mm/slub: add sheaf support for batching kfree_rcu() operations

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@suse.cz>

diff --git a/include/linux/slab.h b/include/linux/slab.h
index b13fb1c1f03c14a5b45bc6a64a2096883aef9f83..23904321992ad2eeb9389d0883cf4d5d5d71d896 100644 (file)
--- 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 (file)
--- 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 @@ unlock_return:
        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 (file)
--- 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 (file)
--- 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 (file)
--- 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 @@ do_free:
        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,