[PATCH 1/9] slub: Make PREEMPT_RT support less convoluted

Sebastian Andrzej Siewior
2022-08-17T16:27:45Z

From: Thomas Gleixner <tglx@linutronix.de>

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@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: linux-mm@kvack.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
mm/slub.c | 66 +++++++++++++++++++++++++------------------------------
1 file changed, 30 insertions(+), 36 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 862dbd9af4f52..5f7c5b5bd49f9 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -100,9 +100,11 @@
* 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
*
@@ -163,8 +165,11 @@
* 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)
+#define slub_local_irq_save(flags) local_irq_save(flags)
+#define slub_local_irq_restore(flags) local_irq_restore(flags)
#else
#define slub_get_cpu_ptr(var) \
({ \
@@ -176,6 +181,9 @@ do { \
(void)(var); \
migrate_enable(); \
} while (0)
+#define use_lockless_fast_path() (false)
+#define slub_local_irq_save(flags) do { } while (0)
+#define slub_local_irq_restore(flags) do { } while (0)
#endif

#ifdef CONFIG_SLUB_DEBUG
@@ -460,16 +468,14 @@ static __always_inline void __slab_unlock(struct slab *slab)

static __always_inline void slab_lock(struct slab *slab, unsigned long *flags)
{
- if (IS_ENABLED(CONFIG_PREEMPT_RT))
- local_irq_save(*flags);
+ slub_local_irq_save(*flags);
__slab_lock(slab);
}

static __always_inline void slab_unlock(struct slab *slab, unsigned long *flags)
{
__slab_unlock(slab);
- if (IS_ENABLED(CONFIG_PREEMPT_RT))
- local_irq_restore(*flags);
+ slub_local_irq_restore(*flags);
}

/*
@@ -482,7 +488,7 @@ static inline bool __cmpxchg_double_slab(struct kmem_cache *s, struct slab *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)
@@ -3197,14 +3203,8 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s, struct list_l

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 {
@@ -3463,6 +3463,7 @@ static __always_inline void do_slab_free(struct kmem_cache *s,
void *tail_obj = tail ? : head;
struct kmem_cache_cpu *c;
unsigned long tid;
+ void **freelist;

redo:
/*
@@ -3477,9 +3478,13 @@ static __always_inline void do_slab_free(struct kmem_cache *s,
/* 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);

@@ -3491,16 +3496,8 @@ static __always_inline void do_slab_free(struct kmem_cache *s,
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)) {
@@ -3515,11 +3512,8 @@ static __always_inline void do_slab_free(struct kmem_cache *s,
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,
--
2.37.2