On 9/6/19 10:25 AM, Alex Kogan wrote: > In CNA, spinning threads are organized in two queues, a main queue for > threads running on the same node as the current lock holder, and a > secondary queue for threads running on other nodes. At the unlock time, > the lock holder scans the main queue looking for a thread running on > the same node. If found (call it thread T), all threads in the main queue > between the current lock holder and T are moved to the end of the > secondary queue, and the lock is passed to T. If such T is not found, the > lock is passed to the first node in the secondary queue. Finally, if the > secondary queue is empty, the lock is passed to the next thread in the > main queue. For more details, see https://arxiv.org/abs/1810.05600. > > Note that this variant of CNA may introduce starvation by continuously > passing the lock to threads running on the same node. 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> > --- > arch/x86/Kconfig | 19 ++++ > arch/x86/include/asm/qspinlock.h | 4 + > arch/x86/kernel/alternative.c | 41 +++++++ > kernel/locking/mcs_spinlock.h | 2 +- > kernel/locking/qspinlock.c | 31 +++++- > kernel/locking/qspinlock_cna.h | 225 +++++++++++++++++++++++++++++++++++++++ > 6 files changed, 317 insertions(+), 5 deletions(-) > create mode 100644 kernel/locking/qspinlock_cna.h > > diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig > index 222855cc0158..9d0d87edff62 100644 > --- a/arch/x86/Kconfig > +++ b/arch/x86/Kconfig > @@ -1567,6 +1567,25 @@ config NUMA > > Otherwise, you should say N. > > +config NUMA_AWARE_SPINLOCKS > + bool "Numa-aware spinlocks" > + depends on NUMA > + depends on QUEUED_SPINLOCKS > + # 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 bd5ac6cc37db..d9b6c34d5eb4 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_queued_spin_lock_slowpath(struct qspinlock *lock, u32 val); > +#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 ccd32013c47a..d5194e342db9 100644 > --- a/arch/x86/kernel/alternative.c > +++ b/arch/x86/kernel/alternative.c > @@ -698,6 +698,33 @@ static void __init int3_selftest(void) > unregister_die_notifier(&int3_exception_nb); > } > > +#if defined(CONFIG_NUMA_AWARE_SPINLOCKS) > +/* > + * 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); > + > +#endif > + > void __init alternative_instructions(void) > { > int3_selftest(); > @@ -738,6 +765,20 @@ void __init alternative_instructions(void) > } > #endif > > +#if defined(CONFIG_NUMA_AWARE_SPINLOCKS) > + /* > + * By default, switch to the NUMA-friendly slow path for > + * spinlocks when we have multiple NUMA nodes in native environment. > + */ > + if ((numa_spinlock_flag == 1) || > + (numa_spinlock_flag == 0 && nr_node_ids > 1 && > + pv_ops.lock.queued_spin_lock_slowpath == > + native_queued_spin_lock_slowpath)) { > + pv_ops.lock.queued_spin_lock_slowpath = > + __cna_queued_spin_lock_slowpath; > + } > +#endif > + > apply_paravirt(__parainstructions, __parainstructions_end); > > restart_nmi(); > diff --git a/kernel/locking/mcs_spinlock.h b/kernel/locking/mcs_spinlock.h > index 84327ca21650..bd127b21b70c 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 070015156a10..e4e482685fc1 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> > @@ -70,7 +70,8 @@ > /* > * 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 > @@ -79,7 +80,7 @@ > */ > struct qnode { > struct mcs_spinlock mcs; > -#ifdef CONFIG_PARAVIRT_SPINLOCKS > +#if defined(CONFIG_PARAVIRT_SPINLOCKS) || defined(CONFIG_NUMA_AWARE_SPINLOCKS) > long reserved[2]; > #endif > }; > @@ -103,6 +104,8 @@ 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]); > > @@ -316,7 +319,7 @@ static __always_inline void __mcs_pass_lock(struct mcs_spinlock *node, > #define try_clear_tail __try_clear_tail > #define mcs_pass_lock __mcs_pass_lock > > -#endif /* _GEN_PV_LOCK_SLOWPATH */ > +#endif /* _GEN_PV_LOCK_SLOWPATH && _GEN_CNA_LOCK_SLOWPATH */ > > /** > * queued_spin_lock_slowpath - acquire the queued spinlock > @@ -589,6 +592,26 @@ 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 try_clear_tail > +#define try_clear_tail cna_try_change_tail > + > +#undef mcs_pass_lock > +#define mcs_pass_lock cna_pass_lock > + > +#undef queued_spin_lock_slowpath > +#define queued_spin_lock_slowpath __cna_queued_spin_lock_slowpath > + > +#include "qspinlock_cna.h" > +#include "qspinlock.c" > + > +#endif > + > +/* > * Generate the paravirt code for queued_spin_unlock_slowpath(). > */ > #if !defined(_GEN_PV_LOCK_SLOWPATH) && defined(CONFIG_PARAVIRT_SPINLOCKS) > diff --git a/kernel/locking/qspinlock_cna.h b/kernel/locking/qspinlock_cna.h > new file mode 100644 > index 000000000000..f983debf20bb > --- /dev/null > +++ b/kernel/locking/qspinlock_cna.h > @@ -0,0 +1,225 @@ > +/* 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 main 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 [Main queue] > + * |mcs:locked| +--------+ +--------+ > + * +----------+ > + * | +--------+ +--------+ > + * +-> |mcs:next| -> ... |mcs:next| -> NULL [Secondary queue] > + * |cna:tail| -+ +--------+ > + * +--------+ | ^ > + * +-------+ > + * > + * N.B. locked = 1 if secondary queue is absent. > + * > + * At the unlock time, the lock holder scans the main queue looking for a thread > + * running on the same node. If found (call it thread T), all threads in the > + * main queue between the current lock holder and T are moved to the end of the > + * secondary queue, and the lock is passed to T. If such T is not found, the > + * lock is passed to the first node in the secondary queue. Finally, if the > + * secondary queue is empty, the lock is passed to the next thread in the > + * main queue. To avoid starvation of threads in the secondary queue, > + * those threads are moved back to the head of the main queue after a certain > + * expected number of intra-node lock hand-offs. > + * > + * > + * 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; > + int numa_node; > + u32 encoded_tail; > + struct cna_node *tail; /* points to the secondary queue tail */ > +}; > + > +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->numa_node = numa_node; > + cn->encoded_tail = encode_tail(cpu, i); > + /* > + * @encoded_tail has to be larger than 1, so we do not confuse > + * it with other valid values for @locked (0 or 1) > + */ > + WARN_ON(cn->encoded_tail <= 1); > + } > +} > + > +static void __init cna_init_nodes(void) > +{ > + unsigned int cpu; > + > + 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); > +} > +early_initcall(cna_init_nodes); > + > +static inline bool cna_try_change_tail(struct qspinlock *lock, u32 val, > + struct mcs_spinlock *node) > +{ > + struct cna_node *succ; > + u32 new; > + > + /* If the secondary queue is empty, do what MCS does. */ > + if (node->locked <= 1) > + return __try_clear_tail(lock, val, node); > + > + /* > + * Try to update the tail value to the last node in the secondary queue. > + * If successful, pass the lock to the first thread in the secondary > + * queue. Doing those two actions effectively moves all nodes from the > + * secondary queue into the main one. > + */ > + succ = (struct cna_node *)decode_tail(node->locked); > + new = succ->tail->encoded_tail + _Q_LOCKED_VAL; > + > + if (atomic_try_cmpxchg_relaxed(&lock->val, &val, new)) { > + arch_mcs_pass_lock(&succ->mcs.locked, 1); > + return true; > + } > + > + return false; > +} > + > +/* > + * cna_splice_tail -- splice nodes in the main queue between [first, last] > + * onto the secondary queue. > + */ > +static void cna_splice_tail(struct cna_node *cn, struct cna_node *first, > + struct cna_node *last) > +{ > + /* remove [first,last] */ > + cn->mcs.next = last->mcs.next; > + last->mcs.next = NULL; > + > + /* stick [first,last] on the secondary queue tail */ > + if (cn->mcs.locked <= 1) { /* if secondary queue is empty */ > + /* create secondary queue */ > + first->tail = last; > + cn->mcs.locked = first->encoded_tail; > + } else { > + /* add to the tail of the secondary queue */ > + struct cna_node *head_2nd = > + (struct cna_node *)decode_tail(cn->mcs.locked); > + head_2nd->tail->mcs.next = &first->mcs; > + head_2nd->tail = last; > + } > +} > + > +/* > + * cna_try_find_next - scan the main waiting queue looking for the first > + * thread running on the same NUMA node as the lock holder. If found (call it > + * thread T), move all threads in the main queue between the lock holder and > + * T to the end of the secondary queue and return T; otherwise, return NULL. > + * > + * Schematically, this may look like the following (nn stands for numa_node and > + * et stands for encoded_tail). > + * > + * when cna_try_find_next() is called (the secondary queue is empty): > + * > + * A+------------+ B+--------+ C+--------+ T+--------+ > + * |mcs:next | -> |mcs:next| -> |mcs:next| -> |mcs:next| -> NULL > + * |mcs:locked=1| |cna:nn=0| |cna:nn=2| |cna:nn=1| > + * |cna:nn=1 | +--------+ +--------+ +--------+ > + * +----------- + > + * > + * when cna_try_find_next() returns (the secondary queue contains B and C): > + * > + * A+----------------+ T+--------+ > + * |mcs:next | -> |mcs:next| -> NULL > + * |mcs:locked=B.et | -+ |cna:nn=1| > + * |cna:nn=1 | | +--------+ > + * +--------------- + | > + * | > + * +-> B+--------+ C+--------+ > + * |mcs:next| -> |mcs:next| > + * |cna:nn=0| |cna:nn=2| > + * |cna:tail| -> +--------+ > + * +--------+ > + * > + * The worst case complexity of the scan is O(n), where n is the number > + * of current waiters. However, the fast path, which is expected to be the > + * common case, is O(1). > + */ > +static struct mcs_spinlock *cna_try_find_next(struct mcs_spinlock *node, > + struct mcs_spinlock *next) > +{ > + struct cna_node *cn = (struct cna_node *)node; > + struct cna_node *cni = (struct cna_node *)next; > + struct cna_node *first, *last = NULL; > + int my_numa_node = cn->numa_node; > + > + /* fast path: immediate successor is on the same NUMA node */ > + if (cni->numa_node == my_numa_node) > + return next; > + > + /* find any next waiter on 'our' NUMA node */ > + for (first = cni; > + cni && cni->numa_node != my_numa_node; > + last = cni, cni = (struct cna_node *)READ_ONCE(cni->mcs.next)) > + ; > + > + /* if found, splice any skipped waiters onto the secondary queue */ > + if (cni && last) > + cna_splice_tail(cn, first, last); > + > + return (struct mcs_spinlock *)cni; > +} At the Linux Plumbers Conference last week, Will has raised the concern about the latency of the O(1) cna_try_find_next() operation that will add to the lock hold time. One way to hide some of the latency is to do a pre-scan before acquiring the lock. The CNA code could override the pv_wait_head_or_lock() function to call cna_try_find_next() as a pre-scan and return 0. What do you think? Cheers, Longman
