[PATCH v2 3/6] locking/pvqspinlock: Collect slowpath lock statistics

[Waiman Long <Waiman.Long@hp.com>]

This patch enables the accumulation of kicking and waiting related
PV qspinlock statistics when the new QUEUED_LOCK_STAT configuration
option is selected. It also enables the collection of kicking and
wakeup latencies which have a heavy dependency on the CPUs being used.

The measured latencies for different CPUs are:

	CPU		Wakeup		Kicking
	---		------		-------
	Haswell-EX	89.8us		 7.4us
	Westmere-EX	67.6us		 9.3us

The measured latencies varied a bit from run-to-run. The wakeup
latency is much higher than the kicking latency.

A sample of statistics counts after a kernel build (no CPU overcommit)
was:

hash_count=93
hash_hops_count=93
kick_latencies=6997535456
kick_time_count=755426
lock_kick_count=755354
pending_fail_count=10619
pending_lock_count=2912098
spurious_wakeup=8
unlock_kick_count=87
wait_head_count=91
wait_node_count=755359
wake_latencies=51052626984

Signed-off-by: Waiman Long <Waiman.Long@hp.com>
---
 arch/x86/Kconfig                    |    7 ++
 kernel/locking/qspinlock_paravirt.h |  165 ++++++++++++++++++++++++++++++++++-
 2 files changed, 170 insertions(+), 2 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 55bced1..299a1c4 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -679,6 +679,13 @@ config PARAVIRT_SPINLOCKS
 
 	  If you are unsure how to answer this question, answer Y.
 
+config QUEUED_LOCK_STAT
+	bool "Paravirt queued lock statistics"
+	depends on PARAVIRT && DEBUG_FS && QUEUED_SPINLOCKS
+	---help---
+	  Enable the collection of statistical data on the behavior of
+	  paravirtualized queued spinlocks and report them on debugfs.
+
 source "arch/x86/xen/Kconfig"
 
 config KVM_GUEST
diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
index c43dec7..c8485c4 100644
--- a/kernel/locking/qspinlock_paravirt.h
+++ b/kernel/locking/qspinlock_paravirt.h
@@ -43,6 +43,153 @@ struct pv_node {
 };
 
 /*
+ * PV qspinlock statistics
+ */
+enum pv_qlock_stat {
+	pvstat_wait_head,
+	pvstat_wait_node,
+	pvstat_kick_time,
+	pvstat_lock_kick,
+	pvstat_unlock_kick,
+	pvstat_pend_lock,
+	pvstat_pend_fail,
+	pvstat_spurious,
+	pvstat_hash,
+	pvstat_hops,
+	pvstat_num	/* Total number of statistics counts */
+};
+
+#ifdef CONFIG_QUEUED_LOCK_STAT
+/*
+ * Collect pvqspinlock statiatics
+ */
+#include <linux/debugfs.h>
+#include <linux/sched.h>
+
+static const char * const stat_fsnames[pvstat_num] = {
+	[pvstat_wait_head]   = "wait_head_count",
+	[pvstat_wait_node]   = "wait_node_count",
+	[pvstat_kick_time]   = "kick_time_count",
+	[pvstat_lock_kick]   = "lock_kick_count",
+	[pvstat_unlock_kick] = "unlock_kick_count",
+	[pvstat_pend_lock]   = "pending_lock_count",
+	[pvstat_pend_fail]   = "pending_fail_count",
+	[pvstat_spurious]    = "spurious_wakeup",
+	[pvstat_hash]	     = "hash_count",
+	[pvstat_hops]	     = "hash_hops_count",
+};
+
+static atomic_t pvstats[pvstat_num];
+
+/*
+ * pv_kick_latencies = sum of all pv_kick latencies in ns
+ * pv_wake_latencies = sum of all wakeup latencies in ns
+ *
+ * Avg kick latency = pv_kick_latencies/(lock_kick_count + unlock_kick_count)
+ * Avg wake latency = pv_wake_latencies/kick_time_count
+ */
+static atomic64_t pv_kick_latencies, pv_wake_latencies;
+static DEFINE_PER_CPU(u64, pv_kick_time);
+
+/*
+ * Reset all the statistics counts if set
+ */
+static bool reset_cnts __read_mostly;
+
+/*
+ * Initialize debugfs for the PV qspinlock statistics
+ */
+static int __init pv_qspinlock_debugfs(void)
+{
+	struct dentry *d_pvqlock = debugfs_create_dir("pv-qspinlock", NULL);
+	int i;
+
+	if (!d_pvqlock)
+		pr_warn("Could not create 'pv-qspinlock' debugfs directory\n");
+
+	for (i = 0; i < pvstat_num; i++)
+		debugfs_create_u32(stat_fsnames[i], 0444, d_pvqlock,
+				  (u32 *)&pvstats[i]);
+	debugfs_create_u64("kick_latencies", 0444, d_pvqlock,
+			   (u64 *)&pv_kick_latencies);
+	debugfs_create_u64("wake_latencies", 0444, d_pvqlock,
+			   (u64 *)&pv_wake_latencies);
+	debugfs_create_bool("reset_cnts", 0644, d_pvqlock, (u32 *)&reset_cnts);
+	return 0;
+}
+fs_initcall(pv_qspinlock_debugfs);
+
+/*
+ * Reset all the counts
+ */
+static noinline void pvstat_reset(void)
+{
+	int i;
+
+	for (i = 0; i < pvstat_num; i++)
+		atomic_set(&pvstats[i], 0);
+	atomic64_set(&pv_kick_latencies, 0);
+	atomic64_set(&pv_wake_latencies, 0);
+	reset_cnts = 0;
+}
+
+/*
+ * Increment the PV qspinlock statistics counts
+ */
+static inline void pvstat_inc(enum pv_qlock_stat stat)
+{
+	atomic_inc(&pvstats[stat]);
+	if (unlikely(reset_cnts))
+		pvstat_reset();
+}
+
+/*
+ * PV hash hop count
+ */
+static inline void pvstat_hop(int hopcnt)
+{
+	atomic_inc(&pvstats[pvstat_hash]);
+	atomic_add(hopcnt, &pvstats[pvstat_hops]);
+}
+
+/*
+ * Replacement function for pv_kick()
+ */
+static inline void __pv_kick(int cpu)
+{
+	u64 start = sched_clock();
+
+	*per_cpu_ptr(&pv_kick_time, cpu) = start;
+	pv_kick(cpu);
+	atomic64_add(sched_clock() - start, &pv_kick_latencies);
+}
+
+/*
+ * Replacement function for pv_wait()
+ */
+static inline void __pv_wait(u8 *ptr, u8 val)
+{
+	u64 *pkick_time = this_cpu_ptr(&pv_kick_time);
+
+	*pkick_time = 0;
+	pv_wait(ptr, val);
+	if (*pkick_time) {
+		atomic64_add(sched_clock() - *pkick_time, &pv_wake_latencies);
+		pvstat_inc(pvstat_kick_time);
+	}
+}
+
+#define pv_kick(c)	__pv_kick(c)
+#define pv_wait(p, v)	__pv_wait(p, v)
+
+#else /* CONFIG_QUEUED_LOCK_STAT */
+
+static inline void pvstat_inc(enum pv_qlock_stat stat)	{ }
+static inline void pvstat_hop(int hopcnt)		{ }
+
+#endif /* CONFIG_QUEUED_LOCK_STAT */
+
+/*
  * Lock and MCS node addresses hash table for fast lookup
  *
  * Hashing is done on a per-cacheline basis to minimize the need to access
@@ -102,10 +249,13 @@ static struct qspinlock **pv_hash(struct qspinlock *lock, struct pv_node *node)
 {
 	unsigned long offset, hash = hash_ptr(lock, pv_lock_hash_bits);
 	struct pv_hash_entry *he;
+	int hopcnt = 0;
 
 	for_each_hash_entry(he, offset, hash) {
+		hopcnt++;
 		if (!cmpxchg(&he->lock, NULL, lock)) {
 			WRITE_ONCE(he->node, node);
+			pvstat_hop(hopcnt);
 			return &he->lock;
 		}
 	}
@@ -208,11 +358,13 @@ static int pv_pending_lock(struct qspinlock *lock, u32 val)
 	 * Clear the pending bit and fall back to queuing
 	 */
 	clear_pending(lock);
+	pvstat_inc(pvstat_pend_fail);
 
 queue:
 	return 0;
 
 gotlock:
+	pvstat_inc(pvstat_pend_lock);
 	return 1;
 }
 
@@ -243,14 +395,18 @@ static void pv_wait_node(struct mcs_spinlock *node)
 		 */
 		smp_store_mb(pn->state, vcpu_halted);
 
-		if (!READ_ONCE(node->locked))
+		if (!READ_ONCE(node->locked)) {
+			pvstat_inc(pvstat_wait_node);
 			pv_wait(&pn->state, vcpu_halted);
+		}
 
 		/*
 		 * Reset the vCPU state to avoid unncessary CPU kicking
 		 */
 		WRITE_ONCE(pn->state, vcpu_running);
 
+		if (READ_ONCE(node->locked))
+			break;
 		/*
 		 * If the locked flag is still not set after wakeup, it is a
 		 * spurious wakeup and the vCPU should wait again. However,
@@ -258,6 +414,7 @@ static void pv_wait_node(struct mcs_spinlock *node)
 		 * So it is better to spin for a while in the hope that the
 		 * MCS lock will be released soon.
 		 */
+		pvstat_inc(pvstat_spurious);
 	}
 	/*
 	 * By now our node->locked should be 1 and our caller will not actually
@@ -283,8 +440,10 @@ static void pv_kick_node(struct mcs_spinlock *node)
 	 *
 	 * See the comment in pv_wait_node().
 	 */
-	if (xchg(&pn->state, vcpu_running) == vcpu_halted)
+	if (xchg(&pn->state, vcpu_running) == vcpu_halted) {
+		pvstat_inc(pvstat_lock_kick);
 		pv_kick(pn->cpu);
+	}
 }
 
 /*
@@ -326,6 +485,7 @@ static void pv_wait_head(struct qspinlock *lock, struct mcs_spinlock *node)
 				return;
 			}
 		}
+		pvstat_inc(pvstat_wait_head);
 		pv_wait(&l->locked, _Q_SLOW_VAL);
 
 		/*
@@ -376,6 +536,7 @@ __visible void __pv_queued_spin_unlock(struct qspinlock *lock)
 	 * At this point the memory pointed at by lock can be freed/reused,
 	 * however we can still use the pv_node to kick the CPU.
 	 */
+	pvstat_inc(pvstat_unlock_kick);
 	pv_kick(node->cpu);
 }
 /*
-- 
1.7.1