linux-kernel - [PATCH] RDMA/restrack: Reorder fields in 'struct rdma_restrack

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Date:   Tue, 14 Feb 2023 13:53:52 +0100
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Jason Gunthorpe <jgg@...pe.ca>, Leon Romanovsky <leon@...nel.org>
Cc:     linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        linux-rdma@...r.kernel.org
Subject: [PATCH] RDMA/restrack: Reorder fields in 'struct rdma_restrack_entry'

Group some variables based on their sizes to reduce hole and avoid padding.
On x86_64, this shrinks the size from 136 to 128 bytes.

Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
---
Using pahole

Before:
======
struct rdma_restrack_entry {
	bool                       valid;                /*     0     1 */
	u8                         no_track:1;           /*     1: 0  1 */

	/* XXX 7 bits hole, try to pack */
	/* XXX 2 bytes hole, try to pack */

	struct kref                kref;                 /*     4     4 */
	struct completion          comp;                 /*     8    96 */
	/* --- cacheline 1 boundary (64 bytes) was 40 bytes ago --- */
	struct task_struct *       task;                 /*   104     8 */
	const char  *              kern_name;            /*   112     8 */
	enum rdma_restrack_type    type;                 /*   120     4 */
	bool                       user;                 /*   124     1 */

	/* XXX 3 bytes hole, try to pack */

	/* --- cacheline 2 boundary (128 bytes) --- */
	u32                        id;                   /*   128     4 */

	/* size: 136, cachelines: 3, members: 9 */
	/* sum members: 126, holes: 2, sum holes: 5 */
	/* sum bitfield members: 1 bits, bit holes: 1, sum bit holes: 7 bits */
	/* padding: 4 */
	/* last cacheline: 8 bytes */
};

After:
=====
struct rdma_restrack_entry {
	bool                       valid;                /*     0     1 */
	u8                         no_track:1;           /*     1: 0  1 */

	/* XXX 7 bits hole, try to pack */

	bool                       user;                 /*     2     1 */

	/* XXX 1 byte hole, try to pack */

	struct kref                kref;                 /*     4     4 */
	struct completion          comp;                 /*     8    96 */
	/* --- cacheline 1 boundary (64 bytes) was 40 bytes ago --- */
	struct task_struct *       task;                 /*   104     8 */
	const char  *              kern_name;            /*   112     8 */
	enum rdma_restrack_type    type;                 /*   120     4 */
	u32                        id;                   /*   124     4 */

	/* size: 128, cachelines: 2, members: 9 */
	/* sum members: 126, holes: 1, sum holes: 1 */
	/* sum bitfield members: 1 bits, bit holes: 1, sum bit holes: 7 bits */
};
---
 include/rdma/restrack.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/rdma/restrack.h b/include/rdma/restrack.h
index 8b7c46daeb07..da53fefe6f9e 100644
--- a/include/rdma/restrack.h
+++ b/include/rdma/restrack.h
@@ -80,6 +80,10 @@ struct rdma_restrack_entry {
 	 * query stage.
 	 */
 	u8			no_track : 1;
+	/**
+	 * @user: user resource
+	 */
+	bool			user;
 	/*
 	 * @kref: Protect destroy of the resource
 	 */
@@ -106,10 +110,6 @@ struct rdma_restrack_entry {
 	 * @type: various objects in restrack database
 	 */
 	enum rdma_restrack_type	type;
-	/**
-	 * @user: user resource
-	 */
-	bool			user;
 	/**
 	 * @id: ID to expose to users
 	 */
-- 
2.34.1