From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
The slub code already has a few helpers depending on PREEMPT_RT. Add a few
more and get rid of the CONFIG_PREEMPT_RT conditionals all over the place.
No functional change.
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxx>
Cc: linux-mm@xxxxxxxxx
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
Vlastimil, does it work for you to include this patch in your series? It
depends now on your series :) It has this USE_LOCKLESS_FAST_PATH() Linus
asked about so we should be good.
mm/slub.c | 56 ++++++++++++++++++++++++--------------------------------
1 file changed, 24 insertions(+), 32 deletions(-)
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -104,9 +104,11 @@
* 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.
- * On PREEMPT_RT, the local lock does not actually disable irqs (and thus
- * prevent the lockless operations), so fastpath operations also need to take
- * the lock and are no longer lockless.
+ *
+ * On PREEMPT_RT, the local lock neither disables interrupts nor preemption
+ * which means the lockless fastpath cannot be used as it might interfere with
+ * an in-progress slow path operations. In this case the local lock is always
+ * taken but it still utilizes the freelist for the common operations.
*
* lockless fastpaths
*
@@ -167,8 +169,9 @@
* function call even on !PREEMPT_RT, use inline preempt_disable() there.
*/
#ifndef CONFIG_PREEMPT_RT
-#define slub_get_cpu_ptr(var) get_cpu_ptr(var)
-#define slub_put_cpu_ptr(var) put_cpu_ptr(var)
+#define slub_get_cpu_ptr(var) get_cpu_ptr(var)
+#define slub_put_cpu_ptr(var) put_cpu_ptr(var)
+#define USE_LOCKLESS_FAST_PATH() (true)
#else
#define slub_get_cpu_ptr(var) \
({ \
@@ -180,6 +183,7 @@ do { \
(void)(var); \
migrate_enable(); \
} while (0)
+#define USE_LOCKLESS_FAST_PATH() (false)
#endif
#ifdef CONFIG_SLUB_DEBUG
@@ -474,7 +478,7 @@ static inline bool __cmpxchg_double_slab
void *freelist_new, unsigned long counters_new,
const char *n)
{
- if (!IS_ENABLED(CONFIG_PREEMPT_RT))
+ if (USE_LOCKLESS_FAST_PATH())
lockdep_assert_irqs_disabled();
#if defined(CONFIG_HAVE_CMPXCHG_DOUBLE) && \
defined(CONFIG_HAVE_ALIGNED_STRUCT_PAGE)
@@ -3287,14 +3291,8 @@ static __always_inline void *slab_alloc_
object = c->freelist;
slab = c->slab;
- /*
- * We cannot use the lockless fastpath on PREEMPT_RT because if a
- * slowpath has taken the local_lock_irqsave(), it is not protected
- * against a fast path operation in an irq handler. So we need to take
- * the slow path which uses local_lock. It is still relatively fast if
- * there is a suitable cpu freelist.
- */
- if (IS_ENABLED(CONFIG_PREEMPT_RT) ||
+
+ if (!USE_LOCKLESS_FAST_PATH() ||
unlikely(!object || !slab || !node_match(slab, node))) {
object = __slab_alloc(s, gfpflags, node, addr, c);
} else {
@@ -3554,6 +3552,7 @@ static __always_inline void do_slab_free
void *tail_obj = tail ? : head;
struct kmem_cache_cpu *c;
unsigned long tid;
+ void **freelist;
redo:
/*
@@ -3568,9 +3567,13 @@ static __always_inline void do_slab_free
/* Same with comment on barrier() in slab_alloc_node() */
barrier();
- if (likely(slab == c->slab)) {
-#ifndef CONFIG_PREEMPT_RT
- void **freelist = READ_ONCE(c->freelist);
+ if (unlikely(slab != c->slab)) {
+ __slab_free(s, slab, head, tail_obj, cnt, addr);
+ return;
+ }
+
+ if (USE_LOCKLESS_FAST_PATH()) {
+ freelist = READ_ONCE(c->freelist);
set_freepointer(s, tail_obj, freelist);
@@ -3582,16 +3585,8 @@ static __always_inline void do_slab_free
note_cmpxchg_failure("slab_free", s, tid);
goto redo;
}
-#else /* CONFIG_PREEMPT_RT */
- /*
- * We cannot use the lockless fastpath on PREEMPT_RT because if
- * a slowpath has taken the local_lock_irqsave(), it is not
- * protected against a fast path operation in an irq handler. So
- * we need to take the local_lock. We shouldn't simply defer to
- * __slab_free() as that wouldn't use the cpu freelist at all.
- */
- void **freelist;
-
+ } else {
+ /* Update the free list under the local lock */
local_lock(&s->cpu_slab->lock);
c = this_cpu_ptr(s->cpu_slab);
if (unlikely(slab != c->slab)) {
@@ -3606,11 +3601,8 @@ static __always_inline void do_slab_free
c->tid = next_tid(tid);
local_unlock(&s->cpu_slab->lock);
-#endif
- stat(s, FREE_FASTPATH);
- } else
- __slab_free(s, slab, head, tail_obj, cnt, addr);
-
+ }
+ stat(s, FREE_FASTPATH);
}
static __always_inline void slab_free(struct kmem_cache *s, struct slab *slab,
