[patch 21/25] debugobjects: Implement batch processing

From: Thomas Gleixner
Date: Mon Oct 07 2024 - 12:55:26 EST


Adding and removing single objects in a loop is bad in terms of lock
contention and cache line accesses.

To implement batching, record the last object in a batch in the object
itself. This is trivialy possible as hlists are strictly stacks. At a batch
boundary, when the first object is added to the list the object stores a
pointer to itself in debug_obj::batch_last. When the next object is added
to the list then the batch_last pointer is retrieved from the first object
in the list and stored in the to be added one.

That means for batch processing the first object always has a pointer to
the last object in a batch, which allows to move batches in a cache line
efficient way and reduces the lock held time.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
lib/debugobjects.c | 61 +++++++++++++++++++++++++++++++++++++++--------------
1 file changed, 46 insertions(+), 15 deletions(-)

--- a/lib/debugobjects.c
+++ b/lib/debugobjects.c
@@ -149,18 +149,31 @@ static __always_inline bool pool_must_re

static bool pool_move_batch(struct obj_pool *dst, struct obj_pool *src)
{
- if (dst->cnt + ODEBUG_BATCH_SIZE > dst->max_cnt || !src->cnt)
+ struct hlist_node *last, *next_batch, *first_batch;
+ struct debug_obj *obj;
+
+ if (dst->cnt >= dst->max_cnt || !src->cnt)
return false;

- for (int i = 0; i < ODEBUG_BATCH_SIZE && src->cnt; i++) {
- struct hlist_node *node = src->objects.first;
+ first_batch = src->objects.first;
+ obj = hlist_entry(first_batch, typeof(*obj), node);
+ last = obj->batch_last;
+ next_batch = last->next;

- WRITE_ONCE(src->cnt, src->cnt - 1);
- WRITE_ONCE(dst->cnt, dst->cnt + 1);
+ /* Move the next batch to the front of the source pool */
+ src->objects.first = next_batch;
+ if (next_batch)
+ next_batch->pprev = &src->objects.first;
+
+ /* Add the extracted batch to the destination pool */
+ last->next = dst->objects.first;
+ if (last->next)
+ last->next->pprev = &last->next;
+ first_batch->pprev = &dst->objects.first;
+ dst->objects.first = first_batch;

- hlist_del(node);
- hlist_add_head(node, &dst->objects);
- }
+ WRITE_ONCE(src->cnt, src->cnt - ODEBUG_BATCH_SIZE);
+ WRITE_ONCE(dst->cnt, dst->cnt + ODEBUG_BATCH_SIZE);
return true;
}

@@ -182,16 +195,27 @@ static bool pool_push_batch(struct obj_p

static bool pool_pop_batch(struct hlist_head *head, struct obj_pool *src)
{
+ struct hlist_node *last, *next;
+ struct debug_obj *obj;
+
if (!src->cnt)
return false;

- for (int i = 0; src->cnt && i < ODEBUG_BATCH_SIZE; i++) {
- struct hlist_node *node = src->objects.first;
+ /* Move the complete list to the head */
+ hlist_move_list(&src->objects, head);

- WRITE_ONCE(src->cnt, src->cnt - 1);
- hlist_del(node);
- hlist_add_head(node, head);
- }
+ obj = hlist_entry(head->first, typeof(*obj), node);
+ last = obj->batch_last;
+ next = last->next;
+ /* Disconnect the batch from the list */
+ last->next = NULL;
+
+ /* Move the node after last back to the source pool. */
+ src->objects.first = next;
+ if (next)
+ next->pprev = &src->objects.first;
+
+ WRITE_ONCE(src->cnt, src->cnt - ODEBUG_BATCH_SIZE);
return true;
}

@@ -226,7 +250,7 @@ static struct debug_obj *pcpu_alloc(void
if (!pool_move_batch(pcp, &pool_global))
return NULL;
}
- obj_pool_used += pcp->cnt;
+ obj_pool_used += ODEBUG_BATCH_SIZE;

if (obj_pool_used > obj_pool_max_used)
obj_pool_max_used = obj_pool_used;
@@ -239,9 +263,16 @@ static struct debug_obj *pcpu_alloc(void
static void pcpu_free(struct debug_obj *obj)
{
struct obj_pool *pcp = this_cpu_ptr(&pool_pcpu);
+ struct debug_obj *first;

lockdep_assert_irqs_disabled();

+ if (!(pcp->cnt % ODEBUG_BATCH_SIZE)) {
+ obj->batch_last = &obj->node;
+ } else {
+ first = hlist_entry(pcp->objects.first, typeof(*first), node);
+ obj->batch_last = first->batch_last;
+ }
hlist_add_head(&obj->node, &pcp->objects);
pcp->cnt++;