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Date:   Wed, 18 Oct 2023 10:18:17 +0800
From:   Chengming Zhou <chengming.zhou@...ux.dev>
To:     "wuqiang.matt" <wuqiang.matt@...edance.com>,
        linux-trace-kernel@...r.kernel.org, mhiramat@...nel.org,
        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
Cc:     linux-kernel@...r.kernel.org, lkp@...el.com, mattwu@....com
Subject: Re: [PATCH v11 1/5] lib: objpool added: ring-array based lockless
 MPMC

On 2023/10/17 21:56, wuqiang.matt wrote:
> objpool is a scalable implementation of high performance queue for
> object allocation and reclamation, such as kretprobe instances.
> 
> With leveraging percpu ring-array to mitigate hot spots of memory
> contention, it delivers near-linear scalability for high parallel
> scenarios. The objpool is best suited for the 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 fixed after objpool creation
> 2) All pre-allocated objects are managed in percpu ring array,
>    which consumes more memory than linked lists
> 
I'm curious why not just extend the existing lockless freelist to
percpu lockless freelists? And the percpu freelist is more flexible
to use than this percpu ring-array? The latter has to be fixed size
when creation.

Thanks.

> Signed-off-by: wuqiang.matt <wuqiang.matt@...edance.com>
> ---
>  include/linux/objpool.h | 176 +++++++++++++++++++++++++
>  lib/Makefile            |   2 +-
>  lib/objpool.c           | 286 ++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 463 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..4df18405420a
> --- /dev/null
> +++ b/include/linux/objpool.h
> @@ -0,0 +1,181 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _LINUX_OBJPOOL_H
> +#define _LINUX_OBJPOOL_H
> +
> +#include <linux/types.h>
> +#include <linux/refcount.h>
> +
> +/*
> + * objpool: ring-array based lockless MPMC queue
> + *
> + * Copyright: wuqiang.matt@...edance.com,mhiramat@...nel.org
> + *
> + * objpool is a scalable implementation of high performance queue for
> + * object allocation and reclamation, such as kretprobe instances.
> + *
> + * With leveraging percpu ring-array to mitigate hot spots of memory
> + * contention, it delivers near-linear scalability for high parallel
> + * scenarios. The objpool is best suited for the 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 fixed after objpool creation
> + * 2) All pre-allocated objects are managed in percpu ring array,
> + *    which consumes more memory than linked lists
> + */
> +
> +/**
> + * struct objpool_slot - percpu ring array of objpool
> + * @head: head sequence of the local ring array (to retrieve at)
> + * @tail: tail sequence of the local ring array (to append at)
> + * @last: the last sequence number marked as ready for retrieve
> + * @mask: bits mask for modulo capacity to compute array indexes
> + * @entries: object entries on this slot
> + *
> + * Represents a cpu-local array-based ring buffer, its size is specialized
> + * during initialization of object pool. The percpu objpool node is to be
> + * allocated from local memory for NUMA system, and to be kept compact in
> + * continuous memory: CPU assigned number of objects are stored just after
> + * the body of objpool_node.
> + *
> + * Real size of the ring array is far too smaller than the value range of
> + * head and tail, typed as uint32_t: [0, 2^32), so only lower bits (mask)
> + * of head and tail are used as the actual position in the ring array. In
> + * general the ring array is acting like a small sliding window, which is
> + * always moving forward in the loop of [0, 2^32).
> + */
> +struct objpool_slot {
> +	uint32_t            head;
> +	uint32_t            tail;
> +	uint32_t            last;
> +	uint32_t            mask;
> +	void               *entries[];
> +} __packed;
> +
> +struct objpool_head;
> +
> +/*
> + * caller-specified callback for object initial setup, it's only called
> + * once for each object (just after the memory allocation of the object)
> + */
> +typedef int (*objpool_init_obj_cb)(void *obj, void *context);
> +
> +/* caller-specified cleanup callback for objpool destruction */
> +typedef int (*objpool_fini_cb)(struct objpool_head *head, void *context);
> +
> +/**
> + * struct objpool_head - object pooling metadata
> + * @obj_size:   object size, aligned to sizeof(void *)
> + * @nr_objs:    total objs (to be pre-allocated with objpool)
> + * @nr_cpus:    local copy of nr_cpu_ids
> + * @capacity:   max objs can be managed by one objpool_slot
> + * @gfp:        gfp flags for kmalloc & vmalloc
> + * @ref:        refcount of objpool
> + * @flags:      flags for objpool management
> + * @cpu_slots:  pointer to the array of objpool_slot
> + * @release:    resource cleanup callback
> + * @context:    caller-provided context
> + */
> +struct objpool_head {
> +	int                     obj_size;
> +	int                     nr_objs;
> +	int                     nr_cpus;
> +	int                     capacity;
> +	gfp_t                   gfp;
> +	refcount_t              ref;
> +	unsigned long           flags;
> +	struct objpool_slot   **cpu_slots;
> +	objpool_fini_cb         release;
> +	void                   *context;
> +};
> +
> +#define OBJPOOL_NR_OBJECT_MAX	(1UL << 24) /* maximum numbers of total objects */
> +#define OBJPOOL_OBJECT_SIZE_MAX	(1UL << 16) /* maximum size of an object */
> +
> +/**
> + * objpool_init() - initialize objpool and pre-allocated objects
> + * @pool:    the object pool to be initialized, declared by caller
> + * @nr_objs: total objects to be pre-allocated by this object pool
> + * @object_size: size of an object (should be > 0)
> + * @gfp:     flags for memory allocation (via kmalloc or vmalloc)
> + * @context: user context for object initialization callback
> + * @objinit: object initialization callback for extra setup
> + * @release: cleanup callback for extra cleanup task
> + *
> + * return value: 0 for success, otherwise error code
> + *
> + * All pre-allocated objects are to be zeroed after memory allocation.
> + * Caller could do extra initialization in objinit callback. objinit()
> + * will be called just after slot allocation and called only once for
> + * each object. After that the objpool won't touch any content of the
> + * objects. It's caller's duty to perform reinitialization after each
> + * pop (object allocation) or do clearance before each push (object
> + * reclamation).
> + */
> +int objpool_init(struct objpool_head *pool, int nr_objs, int object_size,
> +		 gfp_t gfp, void *context, objpool_init_obj_cb objinit,
> +		 objpool_fini_cb release);
> +
> +/**
> + * objpool_pop() - allocate an object from objpool
> + * @pool: object pool
> + *
> + * return value: object ptr or NULL if failed
> + */
> +void *objpool_pop(struct objpool_head *pool);
> +
> +/**
> + * objpool_push() - reclaim the object and return back to objpool
> + * @obj:  object ptr to be pushed to objpool
> + * @pool: object pool
> + *
> + * return: 0 or error code (it fails only when user tries to push
> + * the same object multiple times or wrong "objects" into objpool)
> + */
> +int objpool_push(void *obj, struct objpool_head *pool);
> +
> +/**
> + * objpool_drop() - discard the object and deref objpool
> + * @obj:  object ptr to be discarded
> + * @pool: object pool
> + *
> + * return: 0 if objpool was released; -EAGAIN if there are still
> + *         outstanding objects
> + *
> + * objpool_drop is normally for the release of outstanding objects
> + * after objpool cleanup (objpool_fini). Thinking of this example:
> + * kretprobe is unregistered and objpool_fini() is called to release
> + * all remained objects, but there are still objects being used by
> + * unfinished kretprobes (like blockable function: sys_accept). So
> + * only when the last outstanding object is dropped could the whole
> + * objpool be released along with the call of objpool_drop()
> + */
> +int objpool_drop(void *obj, struct objpool_head *pool);
> +
> +/**
> + * objpool_free() - release objpool forcely (all objects to be freed)
> + * @pool: object pool to be released
> + */
> +void objpool_free(struct objpool_head *pool);
> +
> +/**
> + * objpool_fini() - deref object pool (also releasing unused objects)
> + * @pool: object pool to be dereferenced
> + *
> + * objpool_fini() will try to release all remained free objects and
> + * then drop an extra reference of the objpool. If all objects are
> + * already returned to objpool (so called synchronous use cases),
> + * the objpool itself will be freed together. But if there are still
> + * outstanding objects (so called asynchronous use cases, such like
> + * blockable kretprobe), the objpool won't be released until all
> + * the outstanding objects are dropped, but the caller must assure
> + * there are no concurrent objpool_push() on the fly. Normally RCU
> + * is being required to make sure all ongoing objpool_push() must
> + * be finished before calling objpool_fini(), so does test_objpool,
> + * kretprobe or rethook
> + */
> +void objpool_fini(struct objpool_head *pool);
> +
> +#endif /* _LINUX_OBJPOOL_H */
> diff --git a/lib/Makefile b/lib/Makefile
> index 1ffae65bb7ee..7a84c922d9ff 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..37a71e063f18
> --- /dev/null
> +++ b/lib/objpool.c
> @@ -0,0 +1,280 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/objpool.h>
> +#include <linux/slab.h>
> +#include <linux/vmalloc.h>
> +#include <linux/atomic.h>
> +#include <linux/irqflags.h>
> +#include <linux/cpumask.h>
> +#include <linux/log2.h>
> +
> +/*
> + * objpool: ring-array based lockless MPMC/FIFO queues
> + *
> + * Copyright: wuqiang.matt@...edance.com,mhiramat@...nel.org
> + */
> +
> +/* initialize percpu objpool_slot */
> +static int
> +objpool_init_percpu_slot(struct objpool_head *pool,
> +			 struct objpool_slot *slot,
> +			 int nodes, void *context,
> +			 objpool_init_obj_cb objinit)
> +{
> +	void *obj = (void *)&slot->entries[pool->capacity];
> +	int i;
> +
> +	/* initialize elements of percpu objpool_slot */
> +	slot->mask = pool->capacity - 1;
> +
> +	for (i = 0; i < nodes; i++) {
> +		if (objinit) {
> +			int rc = objinit(obj, context);
> +			if (rc)
> +				return rc;
> +		}
> +		slot->entries[slot->tail & slot->mask] = obj;
> +		obj = obj + pool->obj_size;
> +		slot->tail++;
> +		slot->last = slot->tail;
> +		pool->nr_objs++;
> +	}
> +
> +	return 0;
> +}
> +
> +/* allocate and initialize percpu slots */
> +static int
> +objpool_init_percpu_slots(struct objpool_head *pool, int nr_objs,
> +			  void *context, objpool_init_obj_cb objinit)
> +{
> +	int i, cpu_count = 0;
> +
> +	for (i = 0; i < pool->nr_cpus; i++) {
> +
> +		struct objpool_slot *slot;
> +		int nodes, size, rc;
> +
> +		/* skip the cpu node which could never be present */
> +		if (!cpu_possible(i))
> +			continue;
> +
> +		/* compute how many objects to be allocated with this slot */
> +		nodes = nr_objs / num_possible_cpus();
> +		if (cpu_count < (nr_objs % num_possible_cpus()))
> +			nodes++;
> +		cpu_count++;
> +
> +		size = struct_size(slot, entries, pool->capacity) +
> +			pool->obj_size * nodes;
> +
> +		/*
> +		 * here we allocate percpu-slot & objs together in a single
> +		 * allocation to make it more compact, taking advantage of
> +		 * warm caches and TLB hits. in default vmalloc is used to
> +		 * reduce the pressure of kernel slab system. as we know,
> +		 * mimimal size of vmalloc is one page since vmalloc would
> +		 * always align the requested size to page size
> +		 */
> +		if (pool->gfp & GFP_ATOMIC)
> +			slot = kmalloc_node(size, pool->gfp, cpu_to_node(i));
> +		else
> +			slot = __vmalloc_node(size, sizeof(void *), pool->gfp,
> +				cpu_to_node(i), __builtin_return_address(0));
> +		if (!slot)
> +			return -ENOMEM;
> +		memset(slot, 0, size);
> +		pool->cpu_slots[i] = slot;
> +
> +		/* initialize the objpool_slot of cpu node i */
> +		rc = objpool_init_percpu_slot(pool, slot, nodes, context, objinit);
> +		if (rc)
> +			return rc;
> +	}
> +
> +	return 0;
> +}
> +
> +/* cleanup all percpu slots of the object pool */
> +static void objpool_fini_percpu_slots(struct objpool_head *pool)
> +{
> +	int i;
> +
> +	if (!pool->cpu_slots)
> +		return;
> +
> +	for (i = 0; i < pool->nr_cpus; i++)
> +		kvfree(pool->cpu_slots[i]);
> +	kfree(pool->cpu_slots);
> +}
> +
> +/* initialize object pool and pre-allocate objects */
> +int objpool_init(struct objpool_head *pool, int nr_objs, int object_size,
> +		gfp_t gfp, void *context, objpool_init_obj_cb objinit,
> +		objpool_fini_cb release)
> +{
> +	int rc, capacity, slot_size;
> +
> +	/* check input parameters */
> +	if (nr_objs <= 0 || nr_objs > OBJPOOL_NR_OBJECT_MAX ||
> +	    object_size <= 0 || object_size > OBJPOOL_OBJECT_SIZE_MAX)
> +		return -EINVAL;
> +
> +	/* align up to unsigned long size */
> +	object_size = ALIGN(object_size, sizeof(long));
> +
> +	/* calculate capacity of percpu objpool_slot */
> +	capacity = roundup_pow_of_two(nr_objs);
> +	if (!capacity)
> +		return -EINVAL;
> +
> +	/* initialize objpool pool */
> +	memset(pool, 0, sizeof(struct objpool_head));
> +	pool->nr_cpus = nr_cpu_ids;
> +	pool->obj_size = object_size;
> +	pool->capacity = capacity;
> +	pool->gfp = gfp & ~__GFP_ZERO;
> +	pool->context = context;
> +	pool->release = release;
> +	slot_size = pool->nr_cpus * sizeof(struct objpool_slot);
> +	pool->cpu_slots = kzalloc(slot_size, pool->gfp);
> +	if (!pool->cpu_slots)
> +		return -ENOMEM;
> +
> +	/* initialize per-cpu slots */
> +	rc = objpool_init_percpu_slots(pool, nr_objs, context, objinit);
> +	if (rc)
> +		objpool_fini_percpu_slots(pool);
> +	else
> +		refcount_set(&pool->ref, pool->nr_objs + 1);
> +
> +	return rc;
> +}
> +EXPORT_SYMBOL_GPL(objpool_init);
> +
> +/* adding object to slot, abort if the slot was already full */
> +static inline int
> +objpool_try_add_slot(void *obj, struct objpool_head *pool, int cpu)
> +{
> +	struct objpool_slot *slot = pool->cpu_slots[cpu];
> +	uint32_t head, tail;
> +
> +	/* loading tail and head as a local snapshot, tail first */
> +	tail = READ_ONCE(slot->tail);
> +
> +	do {
> +		head = READ_ONCE(slot->head);
> +		/* fault caught: something must be wrong */
> +		WARN_ON_ONCE(tail - head > pool->nr_objs);
> +	} while (!try_cmpxchg_acquire(&slot->tail, &tail, tail + 1));
> +
> +	/* now the tail position is reserved for the given obj */
> +	WRITE_ONCE(slot->entries[tail & slot->mask], obj);
> +	/* update sequence to make this obj available for pop() */
> +	smp_store_release(&slot->last, tail + 1);
> +
> +	return 0;
> +}
> +
> +/* reclaim an object to object pool */
> +int objpool_push(void *obj, struct objpool_head *pool)
> +{
> +	unsigned long flags;
> +	int rc;
> +
> +	/* disable local irq to avoid preemption & interruption */
> +	raw_local_irq_save(flags);
> +	rc = objpool_try_add_slot(obj, pool, raw_smp_processor_id());
> +	raw_local_irq_restore(flags);
> +
> +	return rc;
> +}
> +EXPORT_SYMBOL_GPL(objpool_push);
> +
> +/* try to retrieve object from slot */
> +static inline void *objpool_try_get_slot(struct objpool_head *pool, int cpu)
> +{
> +	struct objpool_slot *slot = pool->cpu_slots[cpu];
> +	/* load head snapshot, other cpus may change it */
> +	uint32_t head = smp_load_acquire(&slot->head);
> +
> +	while (head != READ_ONCE(slot->last)) {
> +		void *obj;
> +
> +		/* obj must be retrieved before moving forward head */
> +		obj = READ_ONCE(slot->entries[head & slot->mask]);
> +
> +		/* move head forward to mark it's consumption */
> +		if (try_cmpxchg_release(&slot->head, &head, head + 1))
> +			return obj;
> +	}
> +
> +	return NULL;
> +}
> +
> +/* allocate an object from object pool */
> +void *objpool_pop(struct objpool_head *pool)
> +{
> +	void *obj = NULL;
> +	unsigned long flags;
> +	int i, cpu;
> +
> +	/* disable local irq to avoid preemption & interruption */
> +	raw_local_irq_save(flags);
> +
> +	cpu = raw_smp_processor_id();
> +	for (i = 0; i < num_possible_cpus(); i++) {
> +		obj = objpool_try_get_slot(pool, cpu);
> +		if (obj)
> +			break;
> +		cpu = cpumask_next_wrap(cpu, cpu_possible_mask, -1, 1);
> +	}
> +	raw_local_irq_restore(flags);
> +
> +	return obj;
> +}
> +EXPORT_SYMBOL_GPL(objpool_pop);
> +
> +/* release whole objpool forcely */
> +void objpool_free(struct objpool_head *pool)
> +{
> +	if (!pool->cpu_slots)
> +		return;
> +
> +	/* release percpu slots */
> +	objpool_fini_percpu_slots(pool);
> +
> +	/* call user's cleanup callback if provided */
> +	if (pool->release)
> +		pool->release(pool, pool->context);
> +}
> +EXPORT_SYMBOL_GPL(objpool_free);
> +
> +/* drop the allocated object, rather reclaim it to objpool */
> +int objpool_drop(void *obj, struct objpool_head *pool)
> +{
> +	if (!obj || !pool)
> +		return -EINVAL;
> +
> +	if (refcount_dec_and_test(&pool->ref)) {
> +		objpool_free(pool);
> +		return 0;
> +	}
> +
> +	return -EAGAIN;
> +}
> +EXPORT_SYMBOL_GPL(objpool_drop);
> +
> +/* drop unused objects and defref objpool for releasing */
> +void objpool_fini(struct objpool_head *pool)
> +{
> +	int count = 1; /* extra ref for objpool itself */
> +
> +	/* drop all remained objects from objpool */
> +	while (objpool_pop(pool))
> +		count++;
> +
> +	if (refcount_sub_and_test(count, &pool->ref))
> +		objpool_free(pool);
> +}
> +EXPORT_SYMBOL_GPL(objpool_fini);

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