Extend the sheaf infrastructure for more efficient kfree_rcu() handling.
For caches where sheafs are initialized, on each cpu maintain a rcu_free
sheaf in addition to main and spare sheaves.
kfree_rcu() operations will try to put objects on this sheaf. Once full,
the sheaf is detached and submitted to call_rcu() with a handler that
will try to put in on the barn, or flush to slab pages using bulk free,
when the barn is full. Then a new empty sheaf must be obtained to put
more objects there.
It's possible that no free sheafs are available to use for a new
rcu_free sheaf, and the allocation in kfree_rcu() context can only use
GFP_NOWAIT and thus may fail. In that case, fall back to the existing
kfree_rcu() machinery.
Because some intended users will need to perform additonal cleanups
after the grace period and thus have custom rcu_call() callbacks today,
add the possibility to specify a kfree_rcu() specific destructor.
Because of the fall back possibility, the destructor now needs be
invoked also from within RCU, so add __kvfree_rcu() that RCU can use
instead of kvfree().
Expected advantages:
- batching the kfree_rcu() operations, that could eventually replace the
batching done in RCU itself
- sheafs can be reused via barn instead of being flushed to slabs, which
is more effective
- this includes cases where only some cpus are allowed to process rcu
callbacks (Android)
Possible disadvantage:
- objects might be waiting for more than their grace period (it is
determined by the last object freed into the sheaf), increasing memory
usage - but that might be true for the batching done by RCU as well?
RFC LIMITATIONS: - only tree rcu is converted, not tiny
- the rcu fallback might resort to kfree_bulk(), not kvfree(). Instead
of adding a variant of kfree_bulk() with destructors, is there an easy
way to disable the kfree_bulk() path in the fallback case?
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
include/linux/slab.h | 15 +++++
kernel/rcu/tree.c | 8 ++-
mm/slab.h | 25 +++++++
mm/slab_common.c | 3 +
mm/slub.c | 182 +++++++++++++++++++++++++++++++++++++++++++++++++--
5 files changed, 227 insertions(+), 6 deletions(-)
diff --git a/include/linux/slab.h b/include/linux/slab.h
index b13fb1c1f03c14a5b45bc6a64a2096883aef9f83..23904321992ad2eeb9389d0883cf4d5d5d71d896 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -343,6 +343,21 @@ struct kmem_cache_args {
* %0 means no sheaves will be created
*/
unsigned int sheaf_capacity;
+ /**
+ * @sheaf_rcu_dtor: A destructor for objects freed by kfree_rcu()
+ *
+ * Only valid when non-zero @sheaf_capacity is specified. When freeing
+ * objects by kfree_rcu() in a cache with sheaves, the objects are put
+ * to a special percpu sheaf. When that sheaf is full, it's passed to
+ * call_rcu() and after a grace period the sheaf can be reused for new
+ * allocations. In case a cleanup is necessary after the grace period
+ * and before reusal, a pointer to such function can be given as
+ * @sheaf_rcu_dtor and will be called on each object in the rcu sheaf
+ * after the grace period passes and before the sheaf's reuse.
+ *
+ * %NULL means no destructor is called.
+ */
+ void (*sheaf_rcu_dtor)(void *obj);
};
struct kmem_cache *__kmem_cache_create_args(const char *name,
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index b1f883fcd9185a5e22c10102d1024c40688f57fb..42c994fdf9960bfed8d8bd697de90af72c1f4f58 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -65,6 +65,7 @@
#include <linux/kasan.h>
#include <linux/context_tracking.h>
#include "../time/tick-internal.h"
+#include "../../mm/slab.h"
#include "tree.h"
#include "rcu.h"
@@ -3420,7 +3421,7 @@ kvfree_rcu_list(struct rcu_head *head)
trace_rcu_invoke_kvfree_callback(rcu_state.name, head, offset);
if (!WARN_ON_ONCE(!__is_kvfree_rcu_offset(offset)))
- kvfree(ptr);
+ __kvfree_rcu(ptr);
rcu_lock_release(&rcu_callback_map);
cond_resched_tasks_rcu_qs();
@@ -3797,6 +3798,9 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr)
if (!head)
might_sleep();
+ if (kfree_rcu_sheaf(ptr))
+ return;
+
// Queue the object but don't yet schedule the batch.
if (debug_rcu_head_queue(ptr)) {
// Probable double kfree_rcu(), just leak.
@@ -3849,7 +3853,7 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr)
if (!success) {
debug_rcu_head_unqueue((struct rcu_head *) ptr);
synchronize_rcu();
- kvfree(ptr);
+ __kvfree_rcu(ptr);
}
}
EXPORT_SYMBOL_GPL(kvfree_call_rcu);
diff --git a/mm/slab.h b/mm/slab.h
index 001e0d55467bb4803b5dff718ba8e0c775f42b3f..4dc145c14dfd97677c74a767c22f5dd22f5d6451 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -276,6 +276,9 @@ struct kmem_cache {
gfp_t allocflags; /* gfp flags to use on each alloc */
int refcount; /* Refcount for slab cache destroy */
void (*ctor)(void *object); /* Object constructor */
+ void (*rcu_dtor)(void *object); /* Object destructor to execute after
+ * kfree_rcu grace period
+ */
unsigned int inuse; /* Offset to metadata */
unsigned int align; /* Alignment */
unsigned int red_left_pad; /* Left redzone padding size */
@@ -454,6 +457,28 @@ static inline bool is_kmalloc_normal(struct kmem_cache *s)
return !(s->flags & (SLAB_CACHE_DMA|SLAB_ACCOUNT|SLAB_RECLAIM_ACCOUNT));
}
+void __kvfree_rcu(void *obj);
+
+bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj);
+
+static inline bool kfree_rcu_sheaf(void *obj)
+{
+ struct kmem_cache *s;
+ struct folio *folio;
+ struct slab *slab;
+
+ folio = virt_to_folio(obj);
+ if (unlikely(!folio_test_slab(folio)))
+ return false;
+
+ slab = folio_slab(folio);
+ s = slab->slab_cache;
+ if (s->cpu_sheaves)
+ return __kfree_rcu_sheaf(s, obj);
+
+ return false;
+}
+
/* Legal flag mask for kmem_cache_create(), for various configurations */
#define SLAB_CORE_FLAGS (SLAB_HWCACHE_ALIGN | SLAB_CACHE_DMA | \
SLAB_CACHE_DMA32 | SLAB_PANIC | \
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 7939f3f017740e0ac49ffa971c45409d0fbe2f23..d69ed1e7ea34f9657cb9514fb98a48647f01381b 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -236,6 +236,9 @@ static struct kmem_cache *create_cache(const char *name,
!IS_ALIGNED(args->freeptr_offset, sizeof(freeptr_t))))
goto out;
+ if (args->sheaf_rcu_dtor && !args->sheaf_capacity)
+ goto out;
+
err = -ENOMEM;
s = kmem_cache_zalloc(kmem_cache, GFP_KERNEL);
if (!s)
diff --git a/mm/slub.c b/mm/slub.c
index 7da08112213b203993b5159eb35a1ea640d706fe..6811d766c0470cd7066c2574ad86e00405c916bb 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -351,6 +351,8 @@ enum stat_item {
ALLOC_FASTPATH, /* Allocation from cpu slab */
ALLOC_SLOWPATH, /* Allocation by getting a new cpu slab */
FREE_PCS, /* Free to percpu sheaf */
+ FREE_RCU_SHEAF, /* Free to rcu_free sheaf */
+ FREE_RCU_SHEAF_FAIL, /* Failed to free to a rcu_free sheaf */
FREE_FASTPATH, /* Free to cpu slab */
FREE_SLOWPATH, /* Freeing not to cpu slab */
FREE_FROZEN, /* Freeing to frozen slab */
@@ -2557,6 +2559,24 @@ static void sheaf_flush(struct kmem_cache *s, struct slab_sheaf *sheaf)
sheaf->size = 0;
}
+static void __rcu_free_sheaf_prepare(struct kmem_cache *s,
+ struct slab_sheaf *sheaf);
+
+static void rcu_free_sheaf_nobarn(struct rcu_head *head)
+{
+ struct slab_sheaf *sheaf;
+ struct kmem_cache *s;
+
+ sheaf = container_of(head, struct slab_sheaf, rcu_head);
+ s = sheaf->cache;
+
+ __rcu_free_sheaf_prepare(s, sheaf);
+
+ sheaf_flush(s, sheaf);
+
+ free_empty_sheaf(s, sheaf);
+}
+
/*
* Caller needs to make sure migration is disabled in order to fully flush
* single cpu's sheaves
@@ -2586,8 +2606,8 @@ static void pcs_flush_all(struct kmem_cache *s)
free_empty_sheaf(s, spare);
}
- // TODO: handle rcu_free
- BUG_ON(rcu_free);
+ if (rcu_free)
+ call_rcu(&rcu_free->rcu_head, rcu_free_sheaf_nobarn);
sheaf_flush_main(s);
}
@@ -2604,8 +2624,10 @@ static void __pcs_flush_all_cpu(struct kmem_cache *s, unsigned int cpu)
pcs->spare = NULL;
}
- // TODO: handle rcu_free
- BUG_ON(pcs->rcu_free);
+ if (pcs->rcu_free) {
+ call_rcu(&pcs->rcu_free->rcu_head, rcu_free_sheaf_nobarn);
+ pcs->rcu_free = NULL;
+ }
sheaf_flush_main(s);
}
@@ -5161,6 +5183,121 @@ void free_to_pcs(struct kmem_cache *s, void *object)
stat(s, FREE_PCS);
}
+static void __rcu_free_sheaf_prepare(struct kmem_cache *s,
+ struct slab_sheaf *sheaf)
+{
+ bool init = slab_want_init_on_free(s);
+ void **p = &sheaf->objects[0];
+ unsigned int i = 0;
+
+ while (i < sheaf->size) {
+ struct slab *slab = virt_to_slab(p[i]);
+
+ if (s->rcu_dtor)
+ s->rcu_dtor(p[i]);
+
+ memcg_slab_free_hook(s, slab, p + i, 1);
+ alloc_tagging_slab_free_hook(s, slab, p + i, 1);
+
+ if (unlikely(!slab_free_hook(s, p[i], init, false))) {
+ p[i] = p[--sheaf->size];
+ continue;
+ }
+
+ i++;
+ }
+}
+
+static void rcu_free_sheaf(struct rcu_head *head)
+{
+ struct slab_sheaf *sheaf;
+ struct node_barn *barn;
+ struct kmem_cache *s;
+
+ sheaf = container_of(head, struct slab_sheaf, rcu_head);
+
+ s = sheaf->cache;
+
+ __rcu_free_sheaf_prepare(s, sheaf);
+
+ barn = get_node(s, numa_mem_id())->barn;
+
+ /* due to slab_free_hook() */
+ if (unlikely(sheaf->size == 0))
+ goto empty;
+
+ if (!barn_put_full_sheaf(barn, sheaf, false))
+ return;
+
+ sheaf_flush(s, sheaf);
+
+empty:
+ if (!barn_put_empty_sheaf(barn, sheaf, false))
+ return;
+
+ free_empty_sheaf(s, sheaf);
+}
+
+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);
+ pcs = this_cpu_ptr(s->cpu_sheaves);
+
+ if (unlikely(!pcs->rcu_free)) {
+
+ struct slab_sheaf *empty;
+
+ empty = barn_get_empty_sheaf(pcs->barn);
+
+ if (empty) {
+ pcs->rcu_free = empty;
+ goto do_free;
+ }
+
+ local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
+
+ empty = alloc_empty_sheaf(s, GFP_NOWAIT);
+
+ if (!empty) {
+ stat(s, FREE_RCU_SHEAF_FAIL);
+ return false;
+ }
+
+ local_lock_irqsave(&s->cpu_sheaves->lock, flags);
+ pcs = this_cpu_ptr(s->cpu_sheaves);
+
+ if (unlikely(pcs->rcu_free))
+ barn_put_empty_sheaf(pcs->barn, empty, true);
+ else
+ pcs->rcu_free = empty;
+ }
+
+do_free:
+
+ rcu_sheaf = pcs->rcu_free;
+
+ rcu_sheaf->objects[rcu_sheaf->size++] = obj;
+
+ if (likely(rcu_sheaf->size < s->sheaf_capacity)) {
+ local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
+ stat(s, FREE_RCU_SHEAF);
+ return true;
+ }
+
+ pcs->rcu_free = NULL;
+ local_unlock_irqrestore(&s->cpu_sheaves->lock, flags);
+
+ call_rcu(&rcu_sheaf->rcu_head, rcu_free_sheaf);
+
+ stat(s, FREE_RCU_SHEAF);
+
+ return true;
+}
+
/*
* Bulk free objects to the percpu sheaves.
* Unlike free_to_pcs() this includes the calls to all necessary hooks
@@ -5466,6 +5603,32 @@ static void free_large_kmalloc(struct folio *folio, void *object)
folio_put(folio);
}
+void __kvfree_rcu(void *obj)
+{
+ struct folio *folio;
+ struct slab *slab;
+ struct kmem_cache *s;
+
+ if (is_vmalloc_addr(obj)) {
+ vfree(obj);
+ return;
+ }
+
+ folio = virt_to_folio(obj);
+ if (unlikely(!folio_test_slab(folio))) {
+ free_large_kmalloc(folio, obj);
+ return;
+ }
+
+ slab = folio_slab(folio);
+ s = slab->slab_cache;
+
+ if (s->rcu_dtor)
+ s->rcu_dtor(obj);
+
+ slab_free(s, slab, obj, _RET_IP_);
+}
+
/**
* kfree - free previously allocated memory
* @object: pointer returned by kmalloc() or kmem_cache_alloc()
@@ -6326,6 +6489,11 @@ int __kmem_cache_shutdown(struct kmem_cache *s)
struct kmem_cache_node *n;
flush_all_cpus_locked(s);
+
+ /* we might have rcu sheaves in flight */
+ if (s->cpu_sheaves)
+ rcu_barrier();
+
/* Attempt to free all objects */
for_each_kmem_cache_node(s, node, n) {
if (n->barn)
@@ -6887,6 +7055,8 @@ int do_kmem_cache_create(struct kmem_cache *s, const char *name,
}
// TODO: increase capacity to grow slab_sheaf up to next kmalloc size?
s->sheaf_capacity = args->sheaf_capacity;
+
+ s->rcu_dtor = args->sheaf_rcu_dtor;
}
#ifdef CONFIG_NUMA
@@ -7710,6 +7880,8 @@ STAT_ATTR(ALLOC_PCS, alloc_cpu_sheaf);
STAT_ATTR(ALLOC_FASTPATH, alloc_fastpath);
STAT_ATTR(ALLOC_SLOWPATH, alloc_slowpath);
STAT_ATTR(FREE_PCS, free_cpu_sheaf);
+STAT_ATTR(FREE_RCU_SHEAF, free_rcu_sheaf);
+STAT_ATTR(FREE_RCU_SHEAF_FAIL, free_rcu_sheaf_fail);
STAT_ATTR(FREE_FASTPATH, free_fastpath);
STAT_ATTR(FREE_SLOWPATH, free_slowpath);
STAT_ATTR(FREE_FROZEN, free_frozen);
@@ -7805,6 +7977,8 @@ static struct attribute *slab_attrs[] = {
&alloc_fastpath_attr.attr,
&alloc_slowpath_attr.attr,
&free_cpu_sheaf_attr.attr,
+ &free_rcu_sheaf_attr.attr,
+ &free_rcu_sheaf_fail_attr.attr,
&free_fastpath_attr.attr,
&free_slowpath_attr.attr,
&free_frozen_attr.attr,
--
2.47.0
