Re: [PATCH 05/10] lib: add light-weight queuing mechanism.

[Chuck Lever <chuck.lever@oracle.com>]

On Wed, Aug 30, 2023 at 12:54:48PM +1000, NeilBrown wrote:
> lwq is a FIFO single-linked queue that only requires a spinlock
> for dequeueing, which happens in process context.  Enqueueing is atomic
> with no spinlock and can happen in any context.
> 
> Include a unit test for basic functionality - runs at boot time.  Does
> not use kunit framework.
> 
> Signed-off-by: NeilBrown <neilb@suse.de>
> ---
>  include/linux/lwq.h | 120 +++++++++++++++++++++++++++++++++++
>  lib/Kconfig         |   5 ++
>  lib/Makefile        |   2 +-
>  lib/lwq.c           | 149 ++++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 275 insertions(+), 1 deletion(-)
>  create mode 100644 include/linux/lwq.h
>  create mode 100644 lib/lwq.c
> 
> diff --git a/include/linux/lwq.h b/include/linux/lwq.h
> new file mode 100644
> index 000000000000..52b9c81b493a
> --- /dev/null
> +++ b/include/linux/lwq.h
> @@ -0,0 +1,120 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +
> +#ifndef LWQ_H
> +#define LWQ_H
> +/*
> + * light-weight single-linked queue built from llist
> + *
> + * Entries can be enqueued from any context with no locking.
> + * Entries can be dequeued from process context with integrated locking.
> + */
> +#include <linux/container_of.h>
> +#include <linux/spinlock.h>
> +#include <linux/llist.h>
> +
> +struct lwq_node {
> +	struct llist_node node;
> +};
> +
> +struct lwq {
> +	spinlock_t		lock;
> +	struct llist_node	*ready;		/* entries to be dequeued */
> +	struct llist_head	new;		/* entries being enqueued */
> +};
> +
> +/**
> + * lwq_init - initialise a lwq
> + * @q:	the lwq object
> + */
> +static inline void lwq_init(struct lwq *q)
> +{
> +	spin_lock_init(&q->lock);
> +	q->ready = NULL;
> +	init_llist_head(&q->new);
> +}
> +
> +/**
> + * lwq_empty - test if lwq contains any entry
> + * @q:	the lwq object
> + *
> + * This empty test contains an acquire barrier so that if a wakeup
> + * is sent when lwq_dequeue returns true, it is safe to go to sleep after
> + * a test on lwq_empty().
> + */
> +static inline bool lwq_empty(struct lwq *q)
> +{
> +	/* acquire ensures ordering wrt lwq_enqueue() */
> +	return smp_load_acquire(&q->ready) == NULL && llist_empty(&q->new);
> +}
> +
> +struct llist_node *__lwq_dequeue(struct lwq *q);
> +/**
> + * lwq_dequeue - dequeue first (oldest) entry from lwq
> + * @q:		the queue to dequeue from
> + * @type:	the type of object to return
> + * @member:	them member in returned object which is an lwq_node.
> + *
> + * Remove a single object from the lwq and return it.  This will take
> + * a spinlock and so must always be called in the same context, typcially
> + * process contet.
> + */
> +#define lwq_dequeue(q, type, member)					\
> +	({ struct llist_node *_n = __lwq_dequeue(q);			\
> +	  _n ? container_of(_n, type, member.node) : NULL; })
> +
> +struct llist_node *lwq_dequeue_all(struct lwq *q);
> +
> +/**
> + * lwq_for_each_safe - iterate over detached queue allowing deletion
> + * @_n:		iterator variable
> + * @_t1:	temporary struct llist_node **
> + * @_t2:	temporary struct llist_node *
> + * @_l:		address of llist_node pointer from lwq_dequeue_all()
> + * @_member:	member in _n where lwq_node is found.
> + *
> + * Iterate over members in a dequeued list.  If the iterator variable
> + * is set to NULL, the iterator removes that entry from the queue.
> + */
> +#define lwq_for_each_safe(_n, _t1, _t2, _l, _member)			\
> +	for (_t1 = (_l);						\
> +	     *(_t1) ? (_n = container_of(*(_t1), typeof(*(_n)), _member.node),\
> +		       _t2 = ((*_t1)->next),				\
> +		       true)						\
> +	     : false;							\
> +	     (_n) ? (_t1 = &(_n)->_member.node.next, 0)			\
> +	     : ((*(_t1) = (_t2)),  0))
> +
> +/**
> + * lwq_enqueue - add a new item to the end of the queue
> + * @n	- the lwq_node embedded in the item to be added
> + * @q	- the lwq to append to.
> + *
> + * No locking is needed to append to the queue so this can
> + * be called from any context.
> + * Return %true is the list may have previously been empty.
> + */
> +static inline bool lwq_enqueue(struct lwq_node *n, struct lwq *q)
> +{
> +	/* acquire enqures ordering wrt lwq_dequeue */
> +	return llist_add(&n->node, &q->new) &&
> +		smp_load_acquire(&q->ready) == NULL;
> +}
> +
> +/**
> + * lwq_enqueue_batch - add a list of new items to the end of the queue
> + * @n	- the lwq_node embedded in the first item to be added
> + * @q	- the lwq to append to.
> + *
> + * No locking is needed to append to the queue so this can
> + * be called from any context.
> + * Return %true is the list may have previously been empty.
> + */
> +static inline bool lwq_enqueue_batch(struct llist_node *n, struct lwq *q)
> +{
> +	struct llist_node *e = n;
> +
> +	/* acquire enqures ordering wrt lwq_dequeue */
> +	return llist_add_batch(llist_reverse_order(n), e, &q->new) &&
> +		smp_load_acquire(&q->ready) == NULL;
> +}
> +#endif /* LWQ_H */
> diff --git a/lib/Kconfig b/lib/Kconfig
> index 5c2da561c516..6620bdba4f94 100644
> --- a/lib/Kconfig
> +++ b/lib/Kconfig
> @@ -763,3 +763,8 @@ config ASN1_ENCODER
>  
>  config POLYNOMIAL
>         tristate
> +
> +config LWQ_TEST
> +	bool "RPC: enable boot-time test for lwq queuing"
> +	help
> +          Enable boot-time test of lwq functionality.
> diff --git a/lib/Makefile b/lib/Makefile
> index 1ffae65bb7ee..4b67c2d6af62 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -45,7 +45,7 @@ obj-y	+= lockref.o
>  obj-y += bcd.o sort.o parser.o debug_locks.o random32.o \
>  	 bust_spinlocks.o kasprintf.o bitmap.o scatterlist.o \
>  	 list_sort.o uuid.o iov_iter.o clz_ctz.o \
> -	 bsearch.o find_bit.o llist.o memweight.o kfifo.o \
> +	 bsearch.o find_bit.o llist.o lwq.o memweight.o kfifo.o \
>  	 percpu-refcount.o rhashtable.o base64.o \
>  	 once.o refcount.o rcuref.o usercopy.o errseq.o bucket_locks.o \
>  	 generic-radix-tree.o
> diff --git a/lib/lwq.c b/lib/lwq.c
> new file mode 100644
> index 000000000000..d6be6dda3867
> --- /dev/null
> +++ b/lib/lwq.c
> @@ -0,0 +1,149 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Light weight single-linked queue.
> + *
> + * Entries are enqueued to the head of an llist, with no blocking.
> + * This can happen in any context.
> + *
> + * Entries are dequeued using a spinlock to protect against
> + * multiple access.  The llist is staged in reverse order, and refreshed
> + * from the llist when it exhausts.
> + */
> +#include <linux/rcupdate.h>
> +#include <linux/lwq.h>
> +
> +struct llist_node *__lwq_dequeue(struct lwq *q)
> +{
> +	struct llist_node *this;
> +
> +	if (lwq_empty(q))
> +		return NULL;
> +	spin_lock(&q->lock);
> +	this = q->ready;
> +	if (!this && !llist_empty(&q->new)) {
> +		/* ensure queue doesn't appear transiently lwq_empty */
> +		smp_store_release(&q->ready, (void *)1);
> +		this = llist_reverse_order(llist_del_all(&q->new));
> +		if (!this)
> +			q->ready = NULL;
> +	}
> +	if (this)
> +		q->ready = llist_next(this);
> +	spin_unlock(&q->lock);
> +	return this;
> +}
> +
> +/**
> + * lwq_dequeue_all - dequeue all currently enqueued objects
> + * @q:	the queue to dequeue from
> + *
> + * Remove and return a linked list of llist_nodes of all the objects that were
> + * in the queue. The first on the list will be the object that was least
> + * recently enqueued.
> + */
> +struct llist_node *lwq_dequeue_all(struct lwq *q)
> +{
> +	struct llist_node *r, *t, **ep;
> +
> +	if (lwq_empty(q))
> +		return NULL;
> +
> +	spin_lock(&q->lock);
> +	r = q->ready;
> +	q->ready = NULL;
> +	t = llist_del_all(&q->new);
> +	spin_unlock(&q->lock);
> +	ep = &r;
> +	while (*ep)
> +		ep = &(*ep)->next;
> +	*ep = llist_reverse_order(t);
> +	return r;
> +}

ERROR: modpost: "lwq_dequeue_all" [net/sunrpc/sunrpc.ko] undefined!
ERROR: modpost: "__lwq_dequeue" [net/sunrpc/sunrpc.ko] undefined!
make[3]: *** [/home/cel/src/linux/even-releases/scripts/Makefile.modpost:144: Module.symvers] Error 1
make[2]: *** [/home/cel/src/linux/even-releases/Makefile:1984: modpost] Error 2
make[1]: *** [/home/cel/src/linux/even-releases/Makefile:234: __sub-make] Error 2
make: *** [Makefile:234: __sub-make] Error 2

You might need an EXPORT_SYMBOL_GPL or two now.


> +
> +#if IS_ENABLED(CONFIG_LWQ_TEST)
> +
> +#include <linux/module.h>
> +#include <linux/slab.h>
> +#include <linux/wait_bit.h>
> +#include <linux/kthread.h>
> +#include <linux/delay.h>
> +struct tnode {
> +	struct lwq_node n;
> +	int i;
> +	int c;
> +};
> +
> +static int lwq_exercise(void *qv)
> +{
> +	struct lwq *q = qv;
> +	int cnt;
> +	struct tnode *t;
> +
> +	for (cnt = 0; cnt < 10000; cnt++) {
> +		wait_var_event(q, (t = lwq_dequeue(q, struct tnode, n)) != NULL);
> +		t->c++;
> +		if (lwq_enqueue(&t->n, q))
> +			wake_up_var(q);
> +	}
> +	while (!kthread_should_stop())
> +		schedule_timeout_idle(1);
> +	return 0;
> +}
> +
> +static int lwq_test(void)
> +{
> +	int i;
> +	struct lwq q;
> +	struct llist_node *l, **t1, *t2;
> +	struct tnode *t;
> +	struct task_struct *threads[8];
> +
> +	printk(KERN_INFO "testing lwq....\n");
> +	lwq_init(&q);
> +	printk(KERN_INFO " lwq: run some threads\n");
> +	for (i = 0; i < ARRAY_SIZE(threads); i++)
> +		threads[i] = kthread_run(lwq_exercise, &q, "lwq-test-%d", i);
> +	for (i = 0; i < 100; i++) {
> +		t = kmalloc(sizeof(*t), GFP_KERNEL);
> +		t->i = i;
> +		t->c = 0;
> +		if (lwq_enqueue(&t->n, &q))
> +			wake_up_var(&q);
> +	};
> +	/* wait for threads to exit */
> +	for (i = 0; i < ARRAY_SIZE(threads); i++)
> +		if (!IS_ERR_OR_NULL(threads[i]))
> +			kthread_stop(threads[i]);
> +	printk(KERN_INFO " lwq: dequeue first 50:");
> +	for (i = 0; i < 50 ; i++) {
> +		if (i && (i % 10) == 0) {
> +			printk(KERN_CONT "\n");
> +			printk(KERN_INFO " lwq: ... ");
> +		}
> +		t = lwq_dequeue(&q, struct tnode, n);
> +		printk(KERN_CONT " %d(%d)", t->i, t->c);
> +		kfree(t);
> +	}
> +	printk(KERN_CONT "\n");
> +	l = lwq_dequeue_all(&q);
> +	printk(KERN_INFO " lwq: delete the multiples of 3 (test lwq_for_each_safe())\n");
> +	lwq_for_each_safe(t, t1, t2, &l, n) {
> +		if ((t->i % 3) == 0) {
> +			t->i = -1;
> +			kfree(t);
> +			t = NULL;
> +		}
> +	}
> +	if (l)
> +		lwq_enqueue_batch(l, &q);
> +	printk(KERN_INFO " lwq: dequeue remaining:");
> +	while ((t = lwq_dequeue(&q, struct tnode, n)) != NULL) {
> +		printk(KERN_CONT " %d", t->i);
> +		kfree(t);
> +	}
> +	printk(KERN_CONT "\n");
> +	return 0;
> +}
> +
> +module_init(lwq_test);
> +#endif /* CONFIG_LWQ_TEST*/
> -- 
> 2.41.0
> 

-- 
Chuck Lever