svn commit: r332398 - head/sys/kern

[Mateusz Guzik]

Author: mjg
Date: Wed Apr 11 01:43:29 2018
New Revision: 332398
URL: https://svnweb.freebsd.org/changeset/base/332398

Log:
  locks: extend speculative spin waiting for readers to drain
  
  Now that 10 years have passed since the original limit of 10000 was
  committed, bump it a little bit.
  
  Spinning waiting for writers is semi-informed in the sense that we always
  know if the owner is running and base the decision to spin on that.
  However, no such information is provided for read-locking. In particular
  this means that it is possible for a write-spinner to completely waste cpu
  time waiting for the lock to be released, while the reader holding it was
  preempted and is now waiting for the spinner to go off cpu.
  
  Nonetheless, in majority of cases it is an improvement to spin instead of
  instantly giving up and going to sleep.
  
  The current approach is pretty simple: snatch the number of current readers
  and performs that many pauses before checking again. The total number of
  pauses to execute is limited to 10k. If the lock is still not free by
  that time, go to sleep.
  
  Given the previously noted problem of not knowing whether spinning makes
  any sense to begin with the new limit has to remain rather conservative.
  But at the very least it should also be related to the machine. Waiting
  for writers uses parameters selected based on the number of activated
  hardware threads. The upper limit of pause instructions to be executed
  in-between re-reads of the lock is typically 16384 or 32678. It was
  selected as the limit of total spins. The lower bound is set to
  already present 10000 as to not change it for smaller machines.
  
  Bumping the limit reduces system time by few % during benchmarks like
  buildworld, buildkernel and others. Tested on 2 and 4 socket machines
  (Broadwell, Skylake).
  
  Figuring out how to make a more informed decision while not pessimizing
  the fast path is left as an exercise for the reader.

Modified:
  head/sys/kern/kern_rwlock.c
  head/sys/kern/kern_sx.c

Modified: head/sys/kern/kern_rwlock.c
==============================================================================
--- head/sys/kern/kern_rwlock.c	Tue Apr 10 23:38:31 2018	(r332397)
+++ head/sys/kern/kern_rwlock.c	Wed Apr 11 01:43:29 2018	(r332398)
@@ -95,8 +95,8 @@ struct lock_class lock_class_rw = {
 };
 
 #ifdef ADAPTIVE_RWLOCKS
-static int __read_frequently rowner_retries = 10;
-static int __read_frequently rowner_loops = 10000;
+static int __read_frequently rowner_retries;
+static int __read_frequently rowner_loops;
 static SYSCTL_NODE(_debug, OID_AUTO, rwlock, CTLFLAG_RD, NULL,
     "rwlock debugging");
 SYSCTL_INT(_debug_rwlock, OID_AUTO, retry, CTLFLAG_RW, &rowner_retries, 0, "");
@@ -109,7 +109,15 @@ SYSCTL_INT(_debug_rwlock, OID_AUTO, delay_base, CTLFLA
 SYSCTL_INT(_debug_rwlock, OID_AUTO, delay_max, CTLFLAG_RW, &rw_delay.max,
     0, "");
 
-LOCK_DELAY_SYSINIT_DEFAULT(rw_delay);
+static void
+rw_lock_delay_init(void *arg __unused)
+{
+
+	lock_delay_default_init(&rw_delay);
+	rowner_retries = 10;
+	rowner_loops = max(10000, rw_delay.max);
+}
+LOCK_DELAY_SYSINIT(rw_lock_delay_init);
 #endif
 
 /*

Modified: head/sys/kern/kern_sx.c
==============================================================================
--- head/sys/kern/kern_sx.c	Tue Apr 10 23:38:31 2018	(r332397)
+++ head/sys/kern/kern_sx.c	Wed Apr 11 01:43:29 2018	(r332398)
@@ -145,8 +145,8 @@ struct lock_class lock_class_sx = {
 #endif
 
 #ifdef ADAPTIVE_SX
-static __read_frequently u_int asx_retries = 10;
-static __read_frequently u_int asx_loops = 10000;
+static __read_frequently u_int asx_retries;
+static __read_frequently u_int asx_loops;
 static SYSCTL_NODE(_debug, OID_AUTO, sx, CTLFLAG_RD, NULL, "sxlock debugging");
 SYSCTL_UINT(_debug_sx, OID_AUTO, retries, CTLFLAG_RW, &asx_retries, 0, "");
 SYSCTL_UINT(_debug_sx, OID_AUTO, loops, CTLFLAG_RW, &asx_loops, 0, "");
@@ -158,7 +158,15 @@ SYSCTL_INT(_debug_sx, OID_AUTO, delay_base, CTLFLAG_RW
 SYSCTL_INT(_debug_sx, OID_AUTO, delay_max, CTLFLAG_RW, &sx_delay.max,
     0, "");
 
-LOCK_DELAY_SYSINIT_DEFAULT(sx_delay);
+static void
+sx_lock_delay_init(void *arg __unused)
+{
+
+	lock_delay_default_init(&sx_delay);
+	asx_retries = 10;
+	asx_loops = max(10000, sx_delay.max);
+}
+LOCK_DELAY_SYSINIT(sx_lock_delay_init);
 #endif
 
 void