Embed local_lock into struct kmem_cpu_slab and use the irq-safe versions of
local_lock instead of plain local_irq_save/restore. On !PREEMPT_RT that's
equivalent, with better lockdep visibility. On PREEMPT_RT that means better
preemption.
Also update the comment about locking scheme in SLUB to reflect changes done
by this series.
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
include/linux/slub_def.h | 2 +
mm/slub.c | 90 ++++++++++++++++++++++++++++------------
2 files changed, 66 insertions(+), 26 deletions(-)
diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
index dcde82a4434c..b5bcac29b979 100644
--- a/include/linux/slub_def.h
+++ b/include/linux/slub_def.h
@@ -10,6 +10,7 @@
#include <linux/kfence.h>
#include <linux/kobject.h>
#include <linux/reciprocal_div.h>
+#include <linux/local_lock.h>
enum stat_item {
ALLOC_FASTPATH, /* Allocation from cpu slab */
@@ -41,6 +42,7 @@ enum stat_item {
NR_SLUB_STAT_ITEMS };
struct kmem_cache_cpu {
+ local_lock_t lock; /* Protects the fields below except stat */
void **freelist; /* Pointer to next available object */
unsigned long tid; /* Globally unique transaction id */
struct page *page; /* The slab from which we are allocating */
diff --git a/mm/slub.c b/mm/slub.c
index 8818c210cb97..5455c6bda997 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -43,13 +43,22 @@
/*
* Lock order:
* 1. slab_mutex (Global Mutex)
- * 2. node->list_lock
+ * 2. node->list_lock (Spinlock)
+ * OR
+ * kmem_cache->cpu_slab->lock (Local lock)
* 3. slab_lock(page) (Only on some arches and for debugging)
+ * 4. object_map_lock (Only for debugging)
*
* slab_mutex
*
* The role of the slab_mutex is to protect the list of all the slabs
* and to synchronize major metadata changes to slab cache structures.
+ * Also synchronizes memory hotplug callbacks.
+ *
+ * slab_lock
+ *
+ * The slab_lock is a wrapper around the page lock, thus it is a bit
+ * spinlock.
*
* The slab_lock is only used for debugging and on arches that do not
* have the ability to do a cmpxchg_double. It only protects:
@@ -65,6 +74,8 @@
* froze the slab is the only one that can retrieve the objects from the
* page's freelist.
*
+ * list_lock
+ *
* The list_lock protects the partial and full list on each node and
* the partial slab counter. If taken then no new slabs may be added or
* removed from the lists nor make the number of partial slabs be modified.
@@ -76,10 +87,33 @@
* slabs, operations can continue without any centralized lock. F.e.
* allocating a long series of objects that fill up slabs does not require
* the list lock.
- * Interrupts are disabled during allocation and deallocation in order to
- * make the slab allocator safe to use in the context of an irq. In addition
- * interrupts are disabled to ensure that the processor does not change
- * while handling per_cpu slabs, due to kernel preemption.
+ *
+ * cpu_slab->lock local lock
+ *
+ * This locks protect slowpath manipulation of all kmem_cache_cpu fields
+ * except the stat counters. This is a percpu structure manipulated only by
+ * the local cpu, so the lock protects against being preempted or interrupted
+ * by an irq. Fast path operations rely on lockless operations instead.
+ *
+ * lockless fastpaths
+ *
+ * The fast path allocation (slab_alloc_node()) and freeing (do_slab_free())
+ * are fully lockless when satisfied from the percpu slab (and when
+ * cmpxchg_double is possible to use, otherwise slab_lock is taken).
+ * They also don't disable preemption or migration or irqs. They rely on
+ * the transaction id (tid) field to detect being preempted or moved to
+ * another cpu.
+ *
+ * irq, preemption, migration considerations
+ *
+ * Interrupts are disabled as part of list_lock or local_lock operations, or
+ * around the slab_lock operation, in order to make the slab allocator safe
+ * to use in the context of an irq.
+ *
+ * In addition, migration is disabled in the allocation slowpath, bulk
+ * allocation, and put_cpu_partial(), so that the local cpu doesn't change in
+ * the process and e.g. the kmem_cache_cpu pointer doesn't have to be
+ * revalidated in each section protected by the local lock.
*
* SLUB assigns one slab for allocation to each processor.
* Allocations only occur from these slabs called cpu slabs.
@@ -427,7 +461,7 @@ static inline bool cmpxchg_double_slab(struct kmem_cache *s, struct page *page,
page->freelist = freelist_new;
page->counters = counters_new;
slab_unlock(page);
- local_irq_restore(flags);
+ local_unlock_irqrestore(&s->cpu_slab->lock, flags);
return true;
}
slab_unlock(page);
@@ -2157,9 +2191,13 @@ static inline void note_cmpxchg_failure(const char *n,
static void init_kmem_cache_cpus(struct kmem_cache *s)
{
int cpu;
+ struct kmem_cache_cpu *c;
- for_each_possible_cpu(cpu)
- per_cpu_ptr(s->cpu_slab, cpu)->tid = init_tid(cpu);
+ for_each_possible_cpu(cpu) {
+ c = per_cpu_ptr(s->cpu_slab, cpu);
+ local_lock_init(&c->lock);
+ c->tid = init_tid(cpu);
+ }
}
/*
@@ -2708,9 +2746,9 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
goto deactivate_slab;
/* must check again c->page in case of IRQ */
- local_irq_save(flags);
+ local_lock_irqsave(&s->cpu_slab->lock, flags);
if (unlikely(page != c->page)) {
- local_irq_restore(flags);
+ local_unlock_irqrestore(&s->cpu_slab->lock, flags);
goto reread_page;
}
freelist = c->freelist;
@@ -2721,7 +2759,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
if (!freelist) {
c->page = NULL;
- local_irq_restore(flags);
+ local_unlock_irqrestore(&s->cpu_slab->lock, flags);
stat(s, DEACTIVATE_BYPASS);
goto new_slab;
}
@@ -2737,37 +2775,37 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
VM_BUG_ON(!c->page->frozen);
c->freelist = get_freepointer(s, freelist);
c->tid = next_tid(c->tid);
- local_irq_restore(flags);
+ local_unlock_irqrestore(&s->cpu_slab->lock, flags);
return freelist;
deactivate_slab:
- local_irq_save(flags);
+ local_lock_irqsave(&s->cpu_slab->lock, flags);
if (page != c->page) {
- local_irq_restore(flags);
+ local_unlock_irqrestore(&s->cpu_slab->lock, flags);
goto reread_page;
}
freelist = c->freelist;
c->page = NULL;
c->freelist = NULL;
- local_irq_restore(flags);
+ local_unlock_irqrestore(&s->cpu_slab->lock, flags);
deactivate_slab(s, page, freelist);
new_slab:
if (slub_percpu_partial(c)) {
- local_irq_save(flags);
+ local_lock_irqsave(&s->cpu_slab->lock, flags);
if (unlikely(c->page)) {
- local_irq_restore(flags);
+ local_unlock_irqrestore(&s->cpu_slab->lock, flags);
goto reread_page;
}
if (unlikely(!slub_percpu_partial(c))) { /* stolen by IRQ? */
- local_irq_restore(flags);
+ local_unlock_irqrestore(&s->cpu_slab->lock, flags);
goto new_objects;
}
page = c->page = slub_percpu_partial(c);
slub_set_percpu_partial(c, page);
- local_irq_restore(flags);
+ local_unlock_irqrestore(&s->cpu_slab->lock, flags);
stat(s, CPU_PARTIAL_ALLOC);
goto redo;
}
@@ -2820,7 +2858,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
goto return_single;
retry_load_page:
- local_irq_save(flags);
+ local_lock_irqsave(&s->cpu_slab->lock, flags);
if (unlikely(c->page)) {
void *flush_freelist = c->freelist;
struct page *flush_page = c->page;
@@ -2829,7 +2867,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
c->freelist = NULL;
c->tid = next_tid(c->tid);
- local_irq_restore(flags);
+ local_unlock_irqrestore(&s->cpu_slab->lock, flags);
deactivate_slab(s, flush_page, flush_freelist);
@@ -3389,7 +3427,7 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
*/
migrate_disable();
c = this_cpu_ptr(s->cpu_slab);
- local_irq_disable();
+ local_lock_irq(&s->cpu_slab->lock);
for (i = 0; i < size; i++) {
void *object = kfence_alloc(s, s->object_size, flags);
@@ -3410,7 +3448,7 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
*/
c->tid = next_tid(c->tid);
- local_irq_enable();
+ local_unlock_irq(&s->cpu_slab->lock);
/*
* Invoking slow path likely have side-effect
@@ -3424,7 +3462,7 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
c = this_cpu_ptr(s->cpu_slab);
maybe_wipe_obj_freeptr(s, p[i]);
- local_irq_disable();
+ local_lock_irq(&s->cpu_slab->lock);
continue; /* goto for-loop */
}
@@ -3433,7 +3471,7 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
maybe_wipe_obj_freeptr(s, p[i]);
}
c->tid = next_tid(c->tid);
- local_irq_enable();
+ local_unlock_irq(&s->cpu_slab->lock);
migrate_enable();
/*
@@ -3444,7 +3482,7 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
slab_want_init_on_alloc(flags, s));
return i;
error:
- local_irq_enable();
+ local_unlock_irq(&s->cpu_slab->lock);
slab_post_alloc_hook(s, objcg, flags, i, p, false);
__kmem_cache_free_bulk(s, i, p);
return 0;
--
2.31.1
