Switch vfs.nfsd.issue_delegations to TUNABLE ?

[Emmanuel Vadot]

 Hello,

 I would like to switch the vfs.nfsd.issue_delegations sysctl to a
tunable.
 The reason behind it is recent problem at work on some on our filer
related to NFS.
 We use NFSv4 without delegation as we never been able to have good
performance with FreeBSD server and Linux client (we need to do test
again but that's for later). We recently see all of our filers with NFS
enabled perform pourly, resulting in really bad performance on the
service.
 After doing some analyze with pmcstat we've seen that we spend ~50%
of our time in lock delay, called by nfsrv_checkgetattr (See [1]).
 This function is only usefull when using delegation, as it search for
some write delegations issued to client, but it's called anyway when
there so no delegation and cause a lot of problem when having a lot of
activities.
 We've patched the kernel with the included diff and now everything is
fine (tm) (See [2]).
 The problem for upstreaming this patch is that since issue_delegations
is a sysctl we cannot know if the checkgetattr called is needed, hence
the question to switch it to a TUNABLE. Also maybe some other code path
could benefit from it, I haven't read all the source of nfsserver yet.

 Note that I won't MFC the changes as it would break POLA.

 Cheers,

[1] https://people.freebsd.org/~manu/m8.svg
[2] https://people.freebsd.org/~manu/m8-new.svg

>From 0cba277f406d3ccf3c9e943a3d4e17b529e31c89 Mon Sep 17 00:00:00 2001
From: Emmanuel Vadot <manu at freebsd.org>
Date: Fri, 24 Nov 2017 11:17:18 +0100
Subject: [PATCH 2/4] Do not call nfsrv_checkgetttr if delegation isn't
enable.

Signed-off-by: Emmanuel Vadot <manu at freebsd.org>
---
 sys/fs/nfsserver/nfs_nfsdserv.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/sys/fs/nfsserver/nfs_nfsdserv.c
b/sys/fs/nfsserver/nfs_nfsdserv.c index 8102c5810a9..8daf0142360 100644
--- a/sys/fs/nfsserver/nfs_nfsdserv.c
+++ b/sys/fs/nfsserver/nfs_nfsdserv.c
@@ -54,6 +54,7 @@ extern struct timeval nfsboottime;
 extern int nfs_rootfhset;
 extern int nfsrv_enable_crossmntpt;
 extern int nfsrv_statehashsize;
+extern int nfsrv_issuedelegs;
 #endif	/* !APPLEKEXT */
 
 static int	nfs_async = 0;
@@ -240,7 +241,7 @@ nfsrvd_getattr(struct nfsrv_descript *nd, int
isdgram, if (nd->nd_flag & ND_NFSV4) {
 			if (NFSISSET_ATTRBIT(&attrbits,
NFSATTRBIT_FILEHANDLE)) nd->nd_repstat = nfsvno_getfh(vp, &fh, p);
-			if (!nd->nd_repstat)
+			if (nd->nd_repstat == 0 && nfsrv_issuedelegs
== 1) nd->nd_repstat = nfsrv_checkgetattr(nd, vp,
 				    &nva, &attrbits, nd->nd_cred, p);
 			if (nd->nd_repstat == 0) {
-- 
2.14.2


-- 
Emmanuel Vadot <manu at bidouilliste.com> <manu at freebsd.org>