Date:   Tue, 17 Sep 2019 13:44:02 -0400
From:   Waiman Long <longman@...hat.com>
To:     Alex Kogan <alex.kogan@...cle.com>, linux@...linux.org.uk,
        peterz@...radead.org, mingo@...hat.com, will.deacon@....com,
        arnd@...db.de, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        guohanjun@...wei.com, jglauber@...vell.com
Cc:     steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
        dave.dice@...cle.com, rahul.x.yadav@...cle.com
Subject: Re: [PATCH v4 3/5] locking/qspinlock: Introduce CNA into the slow
 path of qspinlock

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@...cle.com>
> Reviewed-by: Steve Sistare <steven.sistare@...cle.com>
> ---
>  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@...radead.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>
> @@ -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@...cle.com>
> + *          Dave Dice <dave.dice@...cle.com>
> + */
> +
> +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

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