Re: [PATCH v2] locking/osq_lock: Optimize osq_lock performance using per-NUMA
From: Waiman Long
Date: Mon Mar 18 2024 - 21:59:57 EST
On 3/18/24 01:50, Guo Hui wrote:
Changes in version v1:
The queue is divided according to NUMA nodes,
but the tail of each NUMA node is still stored
in the structure optimistic_spin_queue.
Changes in version v2:
1,On the basis of v1,
the tail of each NUMA node is separated from
the structure optimistic_spin_queue,
and a separate memory space is allocated.
This memory space is a pre-statically allocated
fixed-size array osnq_queue[1024], array length is 1024.
Each array element of osnq_queue is an array of atomic_t type.
The length of this atomic_t type array is MAX_NUMNODES.
The total memory size of statically allocated arrays is as follows:
4 * MAX_NUMNODES * 1024
When MAX_NUMNODES is 64, the memory is 256K,
and when it is 1024, the memory is 4M.
The relationship between the statically allocated array osnq_queue
and the structure optimistic_spin_queue is to use the hash value of
the optimistic_spin_queue structure type pointer as the index of
the array osnq_queue, and obtain the array element
corresponding to osq_lock from osnq_queue.
This array element stores the tail value of
each NUMA node corresponding to the osq_lock lock.
The form of the osnq_queue array is as follows:
----------------------------------------------------------
| | |
| | MAX_NUMNODES |
| | |
| |-------------------------------------|
| | | | |
| | atomic_t tail | atomic_t tail | ... |
| | | | |
| |-------------------------------------|
| | | | |
| | atomic_t tail | atomic_t tail | ... |
| | | | |
| |-------------------------------------|
| | | | |
| osnq_queue[1024] | atomic_t tail | atomic_t tail | ... |
| | | | |
| |-------------------------------------|
| The hash value ->| | | |
| of osq_lock is | atomic_t tail | atomic_t tail | ... |
| the index | | | |
| |-------------------------------------|
| | | | |
| | ... ... | ... ... | ... |
| | | | |
|------------------|-------------------------------------|
There is a very small probability that different osq_locks
with the same hash value will run concurrently on different CPUs.
After extensive testing, this probability is no greater than 0.01%.
This situation is also a normal situation.
In this case, two different osq_locks will share the same
osnq_queue array element,these two different osq_locks
share the same NUMA linked list.Put different CPU nodes waiting
for different osq_locks into the same NUMA linked list,
which means that CPU nodes with different osq_locks
share the same lock of the same NUMA queue.
This is essentially the same method as using zippers
to resolve hash collisions.
Make an extreme case and set the above osnq_queue array
to an array element.Then all osq_locks in the kernel
will share the same queue on different NUMA nodes.
After verification, the kernel can also run normally.
However, the performance of some test cases will deteriorate.
This patch solution greatly reduces the probability of
shared queues to less than 0.01%,greatly improving
the kernel osq_lock lock performance.
I don't like the idea of using a hash table and having hash collision.
Is it possible to adopt the numa-aware qspinlock's idea of having a
primary queue of the same numa node and a secondary queue for all the
other nodes? I know it will be more complex because of the need to back
out when rescheduling is needed.
Also I will prefer to make this numa-awareness optional so that it is
determined at boot time if the numa-aware version or the original
version is being used. At the beginning, the numa-aware osq_lock will be
default to off unless it is forced on by a command line or kconfig
option. We want to minimize risk due to the introduction of bug in the
new code.
2. Achieve fairness in transferring locks between nodes
and prevent the same NUMA node from holding locks for a long time.
This method borrows from the qspinlock numa-aware scheme.
The effect on the 96-core 4-NUMA ARM64 platform is as follows:
System Benchmarks Partial Index with patch without patch promote
Execl Throughput 7255.8 5632.8 +28.81%
File Copy 1024 bufsize 2000 maxblocks 1817.2 910.9 +99.50%
File Copy 256 bufsize 500 maxblocks 1168.1 570.4 +104.79%
File Copy 4096 bufsize 8000 maxblocks 3321.1 2088.7 +59.00%
The effect on the 128-core 8-NUMA X86_64 platform is as follows:
System Benchmarks Partial Index with patch without patch promote
Execl Throughput 3947 3533.6 +11.70%
File Copy 1024 bufsize 2000 maxblocks 819.1 553 +48.12%
File Copy 256 bufsize 500 maxblocks 508.5 330.2 +54.00%
File Copy 4096 bufsize 8000 maxblocks 1982.2 1377.1 +43.94%
Signed-off-by: Guo Hui <guohui@xxxxxxxxxxxxx>
---
include/linux/osq_lock.h | 29 ++++++++--
kernel/locking/osq_lock.c | 111 ++++++++++++++++++++++++++++++++++----
2 files changed, 126 insertions(+), 14 deletions(-)
diff --git a/include/linux/osq_lock.h b/include/linux/osq_lock.h
index ea8fb31379e3..3433f13276b8 100644
--- a/include/linux/osq_lock.h
+++ b/include/linux/osq_lock.h
@@ -2,6 +2,13 @@
#ifndef __LINUX_OSQ_LOCK_H
#define __LINUX_OSQ_LOCK_H
+#include <linux/nodemask.h>
+#include <linux/hash.h>
+
+struct optimistic_spin_numa_queue {
+ atomic_t tail[MAX_NUMNODES]; /* Store the tail of each NUMA queue */
+};
+
/*
* An MCS like lock especially tailored for optimistic spinning for sleeping
* lock implementations (mutex, rwsem, etc).
@@ -9,12 +16,16 @@
struct optimistic_spin_queue {
/*
- * Stores an encoded value of the CPU # of the tail node in the queue.
- * If the queue is empty, then it's set to OSQ_UNLOCKED_VAL.
+ * Stores the hash value of the structure type pointer.
*/
atomic_t tail;
};
+#define HASH_BITS_LEN 10
+
+/* this value is 2^@HASH_BITS_LEN */
+#define NUMA_QUEUE_SIZE 1024
+
#define OSQ_UNLOCKED_VAL (0)
/* Init macro and function. */
@@ -22,15 +33,25 @@ struct optimistic_spin_queue {
static inline void osq_lock_init(struct optimistic_spin_queue *lock)
{
- atomic_set(&lock->tail, OSQ_UNLOCKED_VAL);
+ atomic_set(&lock->tail, hash_ptr(lock, HASH_BITS_LEN));
}
extern bool osq_lock(struct optimistic_spin_queue *lock);
extern void osq_unlock(struct optimistic_spin_queue *lock);
+extern struct optimistic_spin_numa_queue osnq_queue[NUMA_QUEUE_SIZE];
+
static inline bool osq_is_locked(struct optimistic_spin_queue *lock)
{
- return atomic_read(&lock->tail) != OSQ_UNLOCKED_VAL;
+ int node;
+ atomic_t *numa_tail = osnq_queue[atomic_read(&lock->tail)].tail;
+
+ for_each_online_node(node) {
+ if (atomic_read(&numa_tail[node]) != OSQ_UNLOCKED_VAL)
+ return true;
+ }
+
+ return false;
}
That greatly increase of cost of calling osq_is_locked().
#endif
diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 75a6f6133866..bea6a2784b5e 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -2,6 +2,8 @@
#include <linux/percpu.h>
#include <linux/sched.h>
#include <linux/osq_lock.h>
+#include <linux/topology.h>
+#include <linux/random.h>
/*
* An MCS like lock especially tailored for optimistic spinning for sleeping
@@ -14,12 +16,48 @@
struct optimistic_spin_node {
struct optimistic_spin_node *next, *prev;optimistic_spin_queue
+ atomic_t *tail;
Don't use the name "tail" here as it is also used in
optimistic_spin_queue. It is hard to tell which tail is which when
reading the code. Make them unique.
int locked; /* 1 if lock acquired */
int cpu; /* encoded CPU # + 1 value */
+ int numa_node;
};
static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node);
+/* Use the hash value of the structure optimistic_spin_node type pointer as the index. */
+struct optimistic_spin_numa_queue osnq_queue[NUMA_QUEUE_SIZE];
+
+#define INVALID_NUMA_NODE (-1)
+
+/* Per-CPU pseudo-random number seed */
+static DEFINE_PER_CPU(u32, seed);
+
+/*
+ * Controls the probability for intra-node lock hand-off. It can be
+ * tuned and depend, e.g., on the number of CPUs per node. For now,
+ * choose a value that provides reasonable long-term fairness without
+ * sacrificing performance compared to a version that does not have any
+ * fairness guarantees.
+ */
+#define INTRA_NODE_HANDOFF_PROB_ARG (16)
+
+
+/*
+ * Return false with probability 1 / 2^@num_bits.
+ * Intuitively, the larger @num_bits the less likely false is to be returned.
+ * @num_bits must be a number between 0 and 31.
+ */
+static bool probably(unsigned int num_bits)
+{
+ u32 s;
+
+ s = this_cpu_read(seed);
+ s = next_pseudo_random32(s);
+ this_cpu_write(seed, s);
+
+ return s & ((1 << num_bits) - 1);
+}
+
/*
* We use the value 0 to represent "no CPU", thus the encoded value
* will be the CPU number incremented by 1.
@@ -58,8 +96,8 @@ osq_wait_next(struct optimistic_spin_queue *lock,
int curr = encode_cpu(smp_processor_id());
for (;;) {
- if (atomic_read(&lock->tail) == curr &&
- atomic_cmpxchg_acquire(&lock->tail, curr, old_cpu) == curr) {
+ if (atomic_read(&node->tail[node->numa_node]) == curr &&
+ atomic_cmpxchg_acquire(&node->tail[node->numa_node], curr, old_cpu) == curr) {
/*
* We were the last queued, we moved @lock back. @prev
* will now observe @lock and will complete its
@@ -90,6 +128,19 @@ osq_wait_next(struct optimistic_spin_queue *lock,
}
}
+static atomic_t osq_lock_node = ATOMIC_INIT(-1);
+
+static void osq_wait_numa_node(struct optimistic_spin_node *node)
+{
+ int old_node;
+
+ while (!need_resched() &&
+ ((old_node = atomic_cmpxchg_acquire(&osq_lock_node, INVALID_NUMA_NODE,
+ node->numa_node)) != INVALID_NUMA_NODE) &&
+ (node->numa_node != old_node))
+ cpu_relax();
+}
+
bool osq_lock(struct optimistic_spin_queue *lock)
{
struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
@@ -100,6 +151,8 @@ bool osq_lock(struct optimistic_spin_queue *lock)
node->locked = 0;
node->next = NULL;
node->cpu = curr;
+ node->numa_node = cpu_to_node(smp_processor_id());
+ node->tail = osnq_queue[atomic_read(&lock->tail)].tail;
/*optimistic_spin_queue
* We need both ACQUIRE (pairs with corresponding RELEASE in
@@ -107,9 +160,11 @@ bool osq_lock(struct optimistic_spin_queue *lock)
* the node fields we just initialised) semantics when updating
* the lock tail.
*/
- old = atomic_xchg(&lock->tail, curr);
- if (old == OSQ_UNLOCKED_VAL)
+ old = atomic_xchg(&node->tail[node->numa_node], curr);
+ if (old == OSQ_UNLOCKED_VAL) {
+ osq_wait_numa_node(node);
return true;
+ }
prev = decode_cpu(old);
node->prev = prev;
@@ -144,8 +199,10 @@ bool osq_lock(struct optimistic_spin_queue *lock)
* polling, be careful.
*/
if (smp_cond_load_relaxed(&node->locked, VAL || need_resched() ||
- vcpu_is_preempted(node_cpu(node->prev))))
+ vcpu_is_preempted(node_cpu(node->prev)))) {
+ osq_wait_numa_node(node);
return true;
+ }
/* unqueue */
/*
@@ -170,8 +227,10 @@ bool osq_lock(struct optimistic_spin_queue *lock)
* in which case we should observe @node->locked becoming
* true.
*/
- if (smp_load_acquire(&node->locked))
+ if (smp_load_acquire(&node->locked)) {
+ osq_wait_numa_node(node);
return true;
+ }
cpu_relax();
@@ -207,29 +266,61 @@ bool osq_lock(struct optimistic_spin_queue *lock)
return false;
}
+/*
+ * Pass the lock to the next NUMA node.
+ */
+static void pass_lock_numa_node(struct optimistic_spin_node *node)
+{
+ int curr_node = cpu_to_node(smp_processor_id());
+ int numa_node = curr_node;
+ int num_nodes = num_online_nodes();
+
+ do {
+ numa_node = (numa_node + 1) % num_nodes;
+ if (numa_node == curr_node) {
+ atomic_set(&osq_lock_node, INVALID_NUMA_NODE);
+ return;
+ }
+ } while (atomic_read(&node->tail[numa_node]) == OSQ_UNLOCKED_VAL);
+ atomic_set(&osq_lock_node, numa_node);
+}
+
+static inline void pass_lock_fair(struct optimistic_spin_node *node)
+{
+ if (!probably(INTRA_NODE_HANDOFF_PROB_ARG))
+ pass_lock_numa_node(node);
+}
+
void osq_unlock(struct optimistic_spin_queue *lock)
{
struct optimistic_spin_node *node, *next;
int curr = encode_cpu(smp_processor_id());
+ node = this_cpu_ptr(&osq_node);
+
/*
* Fast path for the uncontended case.
*/
- if (likely(atomic_cmpxchg_release(&lock->tail, curr,
- OSQ_UNLOCKED_VAL) == curr))
+ if (likely(atomic_cmpxchg_release(&node->tail[node->numa_node], curr,
+ OSQ_UNLOCKED_VAL) == curr)) {
+ pass_lock_numa_node(node);
return;
+ }
/*
* Second most likely case.
*/
- node = this_cpu_ptr(&osq_node);
next = xchg(&node->next, NULL);
if (next) {
WRITE_ONCE(next->locked, 1);
+ pass_lock_fair(node);
return;
}
next = osq_wait_next(lock, node, OSQ_UNLOCKED_VAL);
- if (next)
+ if (next) {
WRITE_ONCE(next->locked, 1);
+ pass_lock_fair(node);
+ } else
+ pass_lock_numa_node(node);
}
With numa-aware qspinlock, most of the action is in the unlock code to
scan the queue.
Cheers,
Longman