MFC r285706,r303562,r303563,r303584,r303643,r303652,r303655,r303707:

(by markj)
    Don't increment the spin count until after the first attempt to acquire a
    rwlock read lock. Otherwise the lockstat:::rw-spin probe will fire
    spuriously.

==

    rwlock: s/READER/WRITER/ in wlock lockstat annotation

==

    sx: increment spin_cnt before cpu_spinwait in xlock

    The change is a no-op only done for consistency with the rest of the file.

==

    locks: change sleep_cnt and spin_cnt types to u_int

    Both variables are uint64_t, but they only count spins or sleeps.
    All reasonable values which we can get here comfortably hit in 32-bit range.

==

    Implement trivial backoff for locking primitives.

    All current spinning loops retry an atomic op the first chance they get,
    which leads to performance degradation under load.

    One classic solution to the problem consists of delaying the test to an
    extent. This implementation has a trivial linear increment and a random
    factor for each attempt.

    For simplicity, this first thouch implementation only modifies spinning
    loops where the lock owner is running. spin mutexes and thread lock were
    not modified.

    Current parameters are autotuned on boot based on mp_cpus.

    Autotune factors are very conservative and are subject to change later.

==

    locks: fix up ifdef guards introduced in r303643

    Both sx and rwlocks had copy-pasted ADAPTIVE_MUTEXES instead of the correct
    define.

==

    locks: fix compilation for KDTRACE_HOOKS && !ADAPTIVE_* case

==

    locks: fix sx compilation on mips after r303643

    The kernel.h header is required for the SYSINIT macro, which apparently
    was present on amd64 by accident.

1 files changed, 29 insertions, 0 deletions

diff --git a/sys/kern/subr_lock.c b/sys/kern/subr_lock.c
index 8aec803..cacaf56 100644
--- a/sys/kern/subr_lock.c
+++ b/sys/kern/subr_lock.c
@@ -55,6 +55,7 @@ __FBSDID("$FreeBSD$");
 #endif
 
 #include <machine/cpufunc.h>
+#include <machine/cpu.h>
 
 CTASSERT(LOCK_CLASS_MAX == 15);
 
@@ -103,6 +104,34 @@ lock_destroy(struct lock_object *lock)
 	lock->lo_flags &= ~LO_INITIALIZED;
 }
 
+void
+lock_delay(struct lock_delay_arg *la)
+{
+	u_int i, delay, backoff, min, max;
+	struct lock_delay_config *lc = la->config;
+
+	delay = la->delay;
+
+	if (delay == 0)
+		delay = lc->initial;
+	else {
+		delay += lc->step;
+		max = lc->max;
+		if (delay > max)
+			delay = max;
+	}
+
+	backoff = cpu_ticks() % delay;
+	min = lc->min;
+	if (backoff < min)
+		backoff = min;
+	for (i = 0; i < backoff; i++)
+		cpu_spinwait();
+
+	la->delay = delay;
+	la->spin_cnt += backoff;
+}
+
 #ifdef DDB
 DB_SHOW_COMMAND(lock, db_show_lock)
 {