel(struct io_worker *worker, void *data)
 
 static inline void io_wqe_remove_pending(struct io_wqe *wqe,
 					 struct io_wq_work *work,
-					 struct io_wq_work_node *prev)
+					 struct io_wq_work_node *prev,
+					 struct io_wqe_acct *acct)
 {
-	struct io_wqe_acct *acct = io_work_get_acct(wqe, work);
 	unsigned int hash = io_get_work_hash(work);
 	struct io_wq_work *prev_work = NULL;
 
@@ -1160,7 +1238,7 @@ static bool io_acct_cancel_pending_work(struct io_wqe *wqe,
 		work = container_of(node, struct io_wq_work, list);
 		if (!match->fn(work, match->data))
 			continue;
-		io_wqe_remove_pending(wqe, work, prev);
+		io_wqe_remove_pending(wqe, work, prev, acct);
 		raw_spin_unlock(&acct->lock);
 		io_run_cancel(work, wqe);
 		match->nr_pending++;
@@ -1175,17 +1253,35 @@ static bool io_acct_cancel_pending_work(struct io_wqe *wqe,
 static void io_wqe_cancel_pending_work(struct io_wqe *wqe,
 				       struct io_cb_cancel_data *match)
 {
-	int i;
-retry:
-	for (i = 0; i < IO_WQ_ACCT_NR; i++) {
-		struct io_wqe_acct *acct = io_get_acct(wqe, i == 0, false);
+	int i, j;
+	struct io_wqe_acct *acct, *iw_acct;
 
+retry_public:
+	for (i = 0; i < IO_WQ_ACCT_NR; i++) {
+		acct = io_get_acct(wqe, i == 0, false);
 		if (io_acct_cancel_pending_work(wqe, acct, match)) {
 			if (match->cancel_all)
-				goto retry;
-			break;
+				goto retry_public;
+			return;
 		}
 	}
+
+retry_private:
+	for (i = 0; i < IO_WQ_ACCT_NR; i++) {
+		acct = io_get_acct(wqe, i == 0, true);
+		raw_spin_lock(&acct->lock);
+		for (j = 0; j < acct->nr_fixed; j++) {
+			iw_acct = &acct->fixed_workers[j]->acct;
+			if (io_acct_cancel_pending_work(wqe, iw_acct, match)) {
+				if (match->cancel_all) {
+					raw_spin_unlock(&acct->lock);
+					goto retry_private;
+				}
+				break;
+			}
+		}
+		raw_spin_unlock(&acct->lock);
+	}
 }
 
 static void io_wqe_cancel_running_work(struct io_wqe *wqe,
-- 
2.36.0

[PATCH 8/9] io-wq: batch the handling of fixed worker private works

[Hao Xu]

From: Hao Xu <howeyxu@tencent.com>

Reduce acct->lock contension by batching the handling of private work
list for fixed_workers.

Signed-off-by: Hao Xu <howeyxu@tencent.com>
---
 fs/io-wq.c | 42 +++++++++++++++++++++++++++++++++---------
 fs/io-wq.h |  5 +++++
 2 files changed, 38 insertions(+), 9 deletions(-)

diff --git a/fs/io-wq.c b/fs/io-wq.c
index aaa9cea7d39a..df2d480395e8 100644
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -540,8 +540,23 @@ static bool io_wait_on_hash(struct io_wqe *wqe, unsigned int hash)
 	return ret;
 }
 
+static inline void conditional_acct_lock(struct io_wqe_acct *acct,
+					 bool needs_lock)
+{
+	if (needs_lock)
+		raw_spin_lock(&acct->lock);
+}
+
+static inline void conditional_acct_unlock(struct io_wqe_acct *acct,
+					   bool needs_lock)
+{
+	if (needs_lock)
+		raw_spin_unlock(&acct->lock);
+}
+
 static struct io_wq_work *io_get_next_work(struct io_wqe_acct *acct,
-					   struct io_worker *worker)
+					   struct io_worker *worker,
+					   bool needs_lock)
 	__must_hold(acct->lock)
 {
 	struct io_wq_work_node *node, *prev;
@@ -549,6 +564,7 @@ static struct io_wq_work *io_get_next_work(struct io_wqe_acct *acct,
 	unsigned int stall_hash = -1U;
 	struct io_wqe *wqe = worker->wqe;
 
+	conditional_acct_lock(acct, needs_lock);
 	wq_list_for_each(node, prev, &acct->work_list) {
 		unsigned int hash;
 
@@ -557,6 +573,7 @@ static struct io_wq_work *io_get_next_work(struct io_wqe_acct *acct,
 		/* not hashed, can run anytime */
 		if (!io_wq_is_hashed(work)) {
 			wq_list_del(&acct->work_list, node, prev);
+			conditional_acct_unlock(acct, needs_lock);
 			return work;
 		}
 
@@ -568,6 +585,7 @@ static struct io_wq_work *io_get_next_work(struct io_wqe_acct *acct,
 		if (!test_and_set_bit(hash, &wqe->wq->hash->map)) {
 			wqe->hash_tail[hash] = NULL;
 			wq_list_cut(&acct->work_list, &tail->list, prev);
+			conditional_acct_unlock(acct, needs_lock);
 			return work;
 		}
 		if (stall_hash == -1U)
@@ -584,15 +602,16 @@ static struct io_wq_work *io_get_next_work(struct io_wqe_acct *acct,
 		 * work being added and clearing the stalled bit.
 		 */
 		set_bit(IO_ACCT_STALLED_BIT, &acct->flags);
-		raw_spin_unlock(&acct->lock);
+		conditional_acct_unlock(acct, needs_lock);
 		unstalled = io_wait_on_hash(wqe, stall_hash);
-		raw_spin_lock(&acct->lock);
+		conditional_acct_lock(acct, needs_lock);
 		if (unstalled) {
 			clear_bit(IO_ACCT_STALLED_BIT, &acct->flags);
 			if (wq_has_sleeper(&wqe->wq->hash->wait))
 				wake_up(&wqe->wq->hash->wait);
 		}
 	}
+	conditional_acct_unlock(acct, needs_lock);
 
 	return NULL;
 }
@@ -626,7 +645,7 @@ static void io_assign_current_work(struct io_worker *worker,
 static void io_wqe_enqueue(struct io_wqe *wqe, struct io_wq_work *work);
 
 static void io_worker_handle_work(struct io_worker *worker,
-				  struct io_wqe_acct *acct)
+				  struct io_wqe_acct *acct, bool needs_lock)
 {
 	struct io_wqe *wqe = worker->wqe;
 	struct io_wq *wq = wqe->wq;
@@ -642,9 +661,7 @@ static void io_worker_handle_work(struct io_worker *worker,
 		 * can't make progress, any work completion or insertion will
 		 * clear the stalled flag.
 		 */
-		raw_spin_lock(&acct->lock);
-		work = io_get_next_work(acct, worker);
-		raw_spin_unlock(&acct->lock);
+		work = io_get_next_work(acct, worker, needs_lock);
 		if (work) {
 			__io_worker_busy(wqe, worker);
 
@@ -701,12 +718,19 @@ static void io_worker_handle_work(struct io_worker *worker,
 
 static inline void io_worker_handle_private_work(struct io_worker *worker)
 {
-	io_worker_handle_work(worker, &worker->acct);
+	struct io_wqe_acct acct;
+
+	raw_spin_lock(&worker->acct.lock);
+	acct = worker->acct;
+	wq_list_clean(&worker->acct.work_list);
+	worker->acct.nr_works = 0;
+	raw_spin_unlock(&worker->acct.lock);
+	io_worker_handle_work(worker, &acct, false);
 }
 
 static inline void io_worker_handle_public_work(struct io_worker *worker)
 {
-	io_worker_handle_work(worker, io_wqe_get_acct(worker));
+	io_worker_handle_work(worker, io_wqe_get_acct(worker), true);
 }
 
 static int io_wqe_worker(void *data)
diff --git a/fs/io-wq.h b/fs/io-wq.h
index ba6eee76d028..ef3ce577e6b7 100644
--- a/fs/io-wq.h
+++ b/fs/io-wq.h
@@ -40,6 +40,11 @@ struct io_wq_work_list {
 	(list)->first = NULL;					\
 } while (0)
 
+static inline void wq_list_clean(struct io_wq_work_list *list)
+{
+	list->first = list->last = NULL;
+}
+
 static inline void wq_list_add_after(struct io_wq_work_node *node,
 				     struct io_wq_work_node *pos,
 				     struct io_wq_work_list *list)
-- 
2.36.0

[PATCH 9/9] io_uring: add register fixed worker interface

[Hao Xu]

From: Hao Xu <howeyxu@tencent.com>

Add an io_uring_register() interface to register fixed workers and
indicate its work capacity.
The argument is an array of two elements each is
    struct {
    	__s32 nr_workers;
    	__s32 max_works;
    }
(nr_workers, max_works)                        meaning

nr_workers or max_works <  -1                  invalid
nr_workers or max_works == -1           get the old value back
nr_workers or max_works >=  0        get the old value and set to the
                                     new value

Signed-off-by: Hao Xu <howeyxu@tencent.com>
---
 fs/io-wq.c                    | 101 ++++++++++++++++++++++++++++++++++
 fs/io-wq.h                    |   3 +
 fs/io_uring.c                 |  71 ++++++++++++++++++++++++
 include/uapi/linux/io_uring.h |  11 ++++
 4 files changed, 186 insertions(+)

diff --git a/fs/io-wq.c b/fs/io-wq.c
index df2d480395e8..c1e87b29c960 100644
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -1671,6 +1671,107 @@ int io_wq_max_workers(struct io_wq *wq, int *new_count)
 	return 0;
 }
 
+/*
+ * Set max number of fixed workers and the capacity of private work list,
+ * returns old value. If new_count is -1, then just return the old value.
+ */
+int io_wq_fixed_workers(struct io_wq *wq,
+			struct io_uring_fixed_worker_arg *new_count)
+{
+	struct io_uring_fixed_worker_arg prev[IO_WQ_ACCT_NR];
+	bool first_node = true;
+	int i, node;
+	bool readonly[2] = {
+		(new_count[0].nr_workers == -1 && new_count[0].max_works == -1),
+		(new_count[1].nr_workers == -1 && new_count[1].max_works == -1),
+	};
+
+	BUILD_BUG_ON((int) IO_WQ_ACCT_BOUND   != (int) IO_WQ_BOUND);
+	BUILD_BUG_ON((int) IO_WQ_ACCT_UNBOUND != (int) IO_WQ_UNBOUND);
+	BUILD_BUG_ON((int) IO_WQ_ACCT_NR      != 2);
+
+	for (i = 0; i < IO_WQ_ACCT_NR; i++) {
+		if (new_count[i].nr_workers > task_rlimit(current, RLIMIT_NPROC))
+			new_count[i].nr_workers =
+				task_rlimit(current, RLIMIT_NPROC);
+	}
+
+	rcu_read_lock();
+	for_each_node(node) {
+		int j;
+		struct io_wqe *wqe = wq->wqes[node];
+
+		for (i = 0; i < IO_WQ_ACCT_NR; i++) {
+			struct io_wqe_acct *acct = &wqe->fixed_acct[i];
+			int *nr_fixed, *max_works;
+			struct io_worker **fixed_workers;
+			int nr = new_count[i].nr_workers;
+
+			raw_spin_lock(&acct->lock);
+			nr_fixed = &acct->nr_fixed;
+			max_works = &acct->max_works;
+			fixed_workers = acct->fixed_workers;
+			if (first_node) {
+				prev[i].nr_workers = *nr_fixed;
+				prev[i].max_works = *max_works;
+			}
+			if (readonly[i]) {
+				raw_spin_unlock(&acct->lock);
+				continue;
+			}
+			if (*nr_fixed == nr || nr == -1) {
+				*max_works = new_count[i].max_works;
+				raw_spin_unlock(&acct->lock);
+				continue;
+			}
+			for (j = 0; j < *nr_fixed; j++) {
+				struct io_worker *worker = fixed_workers[j];
+
+				if (!worker)
+					continue;
+				worker->flags |= IO_WORKER_F_EXIT;
+				/*
+				 * Mark index to -1 to avoid false deletion
+				 * in io_fixed_worker_exit()
+				 */
+				worker->index = -1;
+				/*
+				 * Once a worker is in fixed_workers array
+				 * it is definitely there before we release
+				 * the acct->lock below. That's why we don't
+				 * need to increment the worker->ref here.
+				 */
+				wake_up_process(worker->task);
+			}
+			kfree(fixed_workers);
+			acct->fixed_workers = NULL;
+			*nr_fixed = 0;
+			*max_works = new_count[i].max_works;
+			acct->fixed_workers = kzalloc_node(
+						sizeof(*fixed_workers) * nr,
+						GFP_KERNEL, wqe->node);
+			if (!acct->fixed_workers) {
+				raw_spin_unlock(&acct->lock);
+				return -ENOMEM;
+			}
+			raw_spin_unlock(&acct->lock);
+			for (j = 0; j < nr; j++)
+				io_wqe_create_worker(wqe, acct);
+
+			acct->fixed_worker_registered = !!nr;
+		}
+		first_node = false;
+	}
+	rcu_read_unlock();
+
+	for (i = 0; i < IO_WQ_ACCT_NR; i++) {
+		new_count[i].nr_workers = prev[i].nr_workers;
+		new_count[i].max_works = prev[i].max_works;
+	}
+
+	return 0;
+}
+
 static __init int io_wq_init(void)
 {
 	int ret;
diff --git a/fs/io-wq.h b/fs/io-wq.h
index ef3ce577e6b7..bf90488b0283 100644
--- a/fs/io-wq.h
+++ b/fs/io-wq.h
@@ -2,6 +2,7 @@
 #define INTERNAL_IO_WQ_H
 
 #include <linux/refcount.h>
+#include <uapi/linux/io_uring.h>
 
 struct io_wq;
 
@@ -202,6 +203,8 @@ void io_wq_hash_work(struct io_wq_work *work, void *val);
 
 int io_wq_cpu_affinity(struct io_wq *wq, cpumask_var_t mask);
 int io_wq_max_workers(struct io_wq *wq, int *new_count);
+int io_wq_fixed_workers(struct io_wq *wq,
+			struct io_uring_fixed_worker_arg *new_count);
 
 static inline bool io_wq_is_hashed(struct io_wq_work *work)
 {
diff --git a/fs/io_uring.c b/fs/io_uring.c
index 1e7466079af7..c0c7c1fd94fd 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -11806,6 +11806,71 @@ static __cold int io_register_iowq_max_workers(struct io_ring_ctx *ctx,
 	return ret;
 }
 
+static __cold int io_register_iowq_fixed_workers(struct io_ring_ctx *ctx,
+						 void __user *arg)
+	__must_hold(&ctx->uring_lock)
+{
+	struct io_uring_task *tctx = NULL;
+	struct io_sq_data *sqd = NULL;
+	struct io_uring_fixed_worker_arg new_count[2];
+	int i, ret;
+
+	if (copy_from_user(new_count, arg, sizeof(new_count)))
+		return -EFAULT;
+	for (i = 0; i < ARRAY_SIZE(new_count); i++) {
+		int nr_workers = new_count[i].nr_workers;
+		int max_works = new_count[i].max_works;
+
+		if (nr_workers < -1 || max_works < -1)
+			return -EINVAL;
+	}
+
+	if (ctx->flags & IORING_SETUP_SQPOLL) {
+		sqd = ctx->sq_data;
+		if (sqd) {
+			/*
+			 * Observe the correct sqd->lock -> ctx->uring_lock
+			 * ordering. Fine to drop uring_lock here, we hold
+			 * a ref to the ctx.
+			 */
+			refcount_inc(&sqd->refs);
+			mutex_unlock(&ctx->uring_lock);
+			mutex_lock(&sqd->lock);
+			mutex_lock(&ctx->uring_lock);
+			if (sqd->thread)
+				tctx = sqd->thread->io_uring;
+		}
+	} else {
+		tctx = current->io_uring;
+	}
+
+	if (tctx && tctx->io_wq) {
+		ret = io_wq_fixed_workers(tctx->io_wq, new_count);
+		if (ret)
+			goto err;
+	} else {
+		memset(new_count, -1, sizeof(new_count));
+	}
+
+	if (sqd) {
+		mutex_unlock(&sqd->lock);
+		io_put_sq_data(sqd);
+	}
+
+	if (copy_to_user(arg, new_count, sizeof(new_count)))
+		return -EFAULT;
+
+	/* that's it for SQPOLL, only the SQPOLL task creates requests */
+	if (sqd)
+		return 0;
+
+err:
+	if (sqd) {
+		mutex_unlock(&sqd->lock);
+		io_put_sq_data(sqd);
+	}
+	return ret;
+}
 static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
 			       void __user *arg, unsigned nr_args)
 	__releases(ctx->uring_lock)
@@ -11934,6 +11999,12 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
 	case IORING_UNREGISTER_RING_FDS:
 		ret = io_ringfd_unregister(ctx, arg, nr_args);
 		break;
+	case IORING_REGISTER_IOWQ_FIXED_WORKERS:
+		ret = -EINVAL;
+		if (!arg || nr_args != 2)
+			break;
+		ret = io_register_iowq_fixed_workers(ctx, arg);
+		break;
 	default:
 		ret = -EINVAL;
 		break;
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index fad63564678a..f0ec9523ab42 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -360,6 +360,12 @@ enum {
 	IORING_REGISTER_RING_FDS		= 20,
 	IORING_UNREGISTER_RING_FDS		= 21,
 
+	/* set number of fixed workers and number
+	 * of works in a private work list which
+	 * belongs to a fixed worker
+	 */
+	IORING_REGISTER_IOWQ_FIXED_WORKERS	= 22,
+
 	/* this goes last */
 	IORING_REGISTER_LAST
 };
@@ -457,4 +463,9 @@ struct io_uring_getevents_arg {
 	__u64	ts;
 };
 
+struct io_uring_fixed_worker_arg {
+	__s32	nr_workers;
+	__s32	max_works;
+};
+
 #endif
-- 
2.36.0

Re: [RFC v3 0/9] fixed worker

[Jens Axboe]

On 4/29/22 4:18 AM, Hao Xu wrote:
> This is the third version of fixed worker implementation.
> Wrote a nop test program to test it, 3 fixed-workers VS 3 normal workers.
> normal workers:
> ./run_nop_wqe.sh nop_wqe_normal 200000 100 3 1-3
>         time spent: 10464397 usecs      IOPS: 1911242
>         time spent: 9610976 usecs       IOPS: 2080954
>         time spent: 9807361 usecs       IOPS: 2039284
> 
> fixed workers:
> ./run_nop_wqe.sh nop_wqe_fixed 200000 100 3 1-3
>         time spent: 17314274 usecs      IOPS: 1155116
>         time spent: 17016942 usecs      IOPS: 1175299
>         time spent: 17908684 usecs      IOPS: 1116776

I saw these numbers in v2 as well, and I have to admit I don't
understand them. Because on the surface, it sure looks like the first
set of results (labeled "normal") are better than the second "fixed"
set. Am I reading them wrong, or did you transpose them?

I think this patch series would benefit from a higher level description
of what fixed workers mean in this context. How are they different from
the existing workers, and why would it improve things.

> things to be done:
>  - Still need some thinking about the work cancellation

Can you expand? What are the challenges with fixed workers and
cancelation?

>  - not very sure IO_WORKER_F_EXIT is safe enough on synchronization
>  - the iowq hash stuff is not compatible with fixed worker for now

We might need to extract the hashing out a bit so it's not as tied to
the existing implementation.

-- 
Jens Axboe

Re: [PATCH 7/9] io-wq: implement fixed worker logic

[Jens Axboe]

On 4/29/22 4:18 AM, Hao Xu wrote:
> @@ -1030,6 +1101,7 @@ static bool io_wq_work_match_item(struct io_wq_work *work, void *data)
>  static void io_wqe_enqueue(struct io_wqe *wqe, struct io_wq_work *work)
>  {
>  	struct io_wqe_acct *acct = io_work_get_acct(wqe, work);
> +	struct io_wqe_acct *fixed_acct;
>  	struct io_cb_cancel_data match;
>  	unsigned work_flags = work->flags;
>  	bool do_create;
> @@ -1044,8 +1116,14 @@ static void io_wqe_enqueue(struct io_wqe *wqe, struct io_wq_work *work)
>  		return;
>  	}
>  
> +	fixed_acct = io_get_acct(wqe, !acct->index, true);
> +	if (fixed_acct->fixed_worker_registered && !io_wq_is_hashed(work)) {
> +		if (io_wqe_insert_private_work(wqe, work, fixed_acct))
> +			return;
> +	}
> +

As per previous email, I was going to comment back saying "why don't we
just always do hashed work on the non-fixed workers?" - but that's
already what you are doing. Isn't this fine, does anything else need to
get done here in terms of hashed work and fixed workers? If you need
per-iowq serialization, then you don't get a fixed worker.

-- 
Jens Axboe

Re: [RFC v3 0/9] fixed worker

[Hao Xu]

On 4/30/22 21:11, Jens Axboe wrote:
> On 4/29/22 4:18 AM, Hao Xu wrote:
>> This is the third version of fixed worker implementation.
>> Wrote a nop test program to test it, 3 fixed-workers VS 3 normal workers.
>> normal workers:
>> ./run_nop_wqe.sh nop_wqe_normal 200000 100 3 1-3
>>          time spent: 10464397 usecs      IOPS: 1911242
>>          time spent: 9610976 usecs       IOPS: 2080954
>>          time spent: 9807361 usecs       IOPS: 2039284
>>
>> fixed workers:
>> ./run_nop_wqe.sh nop_wqe_fixed 200000 100 3 1-3
>>          time spent: 17314274 usecs      IOPS: 1155116
>>          time spent: 17016942 usecs      IOPS: 1175299
>>          time spent: 17908684 usecs      IOPS: 1116776
> 
> I saw these numbers in v2 as well, and I have to admit I don't
> understand them. Because on the surface, it sure looks like the first
> set of results (labeled "normal") are better than the second "fixed"
> set. Am I reading them wrong, or did you transpose them?
Sorry, I transposed them..
> 
> I think this patch series would benefit from a higher level description
> of what fixed workers mean in this context. How are they different from
> the existing workers, and why would it improve things.
Sure, put that in the Patch 7/9, I'll move it to the cover letter as
well.
> 
>> things to be done:
>>   - Still need some thinking about the work cancellation
> 
> Can you expand? What are the challenges with fixed workers and
> cancelation?
Currently, when a fixed worker fetch all the works from its private work
list, I use a temporary acct struct to hold them. This means at that
moment the cancellation cannot find these works which are going to run
but not in the private work list already. This won't be a big problem,
another acct member in io_worker{} should be good enough to resolve
that.
> 
>>   - not very sure IO_WORKER_F_EXIT is safe enough on synchronization
>>   - the iowq hash stuff is not compatible with fixed worker for now
> 
> We might need to extract the hashing out a bit so it's not as tied to
> the existing implementation.
> 

Re: [PATCH 7/9] io-wq: implement fixed worker logic

[Hao Xu]

On 4/30/22 21:27, Jens Axboe wrote:
> On 4/29/22 4:18 AM, Hao Xu wrote:
>> @@ -1030,6 +1101,7 @@ static bool io_wq_work_match_item(struct io_wq_work *work, void *data)
>>   static void io_wqe_enqueue(struct io_wqe *wqe, struct io_wq_work *work)
>>   {
>>   	struct io_wqe_acct *acct = io_work_get_acct(wqe, work);
>> +	struct io_wqe_acct *fixed_acct;
>>   	struct io_cb_cancel_data match;
>>   	unsigned work_flags = work->flags;
>>   	bool do_create;
>> @@ -1044,8 +1116,14 @@ static void io_wqe_enqueue(struct io_wqe *wqe, struct io_wq_work *work)
>>   		return;
>>   	}
>>   
>> +	fixed_acct = io_get_acct(wqe, !acct->index, true);
>> +	if (fixed_acct->fixed_worker_registered && !io_wq_is_hashed(work)) {
>> +		if (io_wqe_insert_private_work(wqe, work, fixed_acct))
>> +			return;
>> +	}
>> +
> 
> As per previous email, I was going to comment back saying "why don't we
> just always do hashed work on the non-fixed workers?" - but that's
> already what you are doing. Isn't this fine, does anything else need to
> get done here in terms of hashed work and fixed workers? If you need
> per-iowq serialization, then you don't get a fixed worker.

Hmm, seems we cannot accelerate serialized works with fixed worker. So
Let's make it as it is.
>