locking/pvqspinlock: Allow limited lock stealing
This patch allows one attempt for the lock waiter to steal the lock
when entering the PV slowpath. To prevent lock starvation, the pending
bit will be set by the queue head vCPU when it is in the active lock
spinning loop to disable any lock stealing attempt. This helps to
reduce the performance penalty caused by lock waiter preemption while
not having much of the downsides of a real unfair lock.
The pv_wait_head() function was renamed as pv_wait_head_or_lock()
as it was modified to acquire the lock before returning. This is
necessary because of possible lock stealing attempts from other tasks.
Linux kernel builds were run in KVM guest on an 8-socket, 4
cores/socket Westmere-EX system and a 4-socket, 8 cores/socket
Haswell-EX system. Both systems are configured to have 32 physical
CPUs. The kernel build times before and after the patch were:
Westmere Haswell
Patch 32 vCPUs 48 vCPUs 32 vCPUs 48 vCPUs
----- -------- -------- -------- --------
Before patch 3m15.6s 10m56.1s 1m44.1s 5m29.1s
After patch 3m02.3s 5m00.2s 1m43.7s 3m03.5s
For the overcommited case (48 vCPUs), this patch is able to reduce
kernel build time by more than 54% for Westmere and 44% for Haswell.
Signed-off-by: Waiman Long <Waiman.Long@hpe.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Douglas Hatch <doug.hatch@hpe.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Scott J Norton <scott.norton@hpe.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1447190336-53317-1-git-send-email-Waiman.Long@hpe.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/kernel/locking/qspinlock_stat.h b/kernel/locking/qspinlock_stat.h
index b1553ad..94d4533 100644
--- a/kernel/locking/qspinlock_stat.h
+++ b/kernel/locking/qspinlock_stat.h
@@ -22,6 +22,7 @@
* pv_kick_wake - # of vCPU kicks used for computing pv_latency_wake
* pv_latency_kick - average latency (ns) of vCPU kick operation
* pv_latency_wake - average latency (ns) from vCPU kick to wakeup
+ * pv_lock_stealing - # of lock stealing operations
* pv_spurious_wakeup - # of spurious wakeups
* pv_wait_again - # of vCPU wait's that happened after a vCPU kick
* pv_wait_head - # of vCPU wait's at the queue head
@@ -43,6 +44,7 @@
qstat_pv_kick_wake,
qstat_pv_latency_kick,
qstat_pv_latency_wake,
+ qstat_pv_lock_stealing,
qstat_pv_spurious_wakeup,
qstat_pv_wait_again,
qstat_pv_wait_head,
@@ -66,6 +68,7 @@
[qstat_pv_spurious_wakeup] = "pv_spurious_wakeup",
[qstat_pv_latency_kick] = "pv_latency_kick",
[qstat_pv_latency_wake] = "pv_latency_wake",
+ [qstat_pv_lock_stealing] = "pv_lock_stealing",
[qstat_pv_wait_again] = "pv_wait_again",
[qstat_pv_wait_head] = "pv_wait_head",
[qstat_pv_wait_node] = "pv_wait_node",
@@ -273,6 +276,19 @@
#define pv_kick(c) __pv_kick(c)
#define pv_wait(p, v) __pv_wait(p, v)
+/*
+ * PV unfair trylock count tracking function
+ */
+static inline int qstat_spin_steal_lock(struct qspinlock *lock)
+{
+ int ret = pv_queued_spin_steal_lock(lock);
+
+ qstat_inc(qstat_pv_lock_stealing, ret);
+ return ret;
+}
+#undef queued_spin_trylock
+#define queued_spin_trylock(l) qstat_spin_steal_lock(l)
+
#else /* CONFIG_QUEUED_LOCK_STAT */
static inline void qstat_inc(enum qlock_stats stat, bool cond) { }