[PATCH 2/2] io_uring: allow user configurable IO thread CPU affinity

[Jens Axboe <axboe@kernel.dk>]

io-wq defaults to per-node masks for IO workers. This works fine by
default, but isn't particularly handy for workloads that prefer more
specific affinities, for either performance or isolation reasons.

This adds IORING_REGISTER_IOWQ_AFF that allows the user to pass in a CPU
mask that is then applied to IO thread workers, and an
IORING_UNREGISTER_IOWQ_AFF that simply resets the masks back to the
default of per-node.

Note that no care is given to existing IO threads, they will need to go
through a reschedule before the affinity is correct if they are already
running or sleeping.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/io-wq.c                    | 17 ++++++++++++
 fs/io-wq.h                    |  2 ++
 fs/io_uring.c                 | 51 +++++++++++++++++++++++++++++++++++
 include/uapi/linux/io_uring.h |  4 +++
 4 files changed, 74 insertions(+)

diff --git a/fs/io-wq.c b/fs/io-wq.c
index 2af8e1df4646..bb4d3ee9592e 100644
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -1087,6 +1087,23 @@ static int io_wq_cpu_offline(unsigned int cpu, struct hlist_node *node)
 	return __io_wq_cpu_online(wq, cpu, false);
 }
 
+int io_wq_cpu_affinity(struct io_wq *wq, cpumask_var_t mask)
+{
+	int i;
+
+	rcu_read_lock();
+	for_each_node(i) {
+		struct io_wqe *wqe = wq->wqes[i];
+
+		if (mask)
+			cpumask_copy(wqe->cpu_mask, mask);
+		else
+			cpumask_copy(wqe->cpu_mask, cpumask_of_node(i));
+	}
+	rcu_read_unlock();
+	return 0;
+}
+
 static __init int io_wq_init(void)
 {
 	int ret;
diff --git a/fs/io-wq.h b/fs/io-wq.h
index af2df0680ee2..02299cdcf55c 100644
--- a/fs/io-wq.h
+++ b/fs/io-wq.h
@@ -128,6 +128,8 @@ void io_wq_put_and_exit(struct io_wq *wq);
 void io_wq_enqueue(struct io_wq *wq, struct io_wq_work *work);
 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);
+
 static inline bool io_wq_is_hashed(struct io_wq_work *work)
 {
 	return work->flags & IO_WQ_WORK_HASHED;
diff --git a/fs/io_uring.c b/fs/io_uring.c
index d916eb2cef09..46a25a7cb70a 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -9983,6 +9983,43 @@ static int io_register_rsrc(struct io_ring_ctx *ctx, void __user *arg,
 	return -EINVAL;
 }
 
+static int io_register_iowq_aff(struct io_ring_ctx *ctx, void __user *arg,
+				unsigned len)
+{
+	struct io_uring_task *tctx = current->io_uring;
+	cpumask_var_t new_mask;
+	int ret;
+
+	if (!tctx || !tctx->io_wq)
+		return -EINVAL;
+
+	if (!alloc_cpumask_var(&new_mask, GFP_KERNEL))
+		return -ENOMEM;
+
+	cpumask_clear(new_mask);
+	if (len > cpumask_size())
+		len = cpumask_size();
+
+	if (copy_from_user(new_mask, arg, len)) {
+		free_cpumask_var(new_mask);
+		return -EFAULT;
+	}
+
+	ret = io_wq_cpu_affinity(tctx->io_wq, new_mask);
+	free_cpumask_var(new_mask);
+	return ret;
+}
+
+static int io_unregister_iowq_aff(struct io_ring_ctx *ctx)
+{
+	struct io_uring_task *tctx = current->io_uring;
+
+	if (!tctx || !tctx->io_wq)
+		return -EINVAL;
+
+	return io_wq_cpu_affinity(tctx->io_wq, NULL);
+}
+
 static bool io_register_op_must_quiesce(int op)
 {
 	switch (op) {
@@ -9998,6 +10035,8 @@ static bool io_register_op_must_quiesce(int op)
 	case IORING_REGISTER_FILES_UPDATE2:
 	case IORING_REGISTER_BUFFERS2:
 	case IORING_REGISTER_BUFFERS_UPDATE:
+	case IORING_REGISTER_IOWQ_AFF:
+	case IORING_UNREGISTER_IOWQ_AFF:
 		return false;
 	default:
 		return true;
@@ -10137,6 +10176,18 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
 		ret = io_register_rsrc_update(ctx, arg, nr_args,
 					      IORING_RSRC_BUFFER);
 		break;
+	case IORING_REGISTER_IOWQ_AFF:
+		ret = -EINVAL;
+		if (!arg || !nr_args)
+			break;
+		ret = io_register_iowq_aff(ctx, arg, nr_args);
+		break;
+	case IORING_UNREGISTER_IOWQ_AFF:
+		ret = -EINVAL;
+		if (arg || nr_args)
+			break;
+		ret = io_unregister_iowq_aff(ctx);
+		break;
 	default:
 		ret = -EINVAL;
 		break;
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 162ff99ed2cb..f1f9ac114b51 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -306,6 +306,10 @@ enum {
 	IORING_REGISTER_BUFFERS2		= 15,
 	IORING_REGISTER_BUFFERS_UPDATE		= 16,
 
+	/* set/clear io-wq thread affinities */
+	IORING_REGISTER_IOWQ_AFF		= 17,
+	IORING_UNREGISTER_IOWQ_AFF		= 18,
+
 	/* this goes last */
 	IORING_REGISTER_LAST
 };
-- 
2.32.0