Re: [RFC v2 33/34] mm, slub: use migrate_disable() on PREEMPT_RT

[Vlastimil Babka <vbabka@suse.cz>]

On 6/9/21 1:39 PM, Vlastimil Babka wrote:
> We currently use preempt_disable() (directly or via get_cpu_ptr()) to stabilize
> the pointer to kmem_cache_cpu. On PREEMPT_RT this would be incompatible with
> the list_lock spinlock. We can use migrate_disable() instead, but that
> increases overhead on !PREEMPT_RT as it's an unconditional function call even
> though it's ultimately a migrate_disable() there.
> 
> In order to get the best available mechanism on both PREEMPT_RT and
> !PREEMPT_RT, introduce private slub_get_cpu_ptr() and slub_put_cpu_ptr()
> wrappers and use them.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  mm/slub.c | 41 +++++++++++++++++++++++++++++++----------
>  1 file changed, 31 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 12e966f07f19..caa206213e72 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -115,6 +115,26 @@
>   * 			the fast path and disables lockless freelists.
>   */
>  
> +/*
> + * We could simply use migrate_disable()/enable() but as long as it's a
> + * function call even on !PREEMPT_RT, use inline preempt_disable() there.
> + */
> +#ifdef CONFIG_PREEMPT_RT
> +#define slub_get_cpu_ptr(var)	get_cpu_ptr(var)
> +#define slub_put_cpu_ptr(var)	put_cpu_ptr(var)

After Mel's report and bisect pointing to this patch, I realized I got the
#ifdef wrong and it should be #ifnded

Fixed patch follows:

----8<----
From e79e166414b8c22e3bcf73a9383bafe802fa64d2 Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@suse.cz>
Date: Fri, 21 May 2021 14:03:23 +0200
Subject: [PATCH] mm, slub: use migrate_disable() on PREEMPT_RT

We currently use preempt_disable() (directly or via get_cpu_ptr()) to stabilize
the pointer to kmem_cache_cpu. On PREEMPT_RT this would be incompatible with
the list_lock spinlock. We can use migrate_disable() instead, but that
increases overhead on !PREEMPT_RT as it's an unconditional function call even
though it's ultimately a migrate_disable() there.

In order to get the best available mechanism on both PREEMPT_RT and
!PREEMPT_RT, introduce private slub_get_cpu_ptr() and slub_put_cpu_ptr()
wrappers and use them.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
---
 mm/slub.c | 41 +++++++++++++++++++++++++++++++----------
 1 file changed, 31 insertions(+), 10 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 12e966f07f19..f0359b0c8154 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -115,6 +115,26 @@
  * 			the fast path and disables lockless freelists.
  */
 
+/*
+ * We could simply use migrate_disable()/enable() but as long as it's a
+ * 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)
+#else
+#define slub_get_cpu_ptr(var)		\
+({					\
+	migrate_disable();		\
+	this_cpu_ptr(var);		\
+})
+#define slub_put_cpu_ptr(var)		\
+do {					\
+	(void)(var);			\
+	migrate_enable();		\
+} while (0)
+#endif
+
 #ifdef CONFIG_SLUB_DEBUG
 #ifdef CONFIG_SLUB_DEBUG_ON
 DEFINE_STATIC_KEY_TRUE(slub_debug_enabled);
@@ -2419,7 +2439,7 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
 	int pages;
 	int pobjects;
 
-	preempt_disable();
+	slub_get_cpu_ptr(s->cpu_slab);
 	do {
 		pages = 0;
 		pobjects = 0;
@@ -2450,7 +2470,7 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
 
 	} while (this_cpu_cmpxchg(s->cpu_slab->partial, oldpage, page)
 								!= oldpage);
-	preempt_enable();
+	slub_put_cpu_ptr(s->cpu_slab);
 #endif	/* CONFIG_SLUB_CPU_PARTIAL */
 }
 
@@ -2759,7 +2779,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
 	if (unlikely(!pfmemalloc_match(page, gfpflags)))
 		goto deactivate_slab;
 
-	/* must check again c->page in case IRQ handler changed it */
+	/* must check again c->page in case we got preempted and it changed */
 	local_irq_save(flags);
 	if (unlikely(page != c->page)) {
 		local_irq_restore(flags);
@@ -2818,7 +2838,8 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
 		}
 		if (unlikely(!slub_percpu_partial(c))) {
 			local_irq_restore(flags);
-			goto new_objects; /* stolen by an IRQ handler */
+			/* we were preempted and partial list got empty */
+			goto new_objects;
 		}
 
 		page = c->page = slub_percpu_partial(c);
@@ -2834,9 +2855,9 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
 	if (freelist)
 		goto check_new_page;
 
-	put_cpu_ptr(s->cpu_slab);
+	slub_put_cpu_ptr(s->cpu_slab);
 	page = new_slab(s, gfpflags, node);
-	c = get_cpu_ptr(s->cpu_slab);
+	c = slub_get_cpu_ptr(s->cpu_slab);
 
 	if (unlikely(!page)) {
 		slab_out_of_memory(s, gfpflags, node);
@@ -2919,12 +2940,12 @@ static void *__slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
 	 * cpu before disabling preemption. Need to reload cpu area
 	 * pointer.
 	 */
-	c = get_cpu_ptr(s->cpu_slab);
+	c = slub_get_cpu_ptr(s->cpu_slab);
 #endif
 
 	p = ___slab_alloc(s, gfpflags, node, addr, c);
 #ifdef CONFIG_PREEMPTION
-	put_cpu_ptr(s->cpu_slab);
+	slub_put_cpu_ptr(s->cpu_slab);
 #endif
 	return p;
 }
@@ -3445,7 +3466,7 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
 	 * IRQs, which protects against PREEMPT and interrupts
 	 * handlers invoking normal fastpath.
 	 */
-	c = get_cpu_ptr(s->cpu_slab);
+	c = slub_get_cpu_ptr(s->cpu_slab);
 	local_irq_disable();
 
 	for (i = 0; i < size; i++) {
@@ -3491,7 +3512,7 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
 	}
 	c->tid = next_tid(c->tid);
 	local_irq_enable();
-	put_cpu_ptr(s->cpu_slab);
+	slub_put_cpu_ptr(s->cpu_slab);
 
 	/*
 	 * memcg and kmem_cache debug support and memory initialization.
-- 
2.32.0