[RFC PATCH 03/30] Lazy percpu counters

Suren Baghdasaryan
2022-08-30T21:50:04Z

From: Kent Overstreet <kent.overstreet@linux.dev>

This patch adds lib/lazy-percpu-counter.c, which implements counters
that start out as atomics, but lazily switch to percpu mode if the
update rate crosses some threshold (arbitrarily set at 256 per second).

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
---
include/linux/lazy-percpu-counter.h | 67 +++++++++++++
lib/Kconfig | 3 +
lib/Makefile | 2 +
lib/lazy-percpu-counter.c | 141 ++++++++++++++++++++++++++++
4 files changed, 213 insertions(+)
create mode 100644 include/linux/lazy-percpu-counter.h
create mode 100644 lib/lazy-percpu-counter.c

diff --git a/include/linux/lazy-percpu-counter.h b/include/linux/lazy-percpu-counter.h
new file mode 100644
index 000000000000..a22a2b9a9f32
--- /dev/null
+++ b/include/linux/lazy-percpu-counter.h
@@ -0,0 +1,67 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Lazy percpu counters:
+ * (C) 2022 Kent Overstreet
+ *
+ * Lazy percpu counters start out in atomic mode, then switch to percpu mode if
+ * the update rate crosses some threshold.
+ *
+ * This means we don't have to decide between low memory overhead atomic
+ * counters and higher performance percpu counters - we can have our cake and
+ * eat it, too!
+ *
+ * Internally we use an atomic64_t, where the low bit indicates whether we're in
+ * percpu mode, and the high 8 bits are a secondary counter that's incremented
+ * when the counter is modified - meaning 55 bits of precision are available for
+ * the counter itself.
+ *
+ * lazy_percpu_counter is 16 bytes (on 64 bit machines), raw_lazy_percpu_counter
+ * is 8 bytes but requires a separate unsigned long to record when the counter
+ * wraps - because sometimes multiple counters are used together and can share
+ * the same timestamp.
+ */
+
+#ifndef _LINUX_LAZY_PERCPU_COUNTER_H
+#define _LINUX_LAZY_PERCPU_COUNTER_H
+
+struct raw_lazy_percpu_counter {
+ atomic64_t v;
+};
+
+void __lazy_percpu_counter_exit(struct raw_lazy_percpu_counter *c);
+void __lazy_percpu_counter_add(struct raw_lazy_percpu_counter *c,
+ unsigned long *last_wrap, s64 i);
+s64 __lazy_percpu_counter_read(struct raw_lazy_percpu_counter *c);
+
+static inline void __lazy_percpu_counter_sub(struct raw_lazy_percpu_counter *c,
+ unsigned long *last_wrap, s64 i)
+{
+ __lazy_percpu_counter_add(c, last_wrap, -i);
+}
+
+struct lazy_percpu_counter {
+ struct raw_lazy_percpu_counter v;
+ unsigned long last_wrap;
+};
+
+static inline void lazy_percpu_counter_exit(struct lazy_percpu_counter *c)
+{
+ __lazy_percpu_counter_exit(&c->v);
+}
+
+static inline void lazy_percpu_counter_add(struct lazy_percpu_counter *c, s64 i)
+{
+ __lazy_percpu_counter_add(&c->v, &c->last_wrap, i);
+}
+
+static inline void lazy_percpu_counter_sub(struct lazy_percpu_counter *c, s64 i)
+{
+ __lazy_percpu_counter_sub(&c->v, &c->last_wrap, i);
+}
+
+static inline s64 lazy_percpu_counter_read(struct lazy_percpu_counter *c)
+{
+ return __lazy_percpu_counter_read(&c->v);
+}
+
+#endif /* _LINUX_LAZY_PERCPU_COUNTER_H */
diff --git a/lib/Kconfig b/lib/Kconfig
index dc1ab2ed1dc6..fc6dbc425728 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -498,6 +498,9 @@ config ASSOCIATIVE_ARRAY

for more information.

+config LAZY_PERCPU_COUNTER
+ bool
+
config HAS_IOMEM
bool
depends on !NO_IOMEM
diff --git a/lib/Makefile b/lib/Makefile
index ffabc30a27d4..cc7762748708 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -163,6 +163,8 @@ obj-$(CONFIG_DEBUG_PREEMPT) += smp_processor_id.o
obj-$(CONFIG_DEBUG_LIST) += list_debug.o
obj-$(CONFIG_DEBUG_OBJECTS) += debugobjects.o

+obj-$(CONFIG_LAZY_PERCPU_COUNTER) += lazy-percpu-counter.o
+
obj-$(CONFIG_BITREVERSE) += bitrev.o
obj-$(CONFIG_LINEAR_RANGES) += linear_ranges.o
obj-$(CONFIG_PACKING) += packing.o
diff --git a/lib/lazy-percpu-counter.c b/lib/lazy-percpu-counter.c
new file mode 100644
index 000000000000..299ef36137ee
--- /dev/null
+++ b/lib/lazy-percpu-counter.c
@@ -0,0 +1,141 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#include <linux/atomic.h>
+#include <linux/gfp.h>
+#include <linux/jiffies.h>
+#include <linux/lazy-percpu-counter.h>
+#include <linux/percpu.h>
+
+/*
+ * We use the high bits of the atomic counter for a secondary counter, which is
+ * incremented every time the counter is touched. When the secondary counter
+ * wraps, we check the time the counter last wrapped, and if it was recent
+ * enough that means the update frequency has crossed our threshold and we
+ * switch to percpu mode:
+ */
+#define COUNTER_MOD_BITS 8
+#define COUNTER_MOD_MASK ~(~0ULL >> COUNTER_MOD_BITS)
+#define COUNTER_MOD_BITS_START (64 - COUNTER_MOD_BITS)
+
+/*
+ * We use the low bit of the counter to indicate whether we're in atomic mode
+ * (low bit clear), or percpu mode (low bit set, counter is a pointer to actual
+ * percpu counters:
+ */
+#define COUNTER_IS_PCPU_BIT 1
+
+static inline u64 __percpu *lazy_percpu_counter_is_pcpu(u64 v)
+{
+ if (!(v & COUNTER_IS_PCPU_BIT))
+ return NULL;
+
+ v ^= COUNTER_IS_PCPU_BIT;
+ return (u64 __percpu *)(unsigned long)v;
+}
+
+static inline s64 lazy_percpu_counter_atomic_val(s64 v)
+{
+ /* Ensure output is sign extended properly: */
+ return (v << COUNTER_MOD_BITS) >>
+ (COUNTER_MOD_BITS + COUNTER_IS_PCPU_BIT);
+}
+
+static void lazy_percpu_counter_switch_to_pcpu(struct raw_lazy_percpu_counter *c)
+{
+ u64 __percpu *pcpu_v = alloc_percpu_gfp(u64, GFP_ATOMIC|__GFP_NOWARN);
+ u64 old, new, v;
+
+ if (!pcpu_v)
+ return;
+
+ preempt_disable();
+ v = atomic64_read(&c->v);
+ do {
+ if (lazy_percpu_counter_is_pcpu(v)) {
+ free_percpu(pcpu_v);
+ return;
+ }
+
+ old = v;
+ new = (unsigned long)pcpu_v | 1;
+
+ *this_cpu_ptr(pcpu_v) = lazy_percpu_counter_atomic_val(v);
+ } while ((v = atomic64_cmpxchg(&c->v, old, new)) != old);
+ preempt_enable();
+}
+
+/**
+ * __lazy_percpu_counter_exit: Free resources associated with a
+ * raw_lazy_percpu_counter
+ *
+ * @c: counter to exit
+ */
+void __lazy_percpu_counter_exit(struct raw_lazy_percpu_counter *c)
+{
+ free_percpu(lazy_percpu_counter_is_pcpu(atomic64_read(&c->v)));
+}
+EXPORT_SYMBOL_GPL(__lazy_percpu_counter_exit);
+
+/**
+ * __lazy_percpu_counter_read: Read current value of a raw_lazy_percpu_counter
+ *
+ * @c: counter to read
+ */
+s64 __lazy_percpu_counter_read(struct raw_lazy_percpu_counter *c)
+{
+ s64 v = atomic64_read(&c->v);
+ u64 __percpu *pcpu_v = lazy_percpu_counter_is_pcpu(v);
+
+ if (pcpu_v) {
+ int cpu;
+
+ v = 0;
+ for_each_possible_cpu(cpu)
+ v += *per_cpu_ptr(pcpu_v, cpu);
+ } else {
+ v = lazy_percpu_counter_atomic_val(v);
+ }
+
+ return v;
+}
+EXPORT_SYMBOL_GPL(__lazy_percpu_counter_read);
+
+/**
+ * __lazy_percpu_counter_add: Add a value to a lazy_percpu_counter
+ *
+ * @c: counter to modify
+ * @last_wrap: pointer to a timestamp, updated when mod counter wraps
+ * @i: value to add
+ */
+void __lazy_percpu_counter_add(struct raw_lazy_percpu_counter *c,
+ unsigned long *last_wrap, s64 i)
+{
+ u64 atomic_i;
+ u64 old, v = atomic64_read(&c->v);
+ u64 __percpu *pcpu_v;
+
+ atomic_i = i << COUNTER_IS_PCPU_BIT;
+ atomic_i &= ~COUNTER_MOD_MASK;
+ atomic_i |= 1ULL << COUNTER_MOD_BITS_START;
+
+ do {
+ pcpu_v = lazy_percpu_counter_is_pcpu(v);
+ if (pcpu_v) {
+ this_cpu_add(*pcpu_v, i);
+ return;
+ }
+
+ old = v;
+ } while ((v = atomic64_cmpxchg(&c->v, old, old + atomic_i)) != old);
+
+ if (unlikely(!(v & COUNTER_MOD_MASK))) {
+ unsigned long now = jiffies;
+
+ if (*last_wrap &&
+ unlikely(time_after(*last_wrap + HZ, now)))
+ lazy_percpu_counter_switch_to_pcpu(c);
+ else
+ *last_wrap = now;
+ }
+}
+EXPORT_SYMBOL(__lazy_percpu_counter_add);
--
2.37.2.672.g94769d06f0-goog