Introduce support for adaptive spinning in lockmgr.

Actually, as it did receive few tuning, the support is disabled by
default, but it can opt-in with the option ADAPTIVE_LOCKMGRS.
Due to the nature of lockmgrs, adaptive spinning needs to be
selectively enabled for any interested lockmgr.
The support is bi-directional, or, in other ways, it will work in both
cases if the lock is held in read or write way.  In particular, the
read path is passible of further tunning using the sysctls
debug.lockmgr.retries and debug.lockmgr.loops .  Ideally, such sysctls
should be axed or compiled out before release.

Addictionally note that adaptive spinning doesn't cope well with
LK_SLEEPFAIL.  The reason is that many (and probabilly all) consumers
of LK_SLEEPFAIL are mainly interested in knowing if the interlock was
dropped or not in order to reacquire it and re-test initial conditions.
This directly interacts with adaptive spinning because lockmgr needs
to drop the interlock while spinning in order to avoid a deadlock
(further details in the comments inside the patch).

Final note: finding someone willing to help on tuning this with
relevant workloads would be either very important and appreciated.

Tested by:	jeff, pho
Requested by:	many

1 files changed, 1 insertions, 0 deletions

diff --git a/sys/conf/options b/sys/conf/options
index 3c56a81..58bf9b9 100644
--- a/sys/conf/options
+++ b/sys/conf/options
@@ -60,6 +60,7 @@ KDB_UNATTENDED	opt_kdb.h
 SYSCTL_DEBUG	opt_sysctl.h
 
 # Miscellaneous options.
+ADAPTIVE_LOCKMGRS
 ALQ
 AUDIT		opt_global.h
 CODA_COMPAT_5	opt_coda.h