[PATCH RFC v2 05/10] slab: switch percpu sheaves locking to localtry_lock

From: Vlastimil Babka
Date: Fri Feb 14 2025 - 11:28:49 EST

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Instead of local_lock_irqsave(), use localtry_trylock() when potential
callers include irq context, and localtry_lock() otherwise (such as when
we already know the gfp flags allow blocking).

This should reduce the locking (due to irq disabling/enabling) overhead.
Failing to use percpu sheaves in an irq due to preempting an already
locked user of sheaves should be rare so it's a favorable tradeoff.

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
mm/slub.c | 122 ++++++++++++++++++++++++++++++++++++++------------------------
1 file changed, 76 insertions(+), 46 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 40175747212fefb27137309b27571abe8d0966e2..3d7345e7e938d53950ed0d6abe8eb0e93cf8f5b1 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -450,7 +450,7 @@ struct slab_sheaf {
};

struct slub_percpu_sheaves {
- local_lock_t lock;
+ localtry_lock_t lock;
struct slab_sheaf *main; /* never NULL when unlocked */
struct slab_sheaf *spare; /* empty or full, may be NULL */
struct slab_sheaf *rcu_free;
@@ -2529,16 +2529,19 @@ static struct slab_sheaf *alloc_full_sheaf(struct kmem_cache *s, gfp_t gfp)

static void __kmem_cache_free_bulk(struct kmem_cache *s, size_t size, void **p);

-static void sheaf_flush_main(struct kmem_cache *s)
+/* returns true if at least partially flushed */
+static bool sheaf_flush_main(struct kmem_cache *s)
{
struct slub_percpu_sheaves *pcs;
unsigned int batch, remaining;
void *objects[PCS_BATCH_MAX];
struct slab_sheaf *sheaf;
- unsigned long flags;
+ bool ret = false;

next_batch:
- local_lock_irqsave(&s->cpu_sheaves->lock, flags);
+ if (!localtry_trylock(&s->cpu_sheaves->lock))
+ return ret;
+
pcs = this_cpu_ptr(s->cpu_sheaves);
sheaf = pcs->main;

@@ -2549,14 +2552,18 @@ static void sheaf_flush_main(struct kmem_cache *s)

remaining = sheaf->size;

- local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
+ localtry_unlock(&s->cpu_sheaves->lock);

__kmem_cache_free_bulk(s, batch, &objects[0]);

stat_add(s, SHEAF_FLUSH_MAIN, batch);

+ ret = true;
+
if (remaining)
goto next_batch;
+
+ return ret;
}

static void sheaf_flush(struct kmem_cache *s, struct slab_sheaf *sheaf)
@@ -2593,6 +2600,8 @@ static void rcu_free_sheaf_nobarn(struct rcu_head *head)
* Caller needs to make sure migration is disabled in order to fully flush
* single cpu's sheaves
*
+ * must not be called from an irq
+ *
* flushing operations are rare so let's keep it simple and flush to slabs
* directly, skipping the barn
*/
@@ -2600,9 +2609,8 @@ static void pcs_flush_all(struct kmem_cache *s)
{
struct slub_percpu_sheaves *pcs;
struct slab_sheaf *spare, *rcu_free;
- unsigned long flags;

- local_lock_irqsave(&s->cpu_sheaves->lock, flags);
+ localtry_lock(&s->cpu_sheaves->lock);
pcs = this_cpu_ptr(s->cpu_sheaves);

spare = pcs->spare;
@@ -2611,7 +2619,7 @@ static void pcs_flush_all(struct kmem_cache *s)
rcu_free = pcs->rcu_free;
pcs->rcu_free = NULL;

- local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
+ localtry_unlock(&s->cpu_sheaves->lock);

if (spare) {
sheaf_flush(s, spare);
@@ -4554,10 +4562,11 @@ static __fastpath_inline
void *alloc_from_pcs(struct kmem_cache *s, gfp_t gfp)
{
struct slub_percpu_sheaves *pcs;
- unsigned long flags;
void *object;

- local_lock_irqsave(&s->cpu_sheaves->lock, flags);
+ if (!localtry_trylock(&s->cpu_sheaves->lock))
+ return NULL;
+
pcs = this_cpu_ptr(s->cpu_sheaves);

if (unlikely(pcs->main->size == 0)) {
@@ -4590,7 +4599,7 @@ void *alloc_from_pcs(struct kmem_cache *s, gfp_t gfp)
}
}

- local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
+ localtry_unlock(&s->cpu_sheaves->lock);

if (!can_alloc)
return NULL;
@@ -4612,7 +4621,11 @@ void *alloc_from_pcs(struct kmem_cache *s, gfp_t gfp)
if (!full)
return NULL;

- local_lock_irqsave(&s->cpu_sheaves->lock, flags);
+ /*
+ * we can reach here only when gfpflags_allow_blocking
+ * so this must not be an irq
+ */
+ localtry_lock(&s->cpu_sheaves->lock);
pcs = this_cpu_ptr(s->cpu_sheaves);

/*
@@ -4646,7 +4659,7 @@ void *alloc_from_pcs(struct kmem_cache *s, gfp_t gfp)
do_alloc:
object = pcs->main->objects[--pcs->main->size];

- local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
+ localtry_unlock(&s->cpu_sheaves->lock);

stat(s, ALLOC_PCS);

@@ -4658,12 +4671,13 @@ unsigned int alloc_from_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
{
struct slub_percpu_sheaves *pcs;
struct slab_sheaf *main;
- unsigned long flags;
unsigned int allocated = 0;
unsigned int batch;

next_batch:
- local_lock_irqsave(&s->cpu_sheaves->lock, flags);
+ if (!localtry_trylock(&s->cpu_sheaves->lock))
+ return allocated;
+
pcs = this_cpu_ptr(s->cpu_sheaves);

if (unlikely(pcs->main->size == 0)) {
@@ -4683,7 +4697,7 @@ unsigned int alloc_from_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
goto do_alloc;
}

- local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
+ localtry_unlock(&s->cpu_sheaves->lock);

/*
* Once full sheaves in barn are depleted, let the bulk
@@ -4701,7 +4715,7 @@ unsigned int alloc_from_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
main->size -= batch;
memcpy(p, main->objects + main->size, batch * sizeof(void *));

- local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
+ localtry_unlock(&s->cpu_sheaves->lock);

stat_add(s, ALLOC_PCS, batch);

@@ -5121,13 +5135,14 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
* The object is expected to have passed slab_free_hook() already.
*/
static __fastpath_inline
-void free_to_pcs(struct kmem_cache *s, void *object)
+bool free_to_pcs(struct kmem_cache *s, void *object)
{
struct slub_percpu_sheaves *pcs;
- unsigned long flags;

restart:
- local_lock_irqsave(&s->cpu_sheaves->lock, flags);
+ if (!localtry_trylock(&s->cpu_sheaves->lock))
+ return false;
+
pcs = this_cpu_ptr(s->cpu_sheaves);

if (unlikely(pcs->main->size == s->sheaf_capacity)) {
@@ -5162,7 +5177,7 @@ void free_to_pcs(struct kmem_cache *s, void *object)
struct slab_sheaf *to_flush = pcs->spare;

pcs->spare = NULL;
- local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
+ localtry_unlock(&s->cpu_sheaves->lock);

sheaf_flush(s, to_flush);
empty = to_flush;
@@ -5170,17 +5185,27 @@ void free_to_pcs(struct kmem_cache *s, void *object)
}

alloc_empty:
- local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
+ localtry_unlock(&s->cpu_sheaves->lock);

empty = alloc_empty_sheaf(s, GFP_NOWAIT);

if (!empty) {
- sheaf_flush_main(s);
- goto restart;
+ if (sheaf_flush_main(s))
+ goto restart;
+ else
+ return false;
}

got_empty:
- local_lock_irqsave(&s->cpu_sheaves->lock, flags);
+ if (!localtry_trylock(&s->cpu_sheaves->lock)) {
+ struct node_barn *barn;
+
+ barn = get_node(s, numa_mem_id())->barn;
+
+ barn_put_empty_sheaf(barn, empty, true);
+ return false;
+ }
+
pcs = this_cpu_ptr(s->cpu_sheaves);

/*
@@ -5209,9 +5234,11 @@ void free_to_pcs(struct kmem_cache *s, void *object)
do_free:
pcs->main->objects[pcs->main->size++] = object;

- local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
+ localtry_unlock(&s->cpu_sheaves->lock);

stat(s, FREE_PCS);
+
+ return true;
}

static void __rcu_free_sheaf_prepare(struct kmem_cache *s,
@@ -5270,9 +5297,10 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj)
{
struct slub_percpu_sheaves *pcs;
struct slab_sheaf *rcu_sheaf;
- unsigned long flags;

- local_lock_irqsave(&s->cpu_sheaves->lock, flags);
+ if (!localtry_trylock(&s->cpu_sheaves->lock))
+ goto fail;
+
pcs = this_cpu_ptr(s->cpu_sheaves);

if (unlikely(!pcs->rcu_free)) {
@@ -5286,16 +5314,16 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj)
goto do_free;
}

- local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
+ localtry_unlock(&s->cpu_sheaves->lock);

empty = alloc_empty_sheaf(s, GFP_NOWAIT);

- if (!empty) {
- stat(s, FREE_RCU_SHEAF_FAIL);
- return false;
- }
+ if (!empty)
+ goto fail;
+
+ if (!localtry_trylock(&s->cpu_sheaves->lock))
+ goto fail;

- local_lock_irqsave(&s->cpu_sheaves->lock, flags);
pcs = this_cpu_ptr(s->cpu_sheaves);

if (unlikely(pcs->rcu_free))
@@ -5311,19 +5339,22 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj)
rcu_sheaf->objects[rcu_sheaf->size++] = obj;

if (likely(rcu_sheaf->size < s->sheaf_capacity)) {
- local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
+ localtry_unlock(&s->cpu_sheaves->lock);
stat(s, FREE_RCU_SHEAF);
return true;
}

pcs->rcu_free = NULL;
- local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
+ localtry_unlock(&s->cpu_sheaves->lock);

call_rcu(&rcu_sheaf->rcu_head, rcu_free_sheaf);

stat(s, FREE_RCU_SHEAF);
-
return true;
+
+fail:
+ stat(s, FREE_RCU_SHEAF_FAIL);
+ return false;
}

/*
@@ -5335,7 +5366,6 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
{
struct slub_percpu_sheaves *pcs;
struct slab_sheaf *main;
- unsigned long flags;
unsigned int batch, i = 0;
bool init;

@@ -5358,7 +5388,9 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
}

next_batch:
- local_lock_irqsave(&s->cpu_sheaves->lock, flags);
+ if (!localtry_trylock(&s->cpu_sheaves->lock))
+ goto fallback;
+
pcs = this_cpu_ptr(s->cpu_sheaves);

if (unlikely(pcs->main->size == s->sheaf_capacity)) {
@@ -5389,13 +5421,13 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
}

no_empty:
- local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
+ localtry_unlock(&s->cpu_sheaves->lock);

/*
* if we depleted all empty sheaves in the barn or there are too
* many full sheaves, free the rest to slab pages
*/
-
+fallback:
__kmem_cache_free_bulk(s, size, p);
return;
}
@@ -5407,7 +5439,7 @@ static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
memcpy(main->objects + main->size, p, batch * sizeof(void *));
main->size += batch;

- local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
+ localtry_unlock(&s->cpu_sheaves->lock);

stat_add(s, FREE_PCS, batch);

@@ -5507,9 +5539,7 @@ void slab_free(struct kmem_cache *s, struct slab *slab, void *object,
if (unlikely(!slab_free_hook(s, object, slab_want_init_on_free(s), false)))
return;

- if (s->cpu_sheaves)
- free_to_pcs(s, object);
- else
+ if (!s->cpu_sheaves || !free_to_pcs(s, object))
do_slab_free(s, slab, object, object, 1, addr);
}

@@ -6288,7 +6318,7 @@ static int init_percpu_sheaves(struct kmem_cache *s)

pcs = per_cpu_ptr(s->cpu_sheaves, cpu);

- local_lock_init(&pcs->lock);
+ localtry_lock_init(&pcs->lock);

nid = cpu_to_mem(cpu);

--
2.48.1

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