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SubjectRe: [PATCH v3 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock
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On 7/15/19 3:25 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. 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). The CNA variant is patched in
> at the boot time only if we run a multi-node machine, 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.
>
> Signed-off-by: Alex Kogan <alex.kogan@oracle.com>
> Reviewed-by: Steve Sistare <steven.sistare@oracle.com>
> ---
> arch/x86/Kconfig | 18 +++++
> arch/x86/include/asm/qspinlock.h | 4 +
> arch/x86/kernel/alternative.c | 12 +++
> kernel/locking/mcs_spinlock.h | 2 +-
> kernel/locking/qspinlock.c | 41 +++++++---
> kernel/locking/qspinlock_cna.h | 164 +++++++++++++++++++++++++++++++++++++++
> 6 files changed, 229 insertions(+), 12 deletions(-)
> create mode 100644 kernel/locking/qspinlock_cna.h
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 2bbbd4d1ba31..1d8f80c47687 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1548,6 +1548,24 @@ config NUMA
>
> Otherwise, you should say N.
>
> +config NUMA_AWARE_SPINLOCKS
> + bool "Numa-aware spinlocks"
> + depends on NUMA
> + # 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.
> +
> + 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.

You should also add a dependency on QUEUED_SPINLOCKS to highlight the
fact that it is a variant of qspinlock. You should also mention that in
the help text.


> +
> 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 0d57015114e7..1c25f0505ec0 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -649,6 +649,18 @@ void __init alternative_instructions(void)
> (unsigned long)__smp_locks_end);
> #endif
>
> +#if defined(CONFIG_NUMA_AWARE_SPINLOCKS)
> + /*
> + * If we have multiple NUMA nodes, switch from native
> + * to the NUMA-friendly slow path for spin locks.
> + */
> + if (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 bc6d3244e1af..36b802babc88 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 */
> + u64 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 5668466b3006..1ba38f85d0ae 100644
> --- a/kernel/locking/qspinlock.c
> +++ b/kernel/locking/qspinlock.c
> @@ -20,7 +20,7 @@
> * Peter Zijlstra <peterz@infradead.org>
> */
>
> -#ifndef _GEN_PV_LOCK_SLOWPATH
> +#if !defined(_GEN_PV_LOCK_SLOWPATH) && !defined(_GEN_CNA_LOCK_SLOWPATH)
>
> #include <linux/smp.h>
> #include <linux/bug.h>
> @@ -77,18 +77,14 @@
> #define MAX_NODES 4
>
> /*
> - * 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
> - * 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
> - * want to penalize pvqspinlocks to optimize for a rare case in native
> - * qspinlocks.
> + * On 64-bit architectures, the mcs_spinlock structure will be 20 bytes in
> + * size. For pvqspinlock or the NUMA-aware variant, however, we need more
> + * space for extra data. To accommodate that, we insert two more long words
> + * to pad it up to 36 bytes.
> */
The 20 bytes figure is wrong. It is actually 24 bytes for 64-bit as the
mcs_spinlock structure is 8-byte aligned. For better cacheline
alignment, I will like to keep mcs_spinlock to 16 bytes as before.
Instead, you can use encode_tail() to store the CNA node pointer in
"locked". For instance, use (encode_tail() << 1) in locked to
distinguish it from the regular locked=1 value.
> struct qnode {
> struct mcs_spinlock mcs;
> -#ifdef CONFIG_PARAVIRT_SPINLOCKS
> +#if defined(CONFIG_PARAVIRT_SPINLOCKS) || defined(CONFIG_NUMA_AWARE_SPINLOCKS)
> long reserved[2];
> #endif
> };
> @@ -327,7 +323,7 @@ static __always_inline void __pass_mcs_lock(struct mcs_spinlock *node,
> #define set_locked_empty_mcs __set_locked_empty_mcs
> #define pass_mcs_lock __pass_mcs_lock
>
> -#endif /* _GEN_PV_LOCK_SLOWPATH */
> +#endif /* _GEN_PV_LOCK_SLOWPATH && _GEN_CNA_LOCK_SLOWPATH */
>
> /**
> * queued_spin_lock_slowpath - acquire the queued spinlock
> @@ -600,6 +596,29 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> EXPORT_SYMBOL(queued_spin_lock_slowpath);
>
> /*
> + * Generate the code for NUMA-aware spin locks
> + */
> +#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 set_locked_empty_mcs
> +#define set_locked_empty_mcs cna_set_locked_empty_mcs
> +
> +#undef pass_mcs_lock
> +#define pass_mcs_lock cna_pass_mcs_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..efb9b12b2f9b
> --- /dev/null
> +++ b/kernel/locking/qspinlock_cna.h
> @@ -0,0 +1,164 @@
> +/* 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 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. 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@oracle.com>
> + * Dave Dice <dave.dice@oracle.com>
> + */
> +
> +struct cna_node {
> + struct mcs_spinlock mcs;
> + u32 numa_node;
> + u32 encoded_tail;
> + struct cna_node *tail; /* points to the secondary queue tail */
> +};
> +
> +#define CNA_NODE(ptr) ((struct cna_node *)(ptr))
> +
> +static void cna_init_node(struct mcs_spinlock *node)
> +{
> + struct cna_node *cn = CNA_NODE(node);
> + struct mcs_spinlock *base_node;
> + int cpuid;
> +
> + BUILD_BUG_ON(sizeof(struct cna_node) > sizeof(struct qnode));
> + /* we store a pointer in the node's @locked field */
> + BUILD_BUG_ON(sizeof(uintptr_t) > sizeof_field(struct mcs_spinlock, locked));
> +
> + cpuid = smp_processor_id();
> + cn->numa_node = cpu_to_node(cpuid);
> +
> + base_node = this_cpu_ptr(&qnodes[0].mcs);
> + cn->encoded_tail = encode_tail(cpuid, base_node->count - 1);
> +}


I think you can use an early_init call to initialize the numa_node and
encoded_tail values for all the per-cpu CNA nodes instead of doing it
every time a node is used. If it turns out that pv_qspinlock is used,
the pv_node_init() will properly re-initialize it. The only thing left
to do here is perhaps setting tail to NULL.

-Longman

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