Illumos 5008 - lock contention (rrw_exit) while running a read only load

5008 lock contention (rrw_exit) while running a read only load
Reviewed by: Matthew Ahrens <matthew.ahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Alex Reece <alex.reece@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: Richard Yao <ryao@gentoo.org>
Reviewed by: Saso Kiselkov <skiselkov.ml@gmail.com>
Approved by: Garrett D'Amore <garrett@damore.org>

Porting notes:

This patch ported perfectly cleanly to ZoL.  During testing 100% cached
small-block reads, extreme contention was noticed on rrl->rr_lock from
rrw_exit() due to the frequent entering and leaving ZPL.  Illumos picked
up this patch from FreeBSD and it also helps under Linux.

On a 1-minute 4K cached read test with 10 fio processes pinned to a single
socket on a 4-socket (10 thread per socket) NUMA system, contentions on
rrl->rr_lock were reduced from 508799 to 43085.

Ported-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3555

include/sys/zfs_znode.h

@@ -250,7 +250,7 @@ typedef struct znode {
 /* Called on entry to each ZFS vnode and vfs operation  */
 #define	ZFS_ENTER(zsb) \
 	{ \
-		rrw_enter_read(&(zsb)->z_teardown_lock, FTAG); \
+		rrm_enter_read(&(zsb)->z_teardown_lock, FTAG); \
 		if ((zsb)->z_unmounted) { \
 			ZFS_EXIT(zsb); \
 			return (EIO); \
@@ -260,7 +260,7 @@ typedef struct znode {
 /* Must be called before exiting the vop */
 #define	ZFS_EXIT(zsb) \
 	{ \
-		rrw_exit(&(zsb)->z_teardown_lock, FTAG); \
+		rrm_exit(&(zsb)->z_teardown_lock, FTAG); \
 	}
 
 /* Verifies the znode is valid */