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Subject[PATCH 4/7] locking/pvqspinlock: Collect slowpath lock statistics
Date
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 26.4us 9.2us
Westmere-EX 99.4US 25.5us

So Haswell is much faster than Westmere.

The accumulated lock statistics will be reported in debugfs under the
pv-qspinlock directory.

Signed-off-by: Waiman Long <Waiman.Long@hp.com>
---
arch/x86/Kconfig | 7 ++
kernel/locking/qspinlock_paravirt.h | 173 ++++++++++++++++++++++++++++++++++-
2 files changed, 177 insertions(+), 3 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 b3fe5bb..efc9a72 100644
--- a/kernel/locking/qspinlock_paravirt.h
+++ b/kernel/locking/qspinlock_paravirt.h
@@ -60,6 +60,155 @@ struct pv_node {
};

/*
+ * PV qspinlock statistics
+ */
+enum pv_qlock_stat {
+ pvstat_wait_head,
+ pvstat_wait_node,
+ pvstat_wait_early,
+ pvstat_kick_wake,
+ pvstat_kick_cpu,
+ pvstat_kick_ahead,
+ pvstat_no_kick,
+ 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_wait_early] = "wait_early_count",
+ [pvstat_kick_wake] = "kick_wake_count",
+ [pvstat_kick_cpu] = "kick_cpu_count",
+ [pvstat_kick_ahead] = "kick_ahead_count",
+ [pvstat_no_kick] = "no_kick_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/(kick_cpu_count + kick_ahead_count)
+ * Avg wake latency = pv_wake_latencies/kick_wake_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)
+ printk(KERN_WARNING
+ "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 kick_time;
+
+ pv_wait(ptr, val);
+ kick_time = *this_cpu_ptr(&pv_kick_time);
+ if (kick_time) {
+ atomic64_add(sched_clock() - kick_time, &pv_wake_latencies);
+ pvstat_inc(pvstat_kick_wake);
+ *this_cpu_ptr(&pv_kick_time) = 0;
+ }
+}
+
+#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
@@ -146,10 +295,13 @@ 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;
}
}
@@ -221,6 +373,8 @@ static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
prev_halted = (READ_ONCE(pp->state) != vcpu_running);
loop = prev_halted ? QNODE_SPIN_THRESHOLD_SHORT
: QNODE_SPIN_THRESHOLD;
+ if (prev_halted)
+ pvstat_inc(pvstat_wait_early);
while (loop--) {
if (READ_ONCE(node->locked))
return;
@@ -236,6 +390,7 @@ static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
bool halted = (READ_ONCE(pp->state)
!= vcpu_running);
if (!prev_halted && halted) {
+ pvstat_inc(pvstat_wait_early);
break;
} else if (prev_halted && !halted) {
loop = QNODE_SPIN_THRESHOLD;
@@ -256,14 +411,18 @@ static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
*/
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 state except when vcpu_hashed is set.
*/
cmpxchg(&pn->state, vcpu_halted, 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,
@@ -271,6 +430,7 @@ static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
* So it is better to spin for a while in the hope that the
* MCS lock will be released soon.
*/
+ pvstat_inc(pvstat_spurious);
}

/*
@@ -372,6 +532,7 @@ 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);

/*
@@ -459,14 +620,20 @@ __visible void __pv_queued_spin_unlock(struct qspinlock *lock)
* vCPU is harmless other than the additional latency in completing
* the unlock.
*/
- if (READ_ONCE(node->state) == vcpu_hashed)
+ if (READ_ONCE(node->state) == vcpu_hashed) {
+ pvstat_inc(pvstat_kick_cpu);
pv_kick(node->cpu);
+ } else {
+ pvstat_inc(pvstat_no_kick);
+ }

/*
* Kick the next group of vCPUs, if available.
*/
- for (i = 0; i < nr_kick; i++)
+ for (i = 0; i < nr_kick; i++) {
+ pvstat_inc(pvstat_kick_ahead);
pv_kick(next[i]->cpu);
+ }
}
/*
* Include the architecture specific callee-save thunk of the
--
1.7.1


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