Message-Id: <20221115005409.9d83cc3299e8e2d2c32167fa@kernel.org>
Date:   Tue, 15 Nov 2022 00:54:09 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     wuqiang <wuqiang.matt@...edance.com>
Cc:     davem@...emloft.net, anil.s.keshavamurthy@...el.com,
        naveen.n.rao@...ux.ibm.com, rostedt@...dmis.org,
        peterz@...radead.org, akpm@...ux-foundation.org,
        sander@...nheule.net, ebiggers@...gle.com,
        dan.j.williams@...el.com, jpoimboe@...nel.org,
        linux-kernel@...r.kernel.org, lkp@...el.com, mattwu@....com
Subject: Re: [PATCH v6 1/4] lib: objpool added: ring-array based lockless
 MPMC queue

On Tue,  8 Nov 2022 15:14:40 +0800
wuqiang <wuqiang.matt@...edance.com> wrote:

> The object pool is a scalable implementaion of high performance queue
> for objects allocation and reclamation, such as kretprobe instances.
> 
> With leveraging per-cpu ring-array to mitigate the hot spots of memory
> contention, it could deliver near-linear scalability for high parallel
> scenarios. The ring-array is compactly managed in a single cache-line
> to benefit from warmed L1 cache for most cases (<= 4 objects per-core).
> The body of pre-allocated objects is stored in continuous cache-lines
> just after the ring-array.
> 
> The object pool is interrupt safe. Both allocation and reclamation
> (object pop and push operations) can be preemptible or interruptable.
> 
> It's best suited for following cases:
> 1) Memory allocation or reclamation are prohibited or too expensive
> 2) Consumers are of different priorities, such as irqs and threads
> 
> Limitations:
> 1) Maximum objects (capacity) is determined during pool initializing
> 2) The memory of objects won't be freed until the poll is finalized
> 3) Object allocation (pop) may fail after trying all cpu slots
> 4) Object reclamation (push) won't fail but may take long time to
>    finish for imbalanced scenarios. You can try larger max_entries
>    to mitigate, or ( >= CPUS * nr_objs) to avoid
> 
> Signed-off-by: wuqiang <wuqiang.matt@...edance.com>
> ---
>  include/linux/objpool.h | 153 +++++++++++++
>  lib/Makefile            |   2 +-
>  lib/objpool.c           | 487 ++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 641 insertions(+), 1 deletion(-)
>  create mode 100644 include/linux/objpool.h
>  create mode 100644 lib/objpool.c
> 
> diff --git a/include/linux/objpool.h b/include/linux/objpool.h
> new file mode 100644
> index 000000000000..7899b054b50c
> --- /dev/null
> +++ b/include/linux/objpool.h
> @@ -0,0 +1,153 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _LINUX_OBJPOOL_H
> +#define _LINUX_OBJPOOL_H
> +
> +#include <linux/types.h>
> +
> +/*
> + * objpool: ring-array based lockless MPMC queue
> + *
> + * Copyright: wuqiang.matt@...edance.com
> + *
> + * The object pool is a scalable implementaion of high performance queue
> + * for objects allocation and reclamation, such as kretprobe instances.
> + *
> + * With leveraging per-cpu ring-array to mitigate the hot spots of memory
> + * contention, it could deliver near-linear scalability for high parallel
> + * scenarios. The ring-array is compactly managed in a single cache-line
> + * to benefit from warmed L1 cache for most cases (<= 4 objects per-core).
> + * The body of pre-allocated objects is stored in continuous cache-lines
> + * just after the ring-array.
> + *
> + * The object pool is interrupt safe. Both allocation and reclamation
> + * (object pop and push operations) can be preemptible or interruptable.
> + *
> + * It's best suited for following cases:
> + * 1) Memory allocation or reclamation are prohibited or too expensive
> + * 2) Consumers are of different priorities, such as irqs and threads
> + *
> + * Limitations:
> + * 1) Maximum objects (capacity) is determined during pool initializing
> + * 2) The memory of objects won't be freed until the poll is finalized
> + * 3) Object allocation (pop) may fail after trying all cpu slots
> + * 4) Object reclamation (push) won't fail but may take long time to
> + *    finish for imbalanced scenarios. You can try larger max_entries
> + *    to mitigate, or ( >= CPUS * nr_objs) to avoid
> + */
> +
> +/*
> + * objpool_slot: per-cpu ring array
> + *
> + * Represents a cpu-local array-based ring buffer, its size is specialized
> + * during initialization of object pool.
> + *
> + * The objpool_slot is allocated from local memory for NUMA system, and to
> + * be kept compact in a single cacheline. ages[] is stored just after the
> + * body of objpool_slot, and then entries[]. The Array of ages[] describes
> + * revision of each item, solely used to avoid ABA. And array of entries[]
> + * contains the pointers of objects.
> + *
> + * The default size of objpool_slot is a single cache-line, aka. 64 bytes.
> + *
> + * 64bit:
> + *        4      8      12     16        32                 64
> + * | head | tail | size | mask | ages[4] | ents[4]: (8 * 4) | objects
> + *
> + * 32bit:
> + *        4      8      12     16        32        48       64
> + * | head | tail | size | mask | ages[4] | ents[4] | unused | objects
> + *
> + */
> +
> +struct objpool_slot {
> +	uint32_t                head;	/* head of ring array */
> +	uint32_t                tail;	/* tail of ring array */
> +	uint32_t                size;	/* array size, pow of 2 */
> +	uint32_t                mask;	/* size - 1 */
> +} __attribute__((packed));
> +
> +/* caller-specified object initial callback to setup each object, only called once */
> +typedef int (*objpool_init_obj_cb)(void *context, void *obj);

It seems a bit confused that this "initialize object" callback
don't have the @obj as the first argument.

> +
> +/* caller-specified cleanup callback for private objects/pool/context */
> +typedef int (*objpool_release_cb)(void *context, void *ptr, uint32_t flags);

Do you have any use-case for this release callback?
If not, until actual use-case comes up, I recommend you to defer
implementing it.

> +
> +/* called for object releasing: ptr points to an object */
> +#define OBJPOOL_FLAG_NODE        (0x00000001)
> +/* for user pool and context releasing, ptr could be NULL */
> +#define OBJPOOL_FLAG_POOL        (0x00001000)
> +/* the object or pool to be released is user-managed */
> +#define OBJPOOL_FLAG_USER        (0x00008000)

Ditto.

> +
> +/*
> + * objpool_head: object pooling metadata
> + */
> +
> +struct objpool_head {
> +	unsigned int            obj_size;	/* object & element size */
> +	unsigned int            nr_objs;	/* total objs (to be pre-allocated) */
> +	unsigned int            nr_cpus;	/* num of possible cpus */
> +	unsigned int            capacity;	/* max objects per cpuslot */
> +	unsigned long           flags;		/* flags for objpool management */
> +	gfp_t                   gfp;		/* gfp flags for kmalloc & vmalloc */
> +	unsigned int            pool_size;	/* user pool size in byes */
> +	void                   *pool;		/* user managed memory pool */
> +	struct objpool_slot   **cpu_slots;	/* array of percpu slots */
> +	unsigned int           *slot_sizes;	/* size in bytes of slots */
> +	objpool_release_cb      release;	/* resource cleanup callback */
> +	void                   *context;	/* caller-provided context */
> +};
> +
> +#define OBJPOOL_FROM_VMALLOC	(0x800000000)	/* objpool allocated from vmalloc area */
> +#define OBJPOOL_HAVE_OBJECTS	(0x400000000)	/* objects allocated along with objpool */

This also doesn't need at this moment. Please start from simple
design for review.

> +
> +/* initialize object pool and pre-allocate objects */
> +int objpool_init(struct objpool_head *head, unsigned int nr_objs,
> +		 unsigned int max_objs, unsigned int object_size,
> +		 gfp_t gfp, void *context, objpool_init_obj_cb objinit,
> +		 objpool_release_cb release);
> +
> +/* add objects in batch from user provided pool */
> +int objpool_populate(struct objpool_head *head, void *pool,
> +		     unsigned int size, unsigned int object_size,
> +		     void *context, objpool_init_obj_cb objinit);
> +
> +/* add pre-allocated object (managed by user) to objpool */
> +int objpool_add(void *obj, struct objpool_head *head);
> +
> +/* allocate an object from objects pool */
> +void *objpool_pop(struct objpool_head *head);
> +
> +/* reclaim an object to objects pool */
> +int objpool_push(void *node, struct objpool_head *head);
> +
> +/* cleanup the whole object pool (objects including) */
> +void objpool_fini(struct objpool_head *head);
> +
> +/* whether the object is pre-allocated with percpu slots */
> +static inline int objpool_is_inslot(void *obj, struct objpool_head *head)
> +{
> +	void *slot;
> +	int i;
> +
> +	if (!obj || !(head->flags & OBJPOOL_HAVE_OBJECTS))
> +		return 0;
> +
> +	for (i = 0; i < head->nr_cpus; i++) {
> +		slot = head->cpu_slots[i];
> +		if (obj >= slot && obj < slot + head->slot_sizes[i])
> +			return 1;
> +	}
> +
> +	return 0;
> +}

Ditto.

It is too complicated to mix the internal allocated objects
and external ones. This will expose the implementation of the
objpool (users must understand they have to free the object
only outside of slot)

You can add it afterwards if it is really needed :)

> +
> +/* whether the object is from user pool (batched adding) */
> +static inline int objpool_is_inpool(void *obj, struct objpool_head *head)
> +{
> +	return (obj && head->pool && obj >= head->pool &&
> +		obj < head->pool + head->pool_size);
> +}
> +
> +#endif /* _LINUX_OBJPOOL_H */
> diff --git a/lib/Makefile b/lib/Makefile
> index 161d6a724ff7..e938703a321f 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -34,7 +34,7 @@ lib-y := ctype.o string.o vsprintf.o cmdline.o \
>  	 is_single_threaded.o plist.o decompress.o kobject_uevent.o \
>  	 earlycpio.o seq_buf.o siphash.o dec_and_lock.o \
>  	 nmi_backtrace.o win_minmax.o memcat_p.o \
> -	 buildid.o
> +	 buildid.o objpool.o
>  
>  lib-$(CONFIG_PRINTK) += dump_stack.o
>  lib-$(CONFIG_SMP) += cpumask.o
> diff --git a/lib/objpool.c b/lib/objpool.c
> new file mode 100644
> index 000000000000..ecffa0795f3d
> --- /dev/null
> +++ b/lib/objpool.c
> @@ -0,0 +1,487 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/objpool.h>
> +#include <linux/slab.h>
> +#include <linux/vmalloc.h>
> +#include <linux/atomic.h>
> +#include <linux/prefetch.h>
> +
> +/*
> + * objpool: ring-array based lockless MPMC/FIFO queues
> + *
> + * Copyright: wuqiang.matt@...edance.com
> + */
> +
> +/* compute the suitable num of objects to be managed by slot */
> +static inline unsigned int __objpool_num_of_objs(unsigned int size)
> +{
> +	return rounddown_pow_of_two((size - sizeof(struct objpool_slot)) /
> +			(sizeof(uint32_t) + sizeof(void *)));
> +}
> +
> +#define SLOT_AGES(s) ((uint32_t *)((char *)(s) + sizeof(struct objpool_slot)))
> +#define SLOT_ENTS(s) ((void **)((char *)(s) + sizeof(struct objpool_slot) + \
> +			sizeof(uint32_t) * (s)->size))
> +#define SLOT_OBJS(s) ((void *)((char *)(s) + sizeof(struct objpool_slot) + \
> +			(sizeof(uint32_t) + sizeof(void *)) * (s)->size))
> +
> +/* allocate and initialize percpu slots */
> +static inline int
> +__objpool_init_percpu_slots(struct objpool_head *head, unsigned int nobjs,
> +			void *context, objpool_init_obj_cb objinit)
> +{
> +	unsigned int i, j, n, size, objsz, nents = head->capacity;
> +
> +	/* aligned object size by sizeof(void *) */
> +	objsz = ALIGN(head->obj_size, sizeof(void *));
> +	/* shall we allocate objects along with objpool_slot */
> +	if (objsz)
> +		head->flags |= OBJPOOL_HAVE_OBJECTS;
> +
> +	for (i = 0; i < head->nr_cpus; i++) {
> +		struct objpool_slot *os;
> +
> +		/* compute how many objects to be managed by this slot */
> +		n = nobjs / head->nr_cpus;
> +		if (i < (nobjs % head->nr_cpus))
> +			n++;
> +		size = sizeof(struct objpool_slot) + sizeof(void *) * nents +
> +		       sizeof(uint32_t) * nents + objsz * n;
> +
> +		/* decide memory area for cpu-slot allocation */
> +		if (!i && !(head->gfp & GFP_ATOMIC) && size > PAGE_SIZE / 2)
> +			head->flags |= OBJPOOL_FROM_VMALLOC;
> +
> +		/* allocate percpu slot & objects from local memory */
> +		if (head->flags & OBJPOOL_FROM_VMALLOC)
> +			os = __vmalloc_node(size, sizeof(void *), head->gfp,
> +				cpu_to_node(i), __builtin_return_address(0));
> +		else
> +			os = kmalloc_node(size, head->gfp, cpu_to_node(i));
> +		if (!os)
> +			return -ENOMEM;
> +
> +		/* initialize percpu slot for the i-th cpu */
> +		memset(os, 0, size);
> +		os->size = head->capacity;
> +		os->mask = os->size - 1;
> +		head->cpu_slots[i] = os;
> +		head->slot_sizes[i] = size;
> +
> +		/*
> +		 * start from 2nd round to avoid conflict of 1st item.
> +		 * we assume that the head item is ready for retrieval
> +		 * iff head is equal to ages[head & mask]. but ages is
> +		 * initialized as 0, so in view of the caller of pop(),
> +		 * the 1st item (0th) is always ready, but fact could
> +		 * be: push() is stalled before the final update, thus
> +		 * the item being inserted will be lost forever.
> +		 */
> +		os->head = os->tail = head->capacity;
> +
> +		if (!objsz)
> +			continue;
> +
> +		for (j = 0; j < n; j++) {
> +			uint32_t *ages = SLOT_AGES(os);
> +			void **ents = SLOT_ENTS(os);
> +			void *obj = SLOT_OBJS(os) + j * objsz;
> +			uint32_t ie = os->tail & os->mask;
> +
> +			/* perform object initialization */
> +			if (objinit) {
> +				int rc = objinit(context, obj);
> +				if (rc)
> +					return rc;
> +			}
> +
> +			/* add obj into the ring array */
> +			ents[ie] = obj;
> +			ages[ie] = os->tail;
> +			os->tail++;
> +			head->nr_objs++;
> +		}
> +	}
> +
> +	return 0;
> +}
> +
> +/* cleanup all percpu slots of the object pool */
> +static inline void __objpool_fini_percpu_slots(struct objpool_head *head)
> +{
> +	unsigned int i;
> +
> +	if (!head->cpu_slots)
> +		return;
> +
> +	for (i = 0; i < head->nr_cpus; i++) {
> +		if (!head->cpu_slots[i])
> +			continue;
> +		if (head->flags & OBJPOOL_FROM_VMALLOC)
> +			vfree(head->cpu_slots[i]);
> +		else
> +			kfree(head->cpu_slots[i]);
> +	}
> +	kfree(head->cpu_slots);
> +	head->cpu_slots = NULL;
> +	head->slot_sizes = NULL;
> +}
> +
> +/**
> + * objpool_init: initialize object pool and pre-allocate objects
> + *
> + * args:
> + * @head:    the object pool to be initialized, declared by caller
> + * @nr_objs: total objects to be pre-allocated by this object pool
> + * @max_objs: max entries (object pool capacity), use nr_objs if 0
> + * @object_size: size of an object, no objects pre-allocated if 0
> + * @gfp:     flags for memory allocation (via kmalloc or vmalloc)
> + * @context: user context for object initialization callback
> + * @objinit: object initialization callback for extra setting-up
> + * @release: cleanup callback for private objects/pool/context
> + *
> + * return:
> + *         0 for success, otherwise error code
> + *
> + * All pre-allocated objects are to be zeroed. Caller could do extra
> + * initialization in objinit callback. The objinit callback will be
> + * called once and only once after the slot allocation. Then objpool
> + * won't touch any content of the objects since then. It's caller's
> + * duty to perform reinitialization after object allocation (pop) or
> + * clearance before object reclamation (push) if required.
> + */
> +int objpool_init(struct objpool_head *head, unsigned int nr_objs,
> +		unsigned int max_objs, unsigned int object_size,
> +		gfp_t gfp, void *context, objpool_init_obj_cb objinit,
> +		objpool_release_cb release)
> +{
> +	unsigned int nents, ncpus = num_possible_cpus();
> +	int rc;
> +
> +	/* calculate percpu slot size (rounded to pow of 2) */
> +	if (max_objs < nr_objs)

This should be an error case.

	if (!max_objs)

> +		max_objs = nr_objs;

	else if (max_objs < nr_objs)
		return -EINVAL;

But to simplify that, I think it should use only nr_objs.
I mean, if we can pass the @objinit, there seems no reason to
have both nr_objs and max_objs.

> +	nents = max_objs / ncpus;
> +	if (nents < __objpool_num_of_objs(L1_CACHE_BYTES))
> +		nents = __objpool_num_of_objs(L1_CACHE_BYTES);
> +	nents = roundup_pow_of_two(nents);
> +	while (nents * ncpus < nr_objs)
> +		nents = nents << 1;
> +
> +	memset(head, 0, sizeof(struct objpool_head));
> +	head->nr_cpus = ncpus;
> +	head->obj_size = object_size;
> +	head->capacity = nents;
> +	head->gfp = gfp & ~__GFP_ZERO;
> +	head->context = context;
> +	head->release = release;
> +
> +	/* allocate array for percpu slots */
> +	head->cpu_slots = kzalloc(head->nr_cpus * sizeof(void *) +
> +			       head->nr_cpus * sizeof(uint32_t), head->gfp);
> +	if (!head->cpu_slots)
> +		return -ENOMEM;
> +	head->slot_sizes = (uint32_t *)&head->cpu_slots[head->nr_cpus];
> +
> +	/* initialize per-cpu slots */
> +	rc = __objpool_init_percpu_slots(head, nr_objs, context, objinit);
> +	if (rc)
> +		__objpool_fini_percpu_slots(head);
> +
> +	return rc;
> +}
> +EXPORT_SYMBOL_GPL(objpool_init);
> +
> +/* adding object to slot tail, the given slot must NOT be full */
> +static inline int __objpool_add_slot(void *obj, struct objpool_slot *os)
> +{
> +	uint32_t *ages = SLOT_AGES(os);
> +	void **ents = SLOT_ENTS(os);
> +	uint32_t tail = atomic_inc_return((atomic_t *)&os->tail) - 1;
> +
> +	WRITE_ONCE(ents[tail & os->mask], obj);
> +
> +	/* order matters: obj must be updated before tail updating */
> +	smp_store_release(&ages[tail & os->mask], tail);
> +	return 0;
> +}
> +
> +/* adding object to slot, abort if the slot was already full */
> +static inline int __objpool_try_add_slot(void *obj, struct objpool_slot *os)
> +{
> +	uint32_t *ages = SLOT_AGES(os);
> +	void **ents = SLOT_ENTS(os);
> +	uint32_t head, tail;
> +
> +	do {
> +		/* perform memory loading for both head and tail */
> +		head = READ_ONCE(os->head);
> +		tail = READ_ONCE(os->tail);
> +		/* just abort if slot is full */
> +		if (tail >= head + os->size)
> +			return -ENOENT;
> +		/* try to extend tail by 1 using CAS to avoid races */
> +		if (try_cmpxchg_acquire(&os->tail, &tail, tail + 1))
> +			break;
> +	} while (1);
> +
> +	/* the tail-th of slot is reserved for the given obj */
> +	WRITE_ONCE(ents[tail & os->mask], obj);
> +	/* update epoch id to make this object available for pop() */
> +	smp_store_release(&ages[tail & os->mask], tail);
> +	return 0;
> +}
> +
> +/**
> + * objpool_populate: add objects from user provided pool in batch
> + *
> + * args:
> + * @head:  object pool
> + * @pool: user buffer for pre-allocated objects
> + * @size: size of user buffer
> + * @object_size: size of object & element
> + * @context: user context for objinit callback
> + * @objinit: object initialization callback
> + *
> + * return: 0 or error code
> + */
> +int objpool_populate(struct objpool_head *head, void *pool,
> +		unsigned int size, unsigned int object_size,
> +		void *context, objpool_init_obj_cb objinit)
> +{
> +	unsigned int n = head->nr_objs, used = 0, i;
> +
> +	if (head->pool || !pool || size < object_size)
> +		return -EINVAL;
> +	if (head->obj_size && head->obj_size != object_size)
> +		return -EINVAL;
> +	if (head->context && context && head->context != context)
> +		return -EINVAL;
> +	if (head->nr_objs >= head->nr_cpus * head->capacity)
> +		return -ENOENT;
> +
> +	WARN_ON_ONCE(((unsigned long)pool) & (sizeof(void *) - 1));
> +	WARN_ON_ONCE(((uint32_t)object_size) & (sizeof(void *) - 1));
> +
> +	/* align object size by sizeof(void *) */
> +	head->obj_size = object_size;
> +	object_size = ALIGN(object_size, sizeof(void *));
> +	if (object_size == 0)
> +		return -EINVAL;
> +
> +	while (used + object_size <= size) {
> +		void *obj = pool + used;
> +
> +		/* perform object initialization */
> +		if (objinit) {
> +			int rc = objinit(context, obj);
> +			if (rc)
> +				return rc;
> +		}
> +
> +		/* insert obj to its corresponding objpool slot */
> +		i = (n + used * head->nr_cpus/size) % head->nr_cpus;
> +		if (!__objpool_try_add_slot(obj, head->cpu_slots[i]))
> +			head->nr_objs++;
> +
> +		used += object_size;
> +	}
> +
> +	if (!used)
> +		return -ENOENT;
> +
> +	head->context = context;
> +	head->pool = pool;
> +	head->pool_size = size;
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(objpool_populate);
> +
> +/**
> + * objpool_add: add pre-allocated object to objpool during pool
> + * initialization
> + *
> + * args:
> + * @obj:  object pointer to be added to objpool
> + * @head: object pool to be inserted into
> + *
> + * return:
> + *     0 or error code
> + *
> + * objpool_add_node doesn't handle race conditions, can only be
> + * called during objpool initialization
> + */
> +int objpool_add(void *obj, struct objpool_head *head)
> +{
> +	unsigned int i, cpu;
> +
> +	if (!obj)
> +		return -EINVAL;
> +	if (head->nr_objs >= head->nr_cpus * head->capacity)
> +		return -ENOENT;
> +
> +	cpu = head->nr_objs % head->nr_cpus;
> +	for (i = 0; i < head->nr_cpus; i++) {
> +		if (!__objpool_try_add_slot(obj, head->cpu_slots[cpu])) {
> +			head->nr_objs++;
> +			return 0;
> +		}
> +
> +		if (++cpu >= head->nr_cpus)
> +			cpu = 0;
> +	}
> +
> +	return -ENOENT;
> +}
> +EXPORT_SYMBOL_GPL(objpool_add);
> +
> +/**
> + * objpool_push: reclaim the object and return back to objects pool
> + *
> + * args:
> + * @obj:  object pointer to be pushed to object pool
> + * @head: object pool
> + *
> + * return:
> + *     0 or error code: it fails only when objects pool are full
> + *
> + * objpool_push is non-blockable, and can be nested
> + */
> +int objpool_push(void *obj, struct objpool_head *head)
> +{
> +	unsigned int cpu = raw_smp_processor_id() % head->nr_cpus;
> +
> +	do {
> +		if (head->nr_objs > head->capacity) {
> +			if (!__objpool_try_add_slot(obj, head->cpu_slots[cpu]))
> +				return 0;
> +		} else {
> +			if (!__objpool_add_slot(obj, head->cpu_slots[cpu]))
> +				return 0;
> +		}
> +		if (++cpu >= head->nr_cpus)
> +			cpu = 0;
> +	} while (1);
> +
> +	return -ENOENT;
> +}
> +EXPORT_SYMBOL_GPL(objpool_push);
> +
> +/* try to retrieve object from slot */
> +static inline void *__objpool_try_get_slot(struct objpool_slot *os)
> +{
> +	uint32_t *ages = SLOT_AGES(os);
> +	void **ents = SLOT_ENTS(os);
> +	/* do memory load of head to local head */
> +	uint32_t head = smp_load_acquire(&os->head);
> +
> +	/* loop if slot isn't empty */
> +	while (head != READ_ONCE(os->tail)) {
> +		uint32_t id = head & os->mask, prev = head;
> +
> +		/* do prefetching of object ents */
> +		prefetch(&ents[id]);
> +
> +		/*
> +		 * check whether this item was ready for retrieval ? There's
> +		 * possibility * in theory * we might retrieve wrong object,
> +		 * in case ages[id] overflows when current task is sleeping,
> +		 * but it will take very very long to overflow an uint32_t
> +		 */
> +		if (smp_load_acquire(&ages[id]) == head) {
> +			/* node must have been udpated by push() */
> +			void *node = READ_ONCE(ents[id]);
> +			/* commit and move forward head of the slot */
> +			if (try_cmpxchg_release(&os->head, &head, head + 1))
> +				return node;
> +		}
> +
> +		/* re-load head from memory continue trying */
> +		head = READ_ONCE(os->head);
> +		/*
> +		 * head stays unchanged, so it's very likely current pop()
> +		 * just preempted/interrupted an ongoing push() operation
> +		 */
> +		if (head == prev)
> +			break;
> +	}
> +
> +	return NULL;
> +}
> +
> +/**
> + * objpool_pop: allocate an object from objects pool
> + *
> + * args:
> + * @oh:  object pool
> + *
> + * return:
> + *   object: NULL if failed (object pool is empty)
> + *
> + * objpool_pop can be nested, so can be used in any context.
> + */
> +void *objpool_pop(struct objpool_head *head)
> +{
> +	unsigned int i, cpu;
> +	void *obj = NULL;
> +
> +	cpu = raw_smp_processor_id() % head->nr_cpus;

(Not sure, do we really need this?)

Thank you,

> +	for (i = 0; i < head->nr_cpus; i++) {
> +		struct objpool_slot *slot = head->cpu_slots[cpu];
> +		obj = __objpool_try_get_slot(slot);
> +		if (obj)
> +			break;
> +		if (++cpu >= head->nr_cpus)
> +			cpu = 0;
> +	}
> +
> +	return obj;
> +}
> +EXPORT_SYMBOL_GPL(objpool_pop);
> +
> +/**
> + * objpool_fini: cleanup the whole object pool (releasing all objects)
> + *
> + * args:
> + * @head: object pool to be released
> + *
> + */
> +void objpool_fini(struct objpool_head *head)
> +{
> +	uint32_t i, flags;
> +
> +	if (!head->cpu_slots)
> +		return;
> +
> +	if (!head->release) {
> +		__objpool_fini_percpu_slots(head);
> +		return;
> +	}
> +
> +	/* cleanup all objects remained in objpool */
> +	for (i = 0; i < head->nr_cpus; i++) {
> +		void *obj;
> +		do {
> +			flags = OBJPOOL_FLAG_NODE;
> +			obj = __objpool_try_get_slot(head->cpu_slots[i]);
> +			if (!obj)
> +				break;
> +			if (!objpool_is_inpool(obj, head) &&
> +			    !objpool_is_inslot(obj, head)) {
> +				flags |= OBJPOOL_FLAG_USER;
> +			}
> +			head->release(head->context, obj, flags);
> +		} while (obj);
> +	}
> +
> +	/* release percpu slots */
> +	__objpool_fini_percpu_slots(head);
> +
> +	/* cleanup user private pool and related context */
> +	flags = OBJPOOL_FLAG_POOL;
> +	if (head->pool)
> +		flags |= OBJPOOL_FLAG_USER;
> +	head->release(head->context, head->pool, flags);
> +}
> +EXPORT_SYMBOL_GPL(objpool_fini);
> -- 
> 2.34.1
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>

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