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- Add sysctls at debug.rwlock to control the behavior of the speculative

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spinning when readers hold a lock.  This spinning is speculative because,
   unlike the write case, we can not test whether the owners are running.
 - Add speculative read spinning for readers who are blocked by pending
   writers while a read lock is still held.  This allows the thread to
   spin until the write lock succeeds after which it may spin until the
   writer has released the lock.  This prevents excessive context switches
   when readers and writers both hold the lock for brief periods.

Sponsored by:	Nokia
This commit is contained in:
jeff 2008-04-04 10:00:46 +00:00
parent fd80ed9056
commit d6dabc3153
1 changed files with 26 additions and 3 deletions
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29
sys/kern/kern_rwlock.c
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@ -39,10 +39,12 @@ __FBSDID("$FreeBSD$");
#include <sys/param.h>
#include <sys/ktr.h>
#include <sys/kernel.h>
#include <sys/lock.h>
#include <sys/mutex.h>
#include <sys/proc.h>
#include <sys/rwlock.h>
#include <sys/sysctl.h>
#include <sys/systm.h>
#include <sys/turnstile.h>
@ -54,6 +56,14 @@ CTASSERT((RW_RECURSE & LO_CLASSFLAGS) == RW_RECURSE);
#define ADAPTIVE_RWLOCKS
#endif
#ifdef ADAPTIVE_RWLOCKS
static int rowner_retries = 10;
static int rowner_loops = 10000;
SYSCTL_NODE(_debug, OID_AUTO, rwlock, CTLFLAG_RD, NULL, "rwlock debugging");
SYSCTL_INT(_debug_rwlock, OID_AUTO, retry, CTLFLAG_RW, &rowner_retries, 0, "");
SYSCTL_INT(_debug_rwlock, OID_AUTO, loops, CTLFLAG_RW, &rowner_loops, 0, "");
#endif
#ifdef DDB
#include <ddb/ddb.h>
@ -261,6 +271,8 @@ _rw_rlock(struct rwlock *rw, const char *file, int line)
struct turnstile *ts;
#ifdef ADAPTIVE_RWLOCKS
volatile struct thread *owner;
int spintries = 0;
int i;
#endif
uint64_t waittime = 0;
int contested = 0;
@ -324,6 +336,16 @@ _rw_rlock(struct rwlock *rw, const char *file, int line)
cpu_spinwait();
continue;
}
} else if (spintries < rowner_retries) {
spintries++;
for (i = 0; i < rowner_loops; i++) {
v = rw->rw_lock;
if ((v & RW_LOCK_READ) == 0 || RW_CAN_READ(v))
break;
cpu_spinwait();
}
if (i != rowner_loops)
continue;
}
#endif
@ -592,7 +614,8 @@ _rw_wlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line)
cpu_spinwait();
continue;
}
if ((v & RW_LOCK_READ) && RW_READERS(v) && spintries < 100) {
if ((v & RW_LOCK_READ) && RW_READERS(v) &&
spintries < rowner_retries) {
if (!(v & RW_LOCK_WRITE_SPINNER)) {
if (!atomic_cmpset_ptr(&rw->rw_lock, v,
v | RW_LOCK_WRITE_SPINNER)) {
@ -601,12 +624,12 @@ _rw_wlock_hard(struct rwlock *rw, uintptr_t tid, const char *file, int line)
}
}
spintries++;
for (i = 100000; i > 0; i--) {
for (i = 0; i < rowner_loops; i++) {
if ((rw->rw_lock & RW_LOCK_WRITE_SPINNER) == 0)
break;
cpu_spinwait();
}
if (i)
if (i != rowner_loops)
continue;
}
#endif