Re: [PATCH 6/5] slub: Make PREEMPT_RT support less convoluted

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

On Thu, Aug 25, 2022 at 09:51:36AM +0200, Sebastian Andrzej Siewior wrote:
> 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>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
> 
> 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@gmail.com>

-- 
Thanks,
Hyeonggon