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From:   Pavel Begunkov <asml.silence@gmail.com>
To:     io-uring@vger.kernel.org
Cc:     Jens Axboe <axboe@kernel.dk>, asml.silence@gmail.com,
        linux-kernel@vger.kernel.org
Subject: [PATCH 01/11] io_uring/rsrc: use non-pcpu refcounts for nodes
Date:   Thu, 30 Mar 2023 15:53:19 +0100
Message-Id: <243504691227d611f27a0f642ec8982db6ef4ead.1680187408.git.asml.silence@gmail.com>
In-Reply-To: <cover.1680187408.git.asml.silence@gmail.com>
References: <cover.1680187408.git.asml.silence@gmail.com>
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One problem with the current rsrc infra is that often updates will
generates lots of rsrc nodes, each carry pcpu refs. That takes quite a
lot of memory, especially if there is a stall, and takes lots of CPU
cycles. Only pcpu allocations takes >50 of CPU with a naive benchmark
updating files in a loop.

Replace pcpu refs with normal refcounting. There is already a hot path
avoiding atomics / refs, but following patches will further improve it.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 io_uring/rsrc.c | 15 +++++----------
 io_uring/rsrc.h |  6 ++++--
 2 files changed, 9 insertions(+), 12 deletions(-)

diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
index 7a43aed8e395..f2da9e251e3f 100644
--- a/io_uring/rsrc.c
+++ b/io_uring/rsrc.c
@@ -155,7 +155,7 @@ void io_rsrc_refs_refill(struct io_ring_ctx *ctx)
 	__must_hold(&ctx->uring_lock)
 {
 	ctx->rsrc_cached_refs += IO_RSRC_REF_BATCH;
-	percpu_ref_get_many(&ctx->rsrc_node->refs, IO_RSRC_REF_BATCH);
+	refcount_add(IO_RSRC_REF_BATCH, &ctx->rsrc_node->refs);
 }
 
 static void __io_rsrc_put_work(struct io_rsrc_node *ref_node)
@@ -220,13 +220,11 @@ void io_wait_rsrc_data(struct io_rsrc_data *data)
 
 void io_rsrc_node_destroy(struct io_rsrc_node *ref_node)
 {
-	percpu_ref_exit(&ref_node->refs);
 	kfree(ref_node);
 }
 
-static __cold void io_rsrc_node_ref_zero(struct percpu_ref *ref)
+__cold void io_rsrc_node_ref_zero(struct io_rsrc_node *node)
 {
-	struct io_rsrc_node *node = container_of(ref, struct io_rsrc_node, refs);
 	struct io_ring_ctx *ctx = node->rsrc_data->ctx;
 	unsigned long flags;
 	bool first_add = false;
@@ -269,11 +267,7 @@ static struct io_rsrc_node *io_rsrc_node_alloc(void)
 	if (!ref_node)
 		return NULL;
 
-	if (percpu_ref_init(&ref_node->refs, io_rsrc_node_ref_zero,
-			    0, GFP_KERNEL)) {
-		kfree(ref_node);
-		return NULL;
-	}
+	refcount_set(&ref_node->refs, 1);
 	INIT_LIST_HEAD(&ref_node->node);
 	INIT_LIST_HEAD(&ref_node->rsrc_list);
 	ref_node->done = false;
@@ -298,7 +292,8 @@ void io_rsrc_node_switch(struct io_ring_ctx *ctx,
 		spin_unlock_irq(&ctx->rsrc_ref_lock);
 
 		atomic_inc(&data_to_kill->refs);
-		percpu_ref_kill(&rsrc_node->refs);
+		/* put master ref */
+		io_rsrc_put_node(rsrc_node, 1);
 		ctx->rsrc_node = NULL;
 	}
 
diff --git a/io_uring/rsrc.h b/io_uring/rsrc.h
index f27f4975217d..1467b31843bc 100644
--- a/io_uring/rsrc.h
+++ b/io_uring/rsrc.h
@@ -37,7 +37,7 @@ struct io_rsrc_data {
 };
 
 struct io_rsrc_node {
-	struct percpu_ref		refs;
+	refcount_t			refs;
 	struct list_head		node;
 	struct list_head		rsrc_list;
 	struct io_rsrc_data		*rsrc_data;
@@ -54,6 +54,7 @@ struct io_mapped_ubuf {
 };
 
 void io_rsrc_put_tw(struct callback_head *cb);
+void io_rsrc_node_ref_zero(struct io_rsrc_node *node);
 void io_rsrc_put_work(struct work_struct *work);
 void io_rsrc_refs_refill(struct io_ring_ctx *ctx);
 void io_wait_rsrc_data(struct io_rsrc_data *data);
@@ -109,7 +110,8 @@ int io_register_rsrc(struct io_ring_ctx *ctx, void __user *arg,
 
 static inline void io_rsrc_put_node(struct io_rsrc_node *node, int nr)
 {
-	percpu_ref_put_many(&node->refs, nr);
+	if (refcount_sub_and_test(nr, &node->refs))
+		io_rsrc_node_ref_zero(node);
 }
 
 static inline void io_req_put_rsrc(struct io_kiocb *req)
-- 
2.39.1