Commit-ID: cd0272fab785077c121aa91ec2401090965bbc37
Gitweb: http://git.kernel.org/tip/cd0272fab785077c121aa91ec2401090965bbc37
Author: Waiman Long <[email protected]>
AuthorDate: Mon, 9 Nov 2015 19:09:27 -0500
Committer: Ingo Molnar <[email protected]>
CommitDate: Fri, 4 Dec 2015 11:39:51 +0100

locking/pvqspinlock: Queue node adaptive spinning

In an overcommitted guest where some vCPUs have to be halted to make
forward progress in other areas, it is highly likely that a vCPU later
in the spinlock queue will be spinning while the ones earlier in the
queue would have been halted. The spinning in the later vCPUs is then
just a waste of precious CPU cycles because they are not going to
get the lock soon as the earlier ones have to be woken up and take
their turn to get the lock.

This patch implements an adaptive spinning mechanism where the vCPU
will call pv_wait() if the previous vCPU is not running.

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 3m02.3s 5m00.2s 1m43.7s 3m03.5s
After patch 3m03.0s 4m37.5s 1m43.0s 2m47.2s

For 32 vCPUs, this patch doesn't cause any noticeable change in
performance. For 48 vCPUs (over-committed), there is about 8%
performance improvement.

Signed-off-by: Waiman Long <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Douglas Hatch <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Scott J Norton <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>
---
kernel/locking/qspinlock.c | 5 ++--
kernel/locking/qspinlock_paravirt.h | 46 +++++++++++++++++++++++++++++++++++--
kernel/locking/qspinlock_stat.h | 3 +++
3 files changed, 50 insertions(+), 4 deletions(-)

diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index 2ea4299..393d187 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -248,7 +248,8 @@ static __always_inline void set_locked(struct qspinlock *lock)
*/

static __always_inline void __pv_init_node(struct mcs_spinlock *node) { }
-static __always_inline void __pv_wait_node(struct mcs_spinlock *node) { }
+static __always_inline void __pv_wait_node(struct mcs_spinlock *node,
+ struct mcs_spinlock *prev) { }
static __always_inline void __pv_kick_node(struct qspinlock *lock,
struct mcs_spinlock *node) { }
static __always_inline u32 __pv_wait_head_or_lock(struct qspinlock *lock,
@@ -407,7 +408,7 @@ queue:
prev = decode_tail(old);
WRITE_ONCE(prev->next, node);

- pv_wait_node(node);
+ pv_wait_node(node, prev);
arch_mcs_spin_lock_contended(&node->locked);

/*
diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
index ace60a4..87bb235 100644
--- a/kernel/locking/qspinlock_paravirt.h
+++ b/kernel/locking/qspinlock_paravirt.h
@@ -23,6 +23,20 @@
#define _Q_SLOW_VAL (3U << _Q_LOCKED_OFFSET)

/*
+ * Queue Node Adaptive Spinning
+ *
+ * A queue node vCPU will stop spinning if the vCPU in the previous node is
+ * not running. The one lock stealing attempt allowed at slowpath entry
+ * mitigates the slight slowdown for non-overcommitted guest with this
+ * aggressive wait-early mechanism.
+ *
+ * The status of the previous node will be checked at fixed interval
+ * controlled by PV_PREV_CHECK_MASK. This is to ensure that we won't
+ * pound on the cacheline of the previous node too heavily.
+ */
+#define PV_PREV_CHECK_MASK 0xff
+
+/*
* Queue node uses: vcpu_running & vcpu_halted.
* Queue head uses: vcpu_running & vcpu_hashed.
*/
@@ -235,6 +249,20 @@ static struct pv_node *pv_unhash(struct qspinlock *lock)
}

/*
+ * Return true if when it is time to check the previous node which is not
+ * in a running state.
+ */
+static inline bool
+pv_wait_early(struct pv_node *prev, int loop)
+{
+
+ if ((loop & PV_PREV_CHECK_MASK) != 0)
+ return false;
+
+ return READ_ONCE(prev->state) != vcpu_running;
+}
+
+/*
* Initialize the PV part of the mcs_spinlock node.
*/
static void pv_init_node(struct mcs_spinlock *node)
@@ -252,17 +280,23 @@ static void pv_init_node(struct mcs_spinlock *node)
* pv_kick_node() is used to set _Q_SLOW_VAL and fill in hash table on its
* behalf.
*/
-static void pv_wait_node(struct mcs_spinlock *node)
+static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
{
struct pv_node *pn = (struct pv_node *)node;
+ struct pv_node *pp = (struct pv_node *)prev;
int waitcnt = 0;
int loop;
+ bool wait_early;

/* waitcnt processing will be compiled out if !QUEUED_LOCK_STAT */
for (;; waitcnt++) {
- for (loop = SPIN_THRESHOLD; loop; loop--) {
+ for (wait_early = false, loop = SPIN_THRESHOLD; loop; loop--) {
if (READ_ONCE(node->locked))
return;
+ if (pv_wait_early(pp, loop)) {
+ wait_early = true;
+ break;
+ }
cpu_relax();
}

@@ -280,6 +314,7 @@ static void pv_wait_node(struct mcs_spinlock *node)
if (!READ_ONCE(node->locked)) {
qstat_inc(qstat_pv_wait_node, true);
qstat_inc(qstat_pv_wait_again, waitcnt);
+ qstat_inc(qstat_pv_wait_early, wait_early);
pv_wait(&pn->state, vcpu_halted);
}

@@ -365,6 +400,12 @@ pv_wait_head_or_lock(struct qspinlock *lock, struct mcs_spinlock *node)

for (;; waitcnt++) {
/*
+ * Set correct vCPU state to be used by queue node wait-early
+ * mechanism.
+ */
+ WRITE_ONCE(pn->state, vcpu_running);
+
+ /*
* Set the pending bit in the active lock spinning loop to
* disable lock stealing before attempting to acquire the lock.
*/
@@ -402,6 +443,7 @@ pv_wait_head_or_lock(struct qspinlock *lock, struct mcs_spinlock *node)
goto gotlock;
}
}
+ WRITE_ONCE(pn->state, vcpu_halted);
qstat_inc(qstat_pv_wait_head, true);
qstat_inc(qstat_pv_wait_again, waitcnt);
pv_wait(&l->locked, _Q_SLOW_VAL);
diff --git a/kernel/locking/qspinlock_stat.h b/kernel/locking/qspinlock_stat.h
index 94d4533..640dcec 100644
--- a/kernel/locking/qspinlock_stat.h
+++ b/kernel/locking/qspinlock_stat.h
@@ -25,6 +25,7 @@
* 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_early - # of early vCPU wait's
* pv_wait_head - # of vCPU wait's at the queue head
* pv_wait_node - # of vCPU wait's at a non-head queue node
*
@@ -47,6 +48,7 @@ enum qlock_stats {
qstat_pv_lock_stealing,
qstat_pv_spurious_wakeup,
qstat_pv_wait_again,
+ qstat_pv_wait_early,
qstat_pv_wait_head,
qstat_pv_wait_node,
qstat_num, /* Total number of statistical counters */
@@ -70,6 +72,7 @@ static const char * const qstat_names[qstat_num + 1] = {
[qstat_pv_latency_wake] = "pv_latency_wake",
[qstat_pv_lock_stealing] = "pv_lock_stealing",
[qstat_pv_wait_again] = "pv_wait_again",
+ [qstat_pv_wait_early] = "pv_wait_early",
[qstat_pv_wait_head] = "pv_wait_head",
[qstat_pv_wait_node] = "pv_wait_node",
[qstat_reset_cnts] = "reset_counters",