Re: [PATCH v15 3/6] locking/qspinlock: Introduce CNA into the slow path of qspinlock

From: Guo Ren
Date: Wed Aug 02 2023 - 19:14:29 EST


On Fri, May 14, 2021 at 04:07:40PM -0400, Alex Kogan wrote:
> In CNA, spinning threads are organized in two queues, a primary queue for
> threads running on the same node as the current lock holder, and a
> secondary queue for threads running on other nodes. After acquiring the
> MCS lock and before acquiring the spinlock, the MCS lock
> holder checks whether the next waiter in the primary queue (if exists) is
> running on the same NUMA node. If it is not, that waiter is detached from
> the main queue and moved into the tail of the secondary queue. This way,
> we gradually filter the primary queue, leaving only waiters running on
> the same preferred NUMA node. For more details, see
> https://arxiv.org/abs/1810.05600.
>
> Note that this variant of CNA may introduce starvation by continuously
> passing the lock between waiters in the main queue. This issue will be
> addressed later in the series.
>
> Enabling CNA is controlled via a new configuration option
> (NUMA_AWARE_SPINLOCKS). By default, the CNA variant is patched in at the
> boot time only if we run on a multi-node machine in native environment and
> the new config is enabled. (For the time being, the patching requires
> CONFIG_PARAVIRT_SPINLOCKS to be enabled as well. However, this should be
> resolved once static_call() is available.) This default behavior can be
> overridden with the new kernel boot command-line option
> "numa_spinlock=on/off" (default is "auto").
>
> Signed-off-by: Alex Kogan <alex.kogan@xxxxxxxxxx>
> Reviewed-by: Steve Sistare <steven.sistare@xxxxxxxxxx>
> Reviewed-by: Waiman Long <longman@xxxxxxxxxx>
> ---
> .../admin-guide/kernel-parameters.txt | 10 +
> arch/x86/Kconfig | 20 ++
> arch/x86/include/asm/qspinlock.h | 4 +
> arch/x86/kernel/alternative.c | 4 +
> kernel/locking/mcs_spinlock.h | 2 +-
> kernel/locking/qspinlock.c | 42 ++-
> kernel/locking/qspinlock_cna.h | 325 ++++++++++++++++++
> 7 files changed, 402 insertions(+), 5 deletions(-)
> create mode 100644 kernel/locking/qspinlock_cna.h
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index a816935d23d4..94d35507560c 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3515,6 +3515,16 @@
> NUMA balancing.
> Allowed values are enable and disable
>
> + numa_spinlock= [NUMA, PV_OPS] Select the NUMA-aware variant
> + of spinlock. The options are:
> + auto - Enable this variant if running on a multi-node
> + machine in native environment.
> + on - Unconditionally enable this variant.
> + off - Unconditionally disable this variant.
> +
> + Not specifying this option is equivalent to
> + numa_spinlock=auto.
> +
> numa_zonelist_order= [KNL, BOOT] Select zonelist order for NUMA.
> 'node', 'default' can be specified
> This can be set from sysctl after boot.
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 0045e1b44190..819c3dad8afc 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1564,6 +1564,26 @@ config NUMA
>
> Otherwise, you should say N.
>
> +config NUMA_AWARE_SPINLOCKS
> + bool "Numa-aware spinlocks"
> + depends on NUMA
> + depends on QUEUED_SPINLOCKS
> + depends on 64BIT
> + # For now, we depend on PARAVIRT_SPINLOCKS to make the patching work.
> + # This is awkward, but hopefully would be resolved once static_call()
> + # is available.
> + depends on PARAVIRT_SPINLOCKS
> + default y
> + help
> + Introduce NUMA (Non Uniform Memory Access) awareness into
> + the slow path of spinlocks.
> +
> + In this variant of qspinlock, the kernel will try to keep the lock
> + on the same node, thus reducing the number of remote cache misses,
> + while trading some of the short term fairness for better performance.
> +
> + Say N if you want absolute first come first serve fairness.
> +
> config AMD_NUMA
> def_bool y
> prompt "Old style AMD Opteron NUMA detection"
> diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h
> index d86ab942219c..21d09e8db979 100644
> --- a/arch/x86/include/asm/qspinlock.h
> +++ b/arch/x86/include/asm/qspinlock.h
> @@ -27,6 +27,10 @@ static __always_inline u32 queued_fetch_set_pending_acquire(struct qspinlock *lo
> return val;
> }
>
> +#ifdef CONFIG_NUMA_AWARE_SPINLOCKS
> +extern void cna_configure_spin_lock_slowpath(void);
> +#endif
> +
> #ifdef CONFIG_PARAVIRT_SPINLOCKS
> extern void native_queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
> extern void __pv_init_lock_hash(void);
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 6974b5174495..6c73c2dc17fb 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -608,6 +608,10 @@ void __init alternative_instructions(void)
> */
> paravirt_set_cap();
>
> +#if defined(CONFIG_NUMA_AWARE_SPINLOCKS)
> + cna_configure_spin_lock_slowpath();
> +#endif
> +
> /*
> * First patch paravirt functions, such that we overwrite the indirect
> * call with the direct call.
> diff --git a/kernel/locking/mcs_spinlock.h b/kernel/locking/mcs_spinlock.h
> index e794babc519a..3926aad129ed 100644
> --- a/kernel/locking/mcs_spinlock.h
> +++ b/kernel/locking/mcs_spinlock.h
> @@ -17,7 +17,7 @@
>
> struct mcs_spinlock {
> struct mcs_spinlock *next;
> - int locked; /* 1 if lock acquired */
> + unsigned int locked; /* 1 if lock acquired */
> int count; /* nesting count, see qspinlock.c */
> };
>
> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> index e3518709ffdc..8c1a21b53913 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -11,7 +11,7 @@
> * Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> */
>
> -#ifndef _GEN_PV_LOCK_SLOWPATH
> +#if !defined(_GEN_PV_LOCK_SLOWPATH) && !defined(_GEN_CNA_LOCK_SLOWPATH)
>
> #include <linux/smp.h>
> #include <linux/bug.h>
> @@ -71,7 +71,8 @@
> /*
> * On 64-bit architectures, the mcs_spinlock structure will be 16 bytes in
> * size and four of them will fit nicely in one 64-byte cacheline. For
> - * pvqspinlock, however, we need more space for extra data. To accommodate
> + * pvqspinlock, however, we need more space for extra data. The same also
> + * applies for the NUMA-aware variant of spinlocks (CNA). To accommodate
> * that, we insert two more long words to pad it up to 32 bytes. IOW, only
> * two of them can fit in a cacheline in this case. That is OK as it is rare
> * to have more than 2 levels of slowpath nesting in actual use. We don't
> @@ -80,7 +81,7 @@
> */
> struct qnode {
> struct mcs_spinlock mcs;
> -#ifdef CONFIG_PARAVIRT_SPINLOCKS
> +#if defined(CONFIG_PARAVIRT_SPINLOCKS) || defined(CONFIG_NUMA_AWARE_SPINLOCKS)
> long reserved[2];
> #endif
> };
> @@ -104,6 +105,8 @@ struct qnode {
> * Exactly fits one 64-byte cacheline on a 64-bit architecture.
> *
> * PV doubles the storage and uses the second cacheline for PV state.
> + * CNA also doubles the storage and uses the second cacheline for
> + * CNA-specific state.
> */
> static DEFINE_PER_CPU_ALIGNED(struct qnode, qnodes[MAX_NODES]);
>
> @@ -317,7 +320,7 @@ static __always_inline void __mcs_lock_handoff(struct mcs_spinlock *node,
> #define try_clear_tail __try_clear_tail
> #define mcs_lock_handoff __mcs_lock_handoff
>
> -#endif /* _GEN_PV_LOCK_SLOWPATH */
> +#endif /* _GEN_PV_LOCK_SLOWPATH && _GEN_CNA_LOCK_SLOWPATH */
>
> /**
> * queued_spin_lock_slowpath - acquire the queued spinlock
> @@ -589,6 +592,37 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> }
> EXPORT_SYMBOL(queued_spin_lock_slowpath);
>
> +/*
> + * Generate the code for NUMA-aware spinlocks
> + */
> +#if !defined(_GEN_CNA_LOCK_SLOWPATH) && defined(CONFIG_NUMA_AWARE_SPINLOCKS)
> +#define _GEN_CNA_LOCK_SLOWPATH
> +
> +#undef pv_init_node
> +#define pv_init_node cna_init_node
> +
> +#undef pv_wait_head_or_lock
> +#define pv_wait_head_or_lock cna_wait_head_or_lock
> +
> +#undef try_clear_tail
> +#define try_clear_tail cna_try_clear_tail
> +
> +#undef mcs_lock_handoff
> +#define mcs_lock_handoff cna_lock_handoff
> +
> +#undef queued_spin_lock_slowpath
> +/*
> + * defer defining queued_spin_lock_slowpath until after the include to
> + * avoid a name clash with the identically named field in pv_ops.lock
> + * (see cna_configure_spin_lock_slowpath())
> + */
> +#include "qspinlock_cna.h"
> +#define queued_spin_lock_slowpath __cna_queued_spin_lock_slowpath
> +
> +#include "qspinlock.c"
> +
> +#endif
> +
> /*
> * Generate the paravirt code for queued_spin_unlock_slowpath().
> */
> diff --git a/kernel/locking/qspinlock_cna.h b/kernel/locking/qspinlock_cna.h
> new file mode 100644
> index 000000000000..ca564e64e5de
> --- /dev/null
> +++ b/kernel/locking/qspinlock_cna.h
> @@ -0,0 +1,325 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _GEN_CNA_LOCK_SLOWPATH
> +#error "do not include this file"
> +#endif
> +
> +#include <linux/topology.h>
> +
> +/*
> + * Implement a NUMA-aware version of MCS (aka CNA, or compact NUMA-aware lock).
> + *
> + * In CNA, spinning threads are organized in two queues, a primary queue for
> + * threads running on the same NUMA node as the current lock holder, and a
> + * secondary queue for threads running on other nodes. Schematically, it
> + * looks like this:
> + *
> + * cna_node
> + * +----------+ +--------+ +--------+
> + * |mcs:next | --> |mcs:next| --> ... |mcs:next| --> NULL [Primary queue]
> + * |mcs:locked| -. +--------+ +--------+
> + * +----------+ |
> + * `----------------------.
> + * v
> + * +--------+ +--------+
> + * |mcs:next| --> ... |mcs:next| [Secondary queue]
> + * +--------+ +--------+
> + * ^ |
> + * `--------------------'
> + *
> + * N.B. locked := 1 if secondary queue is absent. Otherwise, it contains the
> + * encoded pointer to the tail of the secondary queue, which is organized as a
> + * circular list.
> + *
> + * After acquiring the MCS lock and before acquiring the spinlock, the MCS lock
> + * holder checks whether the next waiter in the primary queue (if exists) is
> + * running on the same NUMA node. If it is not, that waiter is detached from the
> + * main queue and moved into the tail of the secondary queue. This way, we
> + * gradually filter the primary queue, leaving only waiters running on the same
> + * preferred NUMA node.
> + *
> + * For more details, see https://arxiv.org/abs/1810.05600.
> + *
> + * Authors: Alex Kogan <alex.kogan@xxxxxxxxxx>
> + * Dave Dice <dave.dice@xxxxxxxxxx>
> + */
> +
> +struct cna_node {
> + struct mcs_spinlock mcs;
> + u16 numa_node;
> + u16 real_numa_node;
> + u32 encoded_tail; /* self */
> +};
> +
> +static void __init cna_init_nodes_per_cpu(unsigned int cpu)
> +{
> + struct mcs_spinlock *base = per_cpu_ptr(&qnodes[0].mcs, cpu);
> + int numa_node = cpu_to_node(cpu);
> + int i;
> +
> + for (i = 0; i < MAX_NODES; i++) {
> + struct cna_node *cn = (struct cna_node *)grab_mcs_node(base, i);
> +
> + cn->real_numa_node = numa_node;
> + cn->encoded_tail = encode_tail(cpu, i);
> + /*
> + * make sure @encoded_tail is not confused with other valid
> + * values for @locked (0 or 1)
> + */
> + WARN_ON(cn->encoded_tail <= 1);
> + }
> +}
> +
> +static int __init cna_init_nodes(void)
> +{
> + unsigned int cpu;
> +
> + /*
> + * this will break on 32bit architectures, so we restrict
> + * the use of CNA to 64bit only (see arch/x86/Kconfig)
> + */
> + BUILD_BUG_ON(sizeof(struct cna_node) > sizeof(struct qnode));
> + /* we store an ecoded tail word in the node's @locked field */
> + BUILD_BUG_ON(sizeof(u32) > sizeof(unsigned int));
> +
> + for_each_possible_cpu(cpu)
> + cna_init_nodes_per_cpu(cpu);
> +
> + return 0;
> +}
> +
> +static __always_inline void cna_init_node(struct mcs_spinlock *node)
> +{
> + struct cna_node *cn = (struct cna_node *)node;
> +
> + cn->numa_node = cn->real_numa_node;
> +}
> +
> +/*
> + * cna_splice_head -- splice the entire secondary queue onto the head of the
> + * primary queue.
> + *
> + * Returns the new primary head node or NULL on failure.
> + */
> +static struct mcs_spinlock *
> +cna_splice_head(struct qspinlock *lock, u32 val,
> + struct mcs_spinlock *node, struct mcs_spinlock *next)
> +{
> + struct mcs_spinlock *head_2nd, *tail_2nd;
> + u32 new;
> +
> + tail_2nd = decode_tail(node->locked);
> + head_2nd = tail_2nd->next;
> +
> + if (next) {
> + /*
> + * If the primary queue is not empty, the primary tail doesn't
> + * need to change and we can simply link the secondary tail to
> + * the old primary head.
> + */
> + tail_2nd->next = next;
> + } else {
> + /*
> + * When the primary queue is empty, the secondary tail becomes
> + * the primary tail.
> + */
> +
> + /*
> + * Speculatively break the secondary queue's circular link such
> + * that when the secondary tail becomes the primary tail it all
> + * works out.
> + */
> + tail_2nd->next = NULL;
> +
> + /*
> + * tail_2nd->next = NULL; old = xchg_tail(lock, tail);
> + * prev = decode_tail(old);
> + * try_cmpxchg_release(...); WRITE_ONCE(prev->next, node);
> + *
> + * If the following cmpxchg() succeeds, our stores will not
> + * collide.
> + */
> + new = ((struct cna_node *)tail_2nd)->encoded_tail |
> + _Q_LOCKED_VAL;
> + if (!atomic_try_cmpxchg_release(&lock->val, &val, new)) {
> + /* Restore the secondary queue's circular link. */
> + tail_2nd->next = head_2nd;
> + return NULL;
> + }
> + }
> +
> + /* The primary queue head now is what was the secondary queue head. */
> + return head_2nd;
> +}
> +
> +static inline bool cna_try_clear_tail(struct qspinlock *lock, u32 val,
> + struct mcs_spinlock *node)
> +{
> + /*
> + * We're here because the primary queue is empty; check the secondary
> + * queue for remote waiters.
> + */
> + if (node->locked > 1) {
> + struct mcs_spinlock *next;
> +
> + /*
> + * When there are waiters on the secondary queue, try to move
> + * them back onto the primary queue and let them rip.
> + */
> + next = cna_splice_head(lock, val, node, NULL);
> + if (next) {
> + arch_mcs_lock_handoff(&next->locked, 1);
> + return true;
> + }
> +
> + return false;
> + }
> +
> + /* Both queues are empty. Do what MCS does. */
> + return __try_clear_tail(lock, val, node);
> +}
> +
> +/*
> + * cna_splice_next -- splice the next node from the primary queue onto
> + * the secondary queue.
> + */
> +static void cna_splice_next(struct mcs_spinlock *node,
> + struct mcs_spinlock *next,
> + struct mcs_spinlock *nnext)
> +{
> + /* remove 'next' from the main queue */
> + node->next = nnext;
> +
> + /* stick `next` on the secondary queue tail */
> + if (node->locked <= 1) { /* if secondary queue is empty */
> + /* create secondary queue */
> + next->next = next;
> + } else {
> + /* add to the tail of the secondary queue */
> + struct mcs_spinlock *tail_2nd = decode_tail(node->locked);
> + struct mcs_spinlock *head_2nd = tail_2nd->next;
> +
> + tail_2nd->next = next;
> + next->next = head_2nd;
> + }
> +
> + node->locked = ((struct cna_node *)next)->encoded_tail;
> +}
> +
> +/*
> + * cna_order_queue - check whether the next waiter in the main queue is on
> + * the same NUMA node as the lock holder; if not, and it has a waiter behind
> + * it in the main queue, move the former onto the secondary queue.
> + * Returns 1 if the next waiter runs on the same NUMA node; 0 otherwise.
> + */
> +static int cna_order_queue(struct mcs_spinlock *node)
> +{
> + struct mcs_spinlock *next = READ_ONCE(node->next);
> + struct cna_node *cn = (struct cna_node *)node;
> + int numa_node, next_numa_node;
> +
> + if (!next)
> + return 0;
> +
> + numa_node = cn->numa_node;
> + next_numa_node = ((struct cna_node *)next)->numa_node;
> +
> + if (next_numa_node != numa_node) {
> + struct mcs_spinlock *nnext = READ_ONCE(next->next);
> +
> + if (nnext)
> + cna_splice_next(node, next, nnext);
> +
> + return 0;
> + }
> + return 1;
> +}
> +
> +#define LOCK_IS_BUSY(lock) (atomic_read(&(lock)->val) & _Q_LOCKED_PENDING_MASK)
> +
> +/* Abuse the pv_wait_head_or_lock() hook to get some work done */
> +static __always_inline u32 cna_wait_head_or_lock(struct qspinlock *lock,
> + struct mcs_spinlock *node)
> +{
> + /*
> + * Try and put the time otherwise spent spin waiting on
> + * _Q_LOCKED_PENDING_MASK to use by sorting our lists.
> + */
> + while (LOCK_IS_BUSY(lock) && !cna_order_queue(node))
> + cpu_relax();
> +
> + return 0; /* we lied; we didn't wait, go do so now */
> +}
> +
> +static inline void cna_lock_handoff(struct mcs_spinlock *node,
> + struct mcs_spinlock *next)
> +{
> + u32 val = 1;
> +
> + if (node->locked > 1) {
> + struct cna_node *cn = (struct cna_node *)node;
> +
> + val = node->locked; /* preseve secondary queue */
> +
> + /*
> + * We have a local waiter, either real or fake one;
> + * reload @next in case it was changed by cna_order_queue().
> + */
> + next = node->next;
> +
> + /*
> + * Pass over NUMA node id of primary queue, to maintain the
> + * preference even if the next waiter is on a different node.
> + */
> + ((struct cna_node *)next)->numa_node = cn->numa_node;
> + }
> +
> + arch_mcs_lock_handoff(&next->locked, val);
> +}
> +
> +/*
> + * Constant (boot-param configurable) flag selecting the NUMA-aware variant
> + * of spinlock. Possible values: -1 (off) / 0 (auto, default) / 1 (on).
> + */
> +static int numa_spinlock_flag;
> +
> +static int __init numa_spinlock_setup(char *str)
> +{
> + if (!strcmp(str, "auto")) {
> + numa_spinlock_flag = 0;
> + return 1;
> + } else if (!strcmp(str, "on")) {
> + numa_spinlock_flag = 1;
> + return 1;
> + } else if (!strcmp(str, "off")) {
> + numa_spinlock_flag = -1;
> + return 1;
> + }
> +
> + return 0;
> +}
> +__setup("numa_spinlock=", numa_spinlock_setup);
> +
> +void __cna_queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
> +
> +/*
> + * Switch to the NUMA-friendly slow path for spinlocks when we have
> + * multiple NUMA nodes in native environment, unless the user has
> + * overridden this default behavior by setting the numa_spinlock flag.
> + */
> +void __init cna_configure_spin_lock_slowpath(void)
> +{
> +
> + if (numa_spinlock_flag < 0)
> + return;
> +
> + if (numa_spinlock_flag == 0 && (nr_node_ids < 2 ||
> + pv_ops.lock.queued_spin_lock_slowpath !=
> + native_queued_spin_lock_slowpath))
> + return;
> +
> + cna_init_nodes();
> +
> + pv_ops.lock.queued_spin_lock_slowpath = __cna_queued_spin_lock_slowpath;
The pv_ops is belongs to x86 custom frame work, and it prevent other
architectures connect to the CNA spinlock.

I'm working on riscv qspinlock on sg2042 64 cores 2/4 NUMA nodes
platforms. Here are the patches about riscv CNA qspinlock:
https://lore.kernel.org/linux-riscv/20230802164701.192791-19-guoren@xxxxxxxxxx/

What's the next plan for this patch series? I think the two-queue design
has satisfied most platforms with two NUMA nodes.


> +
> + pr_info("Enabling CNA spinlock\n");
> +}
> --
> 2.24.3 (Apple Git-128)
>
>