On Thu, Aug 25, 2022 at 09:51:36AM +0200, Sebastian Andrzej Siewior wrote:
> 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.
Thank you for correction!
> *
> * 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,
I have no strong opinion on its naming, but from view of correctness:
Looks good to me.
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx>
--
Thanks,
Hyeonggon
