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Message-Id: <20231012230237.45726dfad125cf0e8d00ba53@kernel.org>
Date: Thu, 12 Oct 2023 23:02:37 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: wuqiang <wuqiang.matt@...edance.com>
Cc: linux-trace-kernel@...r.kernel.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,
linux-kernel@...r.kernel.org, lkp@...el.com, mattwu@....com
Subject: Re: [PATCH v9 1/5] lib: objpool added: ring-array based lockless
MPMC
Hi Wuqiang,
On Mon, 9 Oct 2023 17:23:34 +0800
wuqiang <wuqiang.matt@...edance.com> 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@...edance.com> 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@...nel.org>
> > 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@...nel.org>
> > ---
> > 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@...edance.com
> > */
> >
> > -#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@...nel.org>
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