On 11/25/19 4:07 PM, 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. After acquiring the > MCS lock and before acquiring the spinlock, the lock holder scans the > main queue looking for a thread running on the same node (pre-scan). 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. > If such T is not found, we make another scan of the main queue when > unlocking the MCS lock (post-scan), starting at the position where > pre-scan stopped. If both scans fail to find such T, the MCS lock is > passed to the first thread in the secondary queue. 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> > --- > .../admin-guide/kernel-parameters.txt | 10 + > arch/x86/Kconfig | 20 ++ > arch/x86/include/asm/qspinlock.h | 4 + > arch/x86/kernel/alternative.c | 43 +++ > kernel/locking/mcs_spinlock.h | 2 +- > kernel/locking/qspinlock.c | 34 ++- > kernel/locking/qspinlock_cna.h | 264 ++++++++++++++++++ > 7 files changed, 372 insertions(+), 5 deletions(-) > create mode 100644 kernel/locking/qspinlock_cna.h > : > diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c > index 9d3a971ea364..6a4ccbf4e09c 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 > + This __init function should be in qspinlock_cna.h. We generally like to put as much related code into as few places as possible instead of spreading them around in different places. > void __init alternative_instructions(void) > { > int3_selftest(); > @@ -738,6 +765,22 @@ 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; > + > + pr_info("Enabling CNA spinlock\n"); > + } > +#endif > + > apply_paravirt(__parainstructions, __parainstructions_end); Encapsulate the logic into another __init function in qspinlock_cna.h and just make a function call here. You can declare the function in arch/x86/include/asm/qspinlock.h. > > restart_nmi(); > diff --git a/kernel/locking/mcs_spinlock.h b/kernel/locking/mcs_spinlock.h > index 52d06ec6f525..e40b9538b79f 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 c06d1e8075d9..6d8c4a52e44e 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 > @@ -588,6 +591,29 @@ 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_wait_head_or_lock > +#define pv_wait_head_or_lock cna_pre_scan > + > +#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(). > */ > diff --git a/kernel/locking/qspinlock_cna.h b/kernel/locking/qspinlock_cna.h > new file mode 100644 > index 000000000000..a638336f9560 > --- /dev/null > +++ b/kernel/locking/qspinlock_cna.h > @@ -0,0 +1,264 @@ > +/* 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| [Secondary queue] > + * +--------+ +--------+ > + * ^ | > + * +--------------------+ > + * > + * N.B. locked = 1 if secondary queue is absent. Othewrise, 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 lock > + * holder scans the main queue looking for a thread running on the same node > + * (pre-scan). 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. If such T is not found, we make another scan of the main queue when > + * unlocking the MCS lock (post-scan), starting at the node where pre-scan > + * stopped. If both scans fail to find such T, the MCS lock is passed to the > + * first thread in the secondary queue. 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. > + * > + * Authors: Alex Kogan <alex.kogan@xxxxxxxxxx> > + * Dave Dice <dave.dice@xxxxxxxxxx> > + */ > + > +struct cna_node { > + struct mcs_spinlock mcs; > + int numa_node; > + u32 encoded_tail; > + u32 pre_scan_result; /* 0 or encoded 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 or @pre_scan_result > + * (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; > +} > +early_initcall(cna_init_nodes); > + Include a comment here saying that the cna_try_change_tail() function is only called when the primary queue is empty. That will make the code easier to read. > +static inline bool cna_try_change_tail(struct qspinlock *lock, u32 val, > + struct mcs_spinlock *node) > +{ > + struct mcs_spinlock *head_2nd, *tail_2nd; > + 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. > + */ > + tail_2nd = decode_tail(node->locked); > + head_2nd = tail_2nd->next; > + new = ((struct cna_node *)tail_2nd)->encoded_tail + _Q_LOCKED_VAL; > + > + if (atomic_try_cmpxchg_relaxed(&lock->val, &val, new)) { > + /* > + * Try to reset @next in tail_2nd to NULL, but no need to check > + * the result - if failed, a new successor has updated it. > + */ > + cmpxchg_relaxed(&tail_2nd->next, head_2nd, NULL); > + arch_mcs_pass_lock(&head_2nd->locked, 1); > + return true; > + } > + > + return false; > +} Cheers, Longman
