Track lock contends

e91ed64d · Silas Boyd-Wickizer · 21daa928 · e91ed64d · e91ed64d · e91ed64d
--- a/lockstat.c
+++ b/lockstat.c
@@ -24,7 +24,7 @@ stats(void)
  if (fd < 0)
    die("lockstat: open failed");

-  printf(1, "## name acquires locking locked\n");
+  printf(1, "## name acquires contends locking locked\n");
  while (1) {
    r = read(fd, &ls, sz);
    if (r < 0)
@@ -34,16 +34,18 @@ stats(void)
    if (r != sz)
      die("lockstat: unexpected read");

-    u64 acquires = 0, locking = 0, locked = 0;
+    u64 acquires = 0, contends = 0,
+      locking = 0, locked = 0;
    
    for (int i = 0; i < NCPU; i++) {
-      acquires += ls.cpu[i].acquires;      
+      acquires += ls.cpu[i].acquires;
+      contends += ls.cpu[i].contends;
      locking += ls.cpu[i].locking;
      locked += ls.cpu[i].locked;
    }
-    if (acquires > 0)
-      printf(1, "%s %lu %lu %lu\n", 
-             ls.name, acquires, locking, locked);
+    if (contends > 0)
+      printf(1, "%s %lu %lu %lu %lu\n", 
+             ls.name, acquires, contends, locking, locked);
  }
 }


--- a/lockstat.h
+++ b/lockstat.h
@@ -2,8 +2,10 @@

 struct cpulockstat {
  u64 acquires;
+  u64 contends;
  u64 locking;
  u64 locked;
+
  u64 locking_ts;
  u64 locked_ts;
  __padout__;

--- a/spinlock.c
+++ b/spinlock.c
@@ -40,7 +40,7 @@ locking(struct spinlock *lk)
 }

 static inline void
-locked(struct spinlock *lk)
+locked(struct spinlock *lk, u64 retries)
 {
  mtacquired(lk);
  
@@ -53,6 +53,8 @@ locked(struct spinlock *lk)
 #if LOCKSTAT
  if (lockstat_enable && lk->stat != NULL) {
    struct cpulockstat *s = mylockstat(lk);
+    if (retries > 0)
+      s->contends++;
    s->acquires++;
    s->locked_ts = rdtsc();
  }
@@ -117,7 +119,7 @@ tryacquire(struct spinlock *lk)
      popcli();
      return 0;
  }
-  locked(lk);
+  locked(lk, 0);
  return 1;
 }

@@ -128,14 +130,15 @@ tryacquire(struct spinlock *lk)
 void
 acquire(struct spinlock *lk)
 {
+  u64 retries;
+
  pushcli();
  locking(lk);
-  // The xchg is atomic.
-  // It also serializes, so that reads after acquire are not
-  // reordered before it.
+
+  retries = 0;
  while(xchg32(&lk->locked, 1) != 0)
-    ;
-  locked(lk);
+    retries++;
+  locked(lk, retries);
 }

 // Release the lock.