Re: [PATCH v9 1/5] lib: objpool added: ring-array based lockless MPMC

[Google]

Hi Wuqiang,

On Mon, 9 Oct 2023 17:23:34 +0800
wuqiang <wuqiang.matt@xxxxxxxxxxxxx> wrote:

> Hello Masami,
> 
> Just got time for the new patch and got that ages[] was removed. ages[] is
> introduced the way like 2-phase commit to keep consitency and must be kept.
> 
> Thinking of the following 2 cases that two cpu nodes are operating the same
> objpool_slot simultaneously:
> 
> Case 1:
> 
>    NODE 1:                  NODE 2:
>    push to an empty slot    pop will get wrong value
> 
>    try_cmpxchg_acquire(&slot->tail, &tail, next)
>                             try_cmpxchg_release(&slot->head, &head, head + 1)
>                             return slot->entries[head & slot->mask]
>    WRITE_ONCE(slot->entries[tail & slot->mask], obj)

Today, I rethink the algorithm.

For this case, we can just add a `commit` to the slot for committing the tail
commit instead of the ages[]. 

CPU1                                       CPU2
push to an empty slot                      pop from the same slot

commit = tail = slot->tail;
next = tail + 1;
try_cmpxchg_acquire(&slot->tail,
                    &tail, next);
                                           while (head != commit) -> NG1
WRITE_ONCE(slot->entries[tail & slot->mask],
            obj)
                                           while (head != commit) -> NG2
WRITE_ONCE(&slot->commit, next);
                                           while (head != commit) -> OK
                                           try_cmpxchg_acquire(&slot->head,
                                                               &head, next);
                                           return slot->entries[head & slot->mask]

So the NG1 and NG2 timing, the pop will fail.

This does not expect the nested "push" case because the reserve-commit block
will be interrupt disabled. This doesn't support NMI but that is OK.

If we need to support such NMI or remove irq-disable, we can do following
(please remember the push operation only happens on the slot owned by that
 CPU. So this is per-cpu process)

---
do {
  commit = tail = slot->tail;
  next = tail + 1;
} while (!try_cmpxchg_acquire(&slot->tail, &tail, next));

WRITE_ONCE(slot->entries[tail & slot->mask], obj);

// At this point, "commit == slot->tail - 1" in this nest level.
// Only outmost (non-nexted) case, "commit == slot->commit"
if (commit != slot->commit)
  return; // do nothing. it must be updated by outmost one.

// catch up commit to tail.
do {
  commit = READ_ONCE(slot->tail);
  WRITE_ONCE(slot->commit, commit);
   // note that someone can interrupt this loop too.
} while (commit != READ_ONCE(slot->tail));
---

For the rethook this may be too much.

Thank you,

> 
> 
> Case 2:
> 
>    NODE 1:                  NODE 2
>    push to slot w/ 1 obj    pop will get wrong value
> 
>                             try_cmpxchg_release(&slot->head, &head, head + 1)
>    try_cmpxchg_acquire(&slot->tail, &tail, next)
>    WRITE_ONCE(slot->entries[tail & slot->mask], obj)
>                             return slot->entries[head & slot->mask]
> 
> 
> Regards,
> wuqiang
> 
> On 2023/9/25 17:42, Masami Hiramatsu (Google) wrote:
> > Hi Wuqiang,
> > 
> > On Tue,  5 Sep 2023 09:52:51 +0800
> > "wuqiang.matt" <wuqiang.matt@xxxxxxxxxxxxx> wrote:
> > 
> >> The object pool is a scalable implementaion of high performance queue
> >> for object allocation and reclamation, such as kretprobe instances.
> >>
> >> With leveraging percpu ring-array to mitigate the hot spot of memory
> >> contention, it could deliver near-linear scalability for high parallel
> >> scenarios. The objpool is 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
> >>     and can't be modified (extended) after objpool creation
> >> 2) The memory of objects won't be freed until objpool is finalized
> >> 3) Object allocation (pop) may fail after trying all cpu slots
> > 
> > I made a simplifying patch on this by (mainly) removing ages array.
> > I also rename local variable to use more readable names, like slot,
> > pool, and obj.
> > 
> > Here the results which I run the test_objpool.ko.
> > 
> > Original:
> > [   50.500235] Summary of testcases:
> > [   50.503139]     duration: 1027135us 	hits:   30628293 	miss:          0 	sync: percpu objpool
> > [   50.510416]     duration: 1047977us 	hits:   30453023 	miss:          0 	sync: percpu objpool from vmalloc
> > [   50.517421]     duration: 1047098us 	hits:   31145034 	miss:          0 	sync & hrtimer: percpu objpool
> > [   50.524671]     duration: 1053233us 	hits:   30919640 	miss:          0 	sync & hrtimer: percpu objpool from vmalloc
> > [   50.531382]     duration: 1055822us 	hits:    3407221 	miss:     830219 	sync overrun: percpu objpool
> > [   50.538135]     duration: 1055998us 	hits:    3404624 	miss:     854160 	sync overrun: percpu objpool from vmalloc
> > [   50.546686]     duration: 1046104us 	hits:   19464798 	miss:          0 	async: percpu objpool
> > [   50.552989]     duration: 1033054us 	hits:   18957836 	miss:          0 	async: percpu objpool from vmalloc
> > [   50.560289]     duration: 1041778us 	hits:   33806868 	miss:          0 	async & hrtimer: percpu objpool
> > [   50.567425]     duration: 1048901us 	hits:   34211860 	miss:          0 	async & hrtimer: percpu objpool from vmalloc
> > 
> > Simplified:
> > [   48.393236] Summary of testcases:
> > [   48.395384]     duration: 1013002us 	hits:   29661448 	miss:          0 	sync: percpu objpool
> > [   48.400351]     duration: 1057231us 	hits:   31035578 	miss:          0 	sync: percpu objpool from vmalloc
> > [   48.405660]     duration: 1043196us 	hits:   30546652 	miss:          0 	sync & hrtimer: percpu objpool
> > [   48.411216]     duration: 1047128us 	hits:   30601306 	miss:          0 	sync & hrtimer: percpu objpool from vmalloc
> > [   48.417231]     duration: 1051695us 	hits:    3468287 	miss:     892881 	sync overrun: percpu objpool
> > [   48.422405]     duration: 1054003us 	hits:    3549491 	miss:     898141 	sync overrun: percpu objpool from vmalloc
> > [   48.428425]     duration: 1052946us 	hits:   19005228 	miss:          0 	async: percpu objpool
> > [   48.433597]     duration: 1051757us 	hits:   19670866 	miss:          0 	async: percpu objpool from vmalloc
> > [   48.439280]     duration: 1042951us 	hits:   37135332 	miss:          0 	async & hrtimer: percpu objpool
> > [   48.445085]     duration: 1029803us 	hits:   37093248 	miss:          0 	async & hrtimer: percpu objpool from vmalloc
> > 
> > Can you test it too?
> > 
> > Thanks,
> > 
> >  From f1f442ff653e329839e5452b8b88463a80a12ff3 Mon Sep 17 00:00:00 2001
> > From: "Masami Hiramatsu (Google)" <mhiramat@xxxxxxxxxx>
> > Date: Mon, 25 Sep 2023 16:07:12 +0900
> > Subject: [PATCH] objpool: Simplify objpool by removing ages array
> > 
> > Simplify the objpool code by removing ages array from per-cpu slot.
> > It chooses enough big capacity (which is a rounded up power of 2 value
> > of nr_objects + 1) for the entries array, the tail never catch up to
> > the head in per-cpu slot. Thus tail == head means the slot is empty.
> > 
> > This also uses consistent local variable names for pool, slot and obj.
> > 
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> > ---
> >   include/linux/objpool.h |  61 ++++----
> >   lib/objpool.c           | 310 ++++++++++++++++------------------------
> >   2 files changed, 147 insertions(+), 224 deletions(-)
> > 
> > diff --git a/include/linux/objpool.h b/include/linux/objpool.h
> > index 33c832216b98..ecd5ecaffcd3 100644
> > --- a/include/linux/objpool.h
> > +++ b/include/linux/objpool.h
> > @@ -38,33 +38,23 @@
> >    * struct objpool_slot - percpu ring array of objpool
> >    * @head: head of the local ring array (to retrieve at)
> >    * @tail: tail of the local ring array (to append at)
> > - * @bits: log2 of capacity (for bitwise operations)
> > - * @mask: capacity - 1
> > + * @mask: capacity of entries - 1
> > + * @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 slot is to be
> >    * allocated from local memory for NUMA system, and to be kept compact in
> > - * continuous memory: ages[] is stored just after the body of objpool_slot,
> > - * and then entries[]. ages[] describes revision of each item, solely used
> > - * to avoid ABA; entries[] contains pointers of the actual objects
> > - *
> > - * Layout of objpool_slot in memory:
> > - *
> > - * 64bit:
> > - *        4      8      12     16        32                 64
> > - * | head | tail | bits | mask | ages[4] | ents[4]: (8 * 4) | objects
> > - *
> > - * 32bit:
> > - *        4      8      12     16        32        48       64
> > - * | head | tail | bits | mask | ages[4] | ents[4] | unused | objects
> > + * continuous memory: CPU assigned number of objects are stored just after
> > + * the body of objpool_slot.
> >    *
> >    */
> >   struct objpool_slot {
> > -	uint32_t                head;
> > -	uint32_t                tail;
> > -	uint32_t                bits;
> > -	uint32_t                mask;
> > -} __packed;
> > +	uint32_t	head;
> > +	uint32_t	tail;
> > +	uint32_t	mask;
> > +	uint32_t	dummyr;
> > +	void *		entries[];
> > +};
> >   
> >   struct objpool_head;
> >   
> > @@ -82,12 +72,11 @@ typedef int (*objpool_fini_cb)(struct objpool_head *head, void *context);
> >    * @obj_size:   object & element size
> >    * @nr_objs:    total objs (to be pre-allocated)
> >    * @nr_cpus:    nr_cpu_ids
> > - * @capacity:   max objects per cpuslot
> > + * @capacity:   max objects on percpu slot
> >    * @gfp:        gfp flags for kmalloc & vmalloc
> >    * @ref:        refcount for objpool
> >    * @flags:      flags for objpool management
> >    * @cpu_slots:  array of percpu slots
> > - * @slot_sizes:	size in bytes of slots
> >    * @release:    resource cleanup callback
> >    * @context:    caller-provided context
> >    */
> > @@ -100,7 +89,6 @@ struct objpool_head {
> >   	refcount_t              ref;
> >   	unsigned long           flags;
> >   	struct objpool_slot   **cpu_slots;
> > -	int                    *slot_sizes;
> >   	objpool_fini_cb         release;
> >   	void                   *context;
> >   };
> > @@ -108,9 +96,12 @@ struct objpool_head {
> >   #define OBJPOOL_FROM_VMALLOC	(0x800000000)	/* objpool allocated from vmalloc area */
> >   #define OBJPOOL_HAVE_OBJECTS	(0x400000000)	/* objects allocated along with objpool */
> >   
> > +#define OBJPOOL_NR_OBJECT_MAX	(1 << 24)
> > +#define OBJPOOL_OBJECT_SIZE_MAX	(1 << 16)
> > +
> >   /**
> >    * objpool_init() - initialize objpool and pre-allocated objects
> > - * @head:    the object pool to be initialized, declared by caller
> > + * @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)
> > @@ -128,47 +119,47 @@ struct objpool_head {
> >    * pop (object allocation) or do clearance before each push (object
> >    * reclamation).
> >    */
> > -int objpool_init(struct objpool_head *head, int nr_objs, int object_size,
> > +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
> > - * @head: object pool
> > + * @pool: object pool
> >    *
> >    * return value: object ptr or NULL if failed
> >    */
> > -void *objpool_pop(struct objpool_head *head);
> > +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
> > - * @head: object pool
> > + * @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 *head);
> > +int objpool_push(void *obj, struct objpool_head *pool);
> >   
> >   /**
> >    * objpool_drop() - discard the object and deref objpool
> >    * @obj:  object ptr to be discarded
> > - * @head: object pool
> > + * @pool: object pool
> >    *
> >    * return: 0 if objpool was released or error code
> >    */
> > -int objpool_drop(void *obj, struct objpool_head *head);
> > +int objpool_drop(void *obj, struct objpool_head *pool);
> >   
> >   /**
> >    * objpool_free() - release objpool forcely (all objects to be freed)
> > - * @head: object pool to be released
> > + * @pool: object pool to be released
> >    */
> > -void objpool_free(struct objpool_head *head);
> > +void objpool_free(struct objpool_head *pool);
> >   
> >   /**
> >    * objpool_fini() - deref object pool (also releasing unused objects)
> > - * @head: object pool to be dereferenced
> > + * @pool: object pool to be dereferenced
> >    */
> > -void objpool_fini(struct objpool_head *head);
> > +void objpool_fini(struct objpool_head *pool);
> >   
> >   #endif /* _LINUX_OBJPOOL_H */
> > diff --git a/lib/objpool.c b/lib/objpool.c
> > index 22e752371820..f8e8f70d7253 100644
> > --- a/lib/objpool.c
> > +++ b/lib/objpool.c
> > @@ -15,104 +15,55 @@
> >    * Copyright: wuqiang.matt@xxxxxxxxxxxxx
> >    */
> >   
> > -#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)->bits)))
> > -#define SLOT_OBJS(s) ((void *)((char *)(s) + sizeof(struct objpool_slot) + \
> > -			((sizeof(uint32_t) + sizeof(void *)) << (s)->bits)))
> > -#define SLOT_CORE(n) cpumask_nth((n) % num_possible_cpus(), cpu_possible_mask)
> > -
> > -/* compute the suitable num of objects to be managed per slot */
> > -static int objpool_nobjs(int size)
> > -{
> > -	return rounddown_pow_of_two((size - sizeof(struct objpool_slot)) /
> > -			(sizeof(uint32_t) + sizeof(void *)));
> > -}
> > -
> >   /* allocate and initialize percpu slots */
> >   static int
> > -objpool_init_percpu_slots(struct objpool_head *head, int nobjs,
> > -			void *context, objpool_init_obj_cb objinit)
> > +objpool_init_percpu_slots(struct objpool_head *pool, void *context,
> > +			  objpool_init_obj_cb objinit)
> >   {
> > -	int i, j, n, size, objsz, cpu = 0, nents = head->capacity;
> > -
> > -	/* aligned object size by sizeof(void *) */
> > -	objsz = ALIGN(head->obj_size, sizeof(void *));
> > -	/* shall we allocate objects along with percpu-slot */
> > -	if (objsz)
> > -		head->flags |= OBJPOOL_HAVE_OBJECTS;
> > -
> > -	/* vmalloc is used in default to allocate percpu-slots */
> > -	if (!(head->gfp & GFP_ATOMIC))
> > -		head->flags |= OBJPOOL_FROM_VMALLOC;
> > -
> > -	for (i = 0; i < head->nr_cpus; i++) {
> > -		struct objpool_slot *os;
> > +	int i, j, n, size, slot_size, cpu_count = 0;
> > +	struct objpool_slot *slot;
> >   
> > +	for (i = 0; i < pool->nr_cpus; i++) {
> >   		/* skip the cpus which could never be present */
> >   		if (!cpu_possible(i))
> >   			continue;
> >   
> >   		/* compute how many objects to be managed by this slot */
> > -		n = nobjs / num_possible_cpus();
> > -		if (cpu < (nobjs % num_possible_cpus()))
> > +		n = pool->nr_objs / num_possible_cpus();
> > +		if (cpu_count < (pool->nr_objs % num_possible_cpus()))
> >   			n++;
> > -		size = sizeof(struct objpool_slot) + sizeof(void *) * nents +
> > -		       sizeof(uint32_t) * nents + objsz * n;
> > +		cpu_count++;
> > +
> > +		slot_size = struct_size(slot, entries, pool->capacity);
> > +		size = slot_size + pool->obj_size * n;
> >   
> >   		/*
> >   		 * here we allocate percpu-slot & objects together in a single
> > -		 * allocation, taking advantage of warm caches and TLB hits as
> > -		 * vmalloc always aligns the request size to pages
> > +		 * allocation, taking advantage on NUMA.
> >   		 */
> > -		if (head->flags & OBJPOOL_FROM_VMALLOC)
> > -			os = __vmalloc_node(size, sizeof(void *), head->gfp,
> > +		if (pool->flags & OBJPOOL_FROM_VMALLOC)
> > +			slot = __vmalloc_node(size, sizeof(void *), pool->gfp,
> >   				cpu_to_node(i), __builtin_return_address(0));
> >   		else
> > -			os = kmalloc_node(size, head->gfp, cpu_to_node(i));
> > -		if (!os)
> > +			slot = kmalloc_node(size, pool->gfp, cpu_to_node(i));
> > +		if (!slot)
> >   			return -ENOMEM;
> >   
> >   		/* initialize percpu slot for the i-th slot */
> > -		memset(os, 0, size);
> > -		os->bits = ilog2(head->capacity);
> > -		os->mask = head->capacity - 1;
> > -		head->cpu_slots[i] = os;
> > -		head->slot_sizes[i] = size;
> > -		cpu = cpu + 1;
> > -
> > -		/*
> > -		 * manually set head & tail to avoid possible conflict:
> > -		 * 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 the reality
> > -		 * 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;
> > +		memset(slot, 0, size);
> > +		slot->mask = pool->capacity - 1;
> > +		pool->cpu_slots[i] = slot;
> >   
> >   		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;
> > +			void *obj = (void *)slot + slot_size + pool->obj_size * j;
> >   
> > -			/* perform object initialization */
> >   			if (objinit) {
> >   				int rc = objinit(obj, context);
> >   				if (rc)
> >   					return rc;
> >   			}
> > -
> > -			/* add obj into the ring array */
> > -			ents[ie] = obj;
> > -			ages[ie] = os->tail;
> > -			os->tail++;
> > -			head->nr_objs++;
> > +			slot->entries[j] = obj;
> > +			slot->tail++;
> >   		}
> >   	}
> >   
> > @@ -120,164 +71,135 @@ objpool_init_percpu_slots(struct objpool_head *head, int nobjs,
> >   }
> >   
> >   /* cleanup all percpu slots of the object pool */
> > -static void objpool_fini_percpu_slots(struct objpool_head *head)
> > +static void objpool_fini_percpu_slots(struct objpool_head *pool)
> >   {
> >   	int i;
> >   
> > -	if (!head->cpu_slots)
> > +	if (!pool->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;
> > +	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 *head, int nr_objs, int object_size,
> > +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 nents, rc;
> >   
> >   	/* check input parameters */
> > -	if (nr_objs <= 0 || object_size <= 0)
> > +	if (nr_objs <= 0 || object_size <= 0 ||
> > +	    nr_objs > OBJPOOL_NR_OBJECT_MAX || object_size > OBJPOOL_OBJECT_SIZE_MAX)
> > +		return -EINVAL;
> > +
> > +	/* Align up to unsigned long size */
> > +	object_size = ALIGN(object_size, sizeof(unsigned long));
> > +
> > +	/*
> > +	 * To avoid filling up the entries array in the per-cpu slot,
> > +	 * use the power of 2 which is more than N + 1. Thus, the tail never
> > +	 * catch up to the pool, and able to use pool/tail as the sequencial
> > +	 * number.
> > +	 */
> > +	nents = roundup_pow_of_two(nr_objs + 1);
> > +	if (!nents)
> >   		return -EINVAL;
> >   
> > -	/* calculate percpu slot size (rounded to pow of 2) */
> > -	nents = max_t(int, roundup_pow_of_two(nr_objs),
> > -			objpool_nobjs(L1_CACHE_BYTES));
> > -
> > -	/* initialize objpool head */
> > -	memset(head, 0, sizeof(struct objpool_head));
> > -	head->nr_cpus = nr_cpu_ids;
> > -	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(int), head->gfp);
> > -	if (!head->cpu_slots)
> > +	/* initialize objpool pool */
> > +	memset(pool, 0, sizeof(struct objpool_head));
> > +	pool->nr_cpus = nr_cpu_ids;
> > +	pool->obj_size = object_size;
> > +	pool->nr_objs = nr_objs;
> > +	/* just prevent to fullfill the per-cpu ring array */
> > +	pool->capacity = nents;
> > +	pool->gfp = gfp & ~__GFP_ZERO;
> > +	pool->context = context;
> > +	pool->release = release;
> > +	/* vmalloc is used in default to allocate percpu-slots */
> > +	if (!(pool->gfp & GFP_ATOMIC))
> > +		pool->flags |= OBJPOOL_FROM_VMALLOC;
> > +
> > +	pool->cpu_slots = kzalloc(pool->nr_cpus * sizeof(void *), pool->gfp);
> > +	if (!pool->cpu_slots)
> >   		return -ENOMEM;
> > -	head->slot_sizes = (int *)&head->cpu_slots[head->nr_cpus];
> >   
> >   	/* initialize per-cpu slots */
> > -	rc = objpool_init_percpu_slots(head, nr_objs, context, objinit);
> > +	rc = objpool_init_percpu_slots(pool, context, objinit);
> >   	if (rc)
> > -		objpool_fini_percpu_slots(head);
> > +		objpool_fini_percpu_slots(pool);
> >   	else
> > -		refcount_set(&head->ref, nr_objs + 1);
> > +		refcount_set(&pool->ref, 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_slot *os)
> > +static inline int objpool_try_add_slot(void *obj, struct objpool_head *pool, int cpu)
> >   {
> > -	uint32_t *ages = SLOT_AGES(os);
> > -	void **ents = SLOT_ENTS(os);
> > -	uint32_t head, tail;
> > +	struct objpool_slot *slot = pool->cpu_slots[cpu];
> > +	uint32_t tail, next;
> >   
> >   	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->mask)
> > -			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);
> > +		uint32_t head = READ_ONCE(slot->head);
> >   
> > -	/* 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);
> > +		tail = READ_ONCE(slot->tail);
> > +		next = tail + 1;
> > +
> > +		/* This must never happen because capacity >= N + 1 */
> > +		if (WARN_ON_ONCE((next > head && next - head > pool->nr_objs) ||
> > +				 (next < head && next > head + pool->nr_objs)))
> > +			return -EINVAL;
> > +
> > +	} while (!try_cmpxchg_acquire(&slot->tail, &tail, next));
> > +
> > +	WRITE_ONCE(slot->entries[tail & slot->mask], obj);
> >   	return 0;
> >   }
> >   
> >   /* reclaim an object to object pool */
> > -int objpool_push(void *obj, struct objpool_head *oh)
> > +int objpool_push(void *obj, struct objpool_head *pool)
> >   {
> >   	unsigned long flags;
> > -	int cpu, rc;
> > +	int rc;
> >   
> >   	/* disable local irq to avoid preemption & interruption */
> >   	raw_local_irq_save(flags);
> > -	cpu = raw_smp_processor_id();
> > -	do {
> > -		rc = objpool_try_add_slot(obj, oh->cpu_slots[cpu]);
> > -		if (!rc)
> > -			break;
> > -		cpu = cpumask_next_wrap(cpu, cpu_possible_mask, -1, 1);
> > -	} while (1);
> > +	rc = objpool_try_add_slot(obj, pool, raw_smp_processor_id());
> >   	raw_local_irq_restore(flags);
> >   
> >   	return rc;
> >   }
> >   EXPORT_SYMBOL_GPL(objpool_push);
> >   
> > -/* drop the allocated object, rather reclaim it to objpool */
> > -int objpool_drop(void *obj, struct objpool_head *head)
> > -{
> > -	if (!obj || !head)
> > -		return -EINVAL;
> > -
> > -	if (refcount_dec_and_test(&head->ref)) {
> > -		objpool_free(head);
> > -		return 0;
> > -	}
> > -
> > -	return -EAGAIN;
> > -}
> > -EXPORT_SYMBOL_GPL(objpool_drop);
> > -
> >   /* try to retrieve object from slot */
> > -static inline void *objpool_try_get_slot(struct objpool_slot *os)
> > +static inline void *objpool_try_get_slot(struct objpool_slot *slot)
> >   {
> > -	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);
> > +	uint32_t head = smp_load_acquire(&slot->head);
> >   
> >   	/* loop if slot isn't empty */
> > -	while (head != READ_ONCE(os->tail)) {
> > -		uint32_t id = head & os->mask, prev = head;
> > +	while (head != READ_ONCE(slot->tail)) {
> >   
> >   		/* do prefetching of object ents */
> > -		prefetch(&ents[id]);
> > -
> > -		/* check whether this item was ready for retrieval */
> > -		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;
> > -			/* head was already updated by others */
> > -		}
> > +		prefetch(&slot->entries[head & slot->mask]);
> > +
> > +		/* commit and move forward head of the slot */
> > +		if (try_cmpxchg_release(&slot->head, &head, head + 1))
> > +			/*
> > +			 * TBD: check overwrap the tail/head counter and warn
> > +			 * if it is broken. But this happens only if this
> > +			 * process slows down a lot and another CPU updates
> > +			 * the haed/tail just 2^32 + 1 times, and this slot
> > +			 * is empty.
> > +			 */
> > +			return slot->entries[head & slot->mask];
> >   
> >   		/* re-load head from memory and continue trying */
> > -		head = READ_ONCE(os->head);
> > -		/*
> > -		 * head stays unchanged, so it's very likely there's an
> > -		 * ongoing push() on other cpu nodes but yet not update
> > -		 * ages[] to mark it's completion
> > -		 */
> > -		if (head == prev)
> > -			break;
> > +		head = READ_ONCE(slot->head);
> >   	}
> >   
> >   	return NULL;
> > @@ -307,32 +229,42 @@ void *objpool_pop(struct objpool_head *head)
> >   EXPORT_SYMBOL_GPL(objpool_pop);
> >   
> >   /* release whole objpool forcely */
> > -void objpool_free(struct objpool_head *head)
> > +void objpool_free(struct objpool_head *pool)
> >   {
> > -	if (!head->cpu_slots)
> > +	if (!pool->cpu_slots)
> >   		return;
> >   
> >   	/* release percpu slots */
> > -	objpool_fini_percpu_slots(head);
> > +	objpool_fini_percpu_slots(pool);
> >   
> >   	/* call user's cleanup callback if provided */
> > -	if (head->release)
> > -		head->release(head, head->context);
> > +	if (pool->release)
> > +		pool->release(pool, pool->context);
> >   }
> >   EXPORT_SYMBOL_GPL(objpool_free);
> >   
> > -/* drop unused objects and defref objpool for releasing */
> > -void objpool_fini(struct objpool_head *head)
> > +/* drop the allocated object, rather reclaim it to objpool */
> > +int objpool_drop(void *obj, struct objpool_head *pool)
> >   {
> > -	void *nod;
> > +	if (!obj || !pool)
> > +		return -EINVAL;
> >   
> > -	do {
> > -		/* grab object from objpool and drop it */
> > -		nod = objpool_pop(head);
> > +	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)
> > +{
> > +	void *obj;
> >   
> > -		/* drop the extra ref of objpool */
> > -		if (refcount_dec_and_test(&head->ref))
> > -			objpool_free(head);
> > -	} while (nod);
> > +	/* grab object from objpool and drop it */
> > +	while ((obj = objpool_pop(pool)))
> > +		objpool_drop(obj, pool);
> >   }
> >   EXPORT_SYMBOL_GPL(objpool_fini);
> 


-- 
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>