linux-kernel - [PATCH] pid: reorder fields in pid_namespace to reduce false sharing

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Message-ID: <20260120204820.1497002-1-mjguzik@gmail.com>
Date: Tue, 20 Jan 2026 21:48:20 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: brauner@...nel.org
Cc: viro@...iv.linux.org.uk,
	jack@...e.cz,
	linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	Mateusz Guzik <mjguzik@...il.com>
Subject: [PATCH] pid: reorder fields in pid_namespace to reduce false sharing

alloc_pid() loads pid_cachep, level and pid_max prior to taking the
lock.

It dirties idr and pid_allocated with the lock.

Some of these fields share the cacheline as is, split them up.

No change in the size of the struct.

Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
---

this is independent of other patches

i got an inconsistent win in terms of throughput rate, but relative
contention between pidmap lock and the rest went down

 include/linux/pid_namespace.h | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
index 0e7ae12c96d2..b20baaa7e62b 100644
--- a/include/linux/pid_namespace.h
+++ b/include/linux/pid_namespace.h
@@ -27,6 +27,13 @@ struct pid_namespace {
 	struct idr idr;
 	struct rcu_head rcu;
 	unsigned int pid_allocated;
+#ifdef CONFIG_SYSCTL
+#if defined(CONFIG_MEMFD_CREATE)
+	int memfd_noexec_scope;
+#endif
+	struct ctl_table_set	set;
+	struct ctl_table_header *sysctls;
+#endif
 	struct task_struct *child_reaper;
 	struct kmem_cache *pid_cachep;
 	unsigned int level;
@@ -40,13 +47,6 @@ struct pid_namespace {
 	int reboot;	/* group exit code if this pidns was rebooted */
 	struct ns_common ns;
 	struct work_struct	work;
-#ifdef CONFIG_SYSCTL
-	struct ctl_table_set	set;
-	struct ctl_table_header *sysctls;
-#if defined(CONFIG_MEMFD_CREATE)
-	int memfd_noexec_scope;
-#endif
-#endif
 } __randomize_layout;
 
 extern struct pid_namespace init_pid_ns;
-- 
2.48.1