ovl: ovl_fs::creator_cred::usage scalability issues

[Chen Hu <hu1.chen@intel.com>]

*Problem*
ovl_permission() checks the underlying inode with the credential of mounter.
The cred, struct ovl_fs::creator_cred, is somewhat global per overlayfs
superblock. Performance degrades when concurrency increases on the cred, to be
specific, on ovl_fs::creator_cred::usage.

This happens when there are massive file access inside container, especially
on SoC with many cores. With Linux 6.6.0-rc2, we run a web workload container
on Intel 4th Xeon Sapphire Rapids which has 56 cores. Perf reports that 5.7%
(2.50% + 1.87% + 1.33%) CPU stall in overlayfs:
Self    Command       Shared Object            Symbol
2.50%   foo           [kernel.vmlinux]         [k] override_creds
1.87%   foo           [kernel.vmlinux]         [k] revert_creds
1.33%   foo           [kernel.vmlinux]         [k] generic_permission

On Soc with more than 100 cores, we can even observe ~30% CPU stalled!

This scalability issue is caused by two factors:
1) Contention on creator_cred::usage
   creator_cred::usage is atomic_t and is inc/dec atomically during every file
   access. So HW acquires the corresponding cache line exclusively. This
   operataiton is expensive and gets worse when contention is heavy.
   Call chain:
      ovl_permission()
      -> ovl_override_creds()
      -> override_creds()
      -> get_new_cred()
      -> atomic_inc(&cred->usage);

      ovl_permission()
      -> revert_creds()
      -> put_cred()
      -> atomic_dec_and_test(&(cred)->usage))

2) False sharing
   `perf c2c` shows false sharing issue between cred::usage and cred::fsuid.
   This is why generic_permission() stalls 1.33% CPU in above perf report.
   ovl_permission() updates cred::usage and it also reads cred::fsuid.
   Unfortunately, they locate in the same cache line and thus false sharing
   occurs. cred::fsuid is read at:
      ovl_permission()
      -> inode_permission()
      -> generic_permission()
      -> acl_permission_check()
      -> current_fsuid()

*Mitigations we tried*
We tried several mitigations but are not sure if it can be a fix or just
workaround / hack. So we report this and want to have some discussions.

Our mitigations aims to eliminate the contention on creator_cred->usage.
Without contention, the false sharing will be tiny and no need to handle. The
mitigations we tested are:
   1) Check underlying inode once in its lifetime. 
   OR
   2) In ovl_permission(), copy global creator_cred to a local variable to
      avoid concurrency.

With any mitigations above, CPU will not stall on overlayfs.

Paste mitigation 1 below.

From 472bd18eaabcde0d41e450f556691151b1bdb64e Mon Sep 17 00:00:00 2001
From: Chen Hu <hu1.chen@intel.com>
Date: Fri, 1 Sep 2023 15:03:28 +0800
Subject: [RFC PATCH] ovl: check underlying upper inode once in its lifetime

ovl_permission() checks the underlying inode with the credential of
mounter. The cred, struct ovl_fs::creator_cred, is global per overlayfs
superblock. Performance degrades when concurrency increases on the cred,
to be specific, on ovl_fs::creator_cred::usage.

This patch (or hack to some extent) checks underlying upper inode once
in its lifetime, eliminates the cache line contention on
creator_cred::usage and gets 40%+ perf improvement on a 128 cores CPU.

CAUTION:
this may compromise the file permission check. Need to talk with
overlayfs experts.

Signed-off-by: Chen Hu <hu1.chen@intel.com>
---
 fs/overlayfs/inode.c     | 5 ++++-
 fs/overlayfs/overlayfs.h | 1 +
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index 83ef66644c21..62ec99316c7a 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -307,7 +307,7 @@ int ovl_permission(struct mnt_idmap *idmap,
 	 * with creds of mounter
 	 */
 	err = generic_permission(&nop_mnt_idmap, inode, mask);
-	if (err)
+	if (err || ovl_test_flag(OVL_FASTPERM, inode))
 		return err;
 
 	old_cred = ovl_override_creds(inode->i_sb);
@@ -318,6 +318,9 @@ int ovl_permission(struct mnt_idmap *idmap,
 		mask |= MAY_READ;
 	}
 	err = inode_permission(mnt_idmap(realpath.mnt), realinode, mask);
+	if (err == 0 && upperinode)
+		/* This gets set once for the upper inode lifetime */
+		ovl_set_flag(OVL_FASTPERM, inode);
 	revert_creds(old_cred);
 
 	return err;
diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index 9817b2dcb132..5b71aaa8f77c 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -53,6 +53,7 @@ enum ovl_inode_flag {
 	OVL_CONST_INO,
 	OVL_HAS_DIGEST,
 	OVL_VERIFIED_DIGEST,
+	OVL_FASTPERM,
 };
 
 enum ovl_entry_flag {
-- 
2.34.1