Re: [RFC][PATCH] mm/slub: Skip CPU slab activation when debugging

[Laura Abbott <labbott@redhat.com>]

On 03/28/2016 06:52 PM, Laura Abbott wrote:
> On 03/28/2016 03:53 PM, Laura Abbott wrote:
>> The per-cpu slab is designed to be the primary path for allocation in SLUB
>> since it assumed allocations will go through the fast path if possible.
>> When debugging is enabled, the fast path is disabled and per-cpu
>> allocations are not used. The current debugging code path still activates
>> the cpu slab for allocations and then immediately deactivates it. This
>> is useless work. When a slab is enabled for debugging, skip cpu
>> activation.
>>
>> Signed-off-by: Laura Abbott <labbott@fedoraproject.org>
>> ---
>> This is a follow on to the optimization of the debug paths for poisoning
>> With this I get ~2 second drop on hackbench -g 20 -l 1000 with slub_debug=P
>> and no noticable change with slub_debug=- .
>
> zero day robot pointed out this is triggering one of the BUG_ON on bootup.
> I'll take a deeper look tomorrow unless the approach is actually worthless.
>> ---
>>   mm/slub.c | 82 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++----
>>   1 file changed, 77 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 7277413..4507bd8 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -1482,8 +1482,8 @@ static struct page *allocate_slab(struct kmem_cache *s, gfp_t flags, int node)
>>       }
>>
>>       page->freelist = fixup_red_left(s, start);
>> -    page->inuse = page->objects;
>> -    page->frozen = 1;
>> +    page->inuse = kmem_cache_debug(s) ? 1 : page->objects;
>> +    page->frozen = kmem_cache_debug(s) ? 0 : 1;
>>
>>   out:
>>       if (gfpflags_allow_blocking(flags))
>> @@ -1658,6 +1658,64 @@ static inline void *acquire_slab(struct kmem_cache *s,
>>       return freelist;
>>   }
>>
>> +
>> +static inline void *acquire_slab_debug(struct kmem_cache *s,
>> +        struct kmem_cache_node *n, struct page *page,
>> +        int mode, int *objects)
>> +{
>> +    void *freelist;
>> +    unsigned long counters;
>> +    struct page new;
>> +    void *next;
>> +
>> +    lockdep_assert_held(&n->list_lock);
>> +
>> +
>> +    /*
>> +     * Zap the freelist and set the frozen bit.
>> +     * The old freelist is the list of objects for the
>> +     * per cpu allocation list.
>> +     */
>> +    freelist = page->freelist;
>> +    counters = page->counters;
>> +
>> +    BUG_ON(!freelist);
>> +
>> +    next = get_freepointer_safe(s, freelist);
>> +
>> +    new.counters = counters;
>> +    *objects = new.objects - new.inuse;
>> +    if (mode) {
>> +        new.inuse++;
>> +        new.freelist = next;
>> +    } else {
>> +        BUG();
>> +    }
>> +
>> +    VM_BUG_ON(new.frozen);
>> +
>> +    if (!new.freelist) {
>> +        remove_partial(n, page);
>> +        add_full(s, n, page);
>> +    }
>> +
>> +    if (!__cmpxchg_double_slab(s, page,
>> +            freelist, counters,
>> +            new.freelist, new.counters,
>> +            "acquire_slab")) {
>> +        if (!new.freelist) {
>> +            remove_full(s, n, page);
>> +            add_partial(n, page, DEACTIVATE_TO_HEAD);
>> +        }
>> +        return NULL;
>> +    }
>> +
>> +    WARN_ON(!freelist);
>> +    return freelist;
>> +}
>> +
>> +
>> +
>>   static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain);
>>   static inline bool pfmemalloc_match(struct page *page, gfp_t gfpflags);
>>
>> @@ -1688,7 +1746,11 @@ static void *get_partial_node(struct kmem_cache *s, struct kmem_cache_node *n,
>>           if (!pfmemalloc_match(page, flags))
>>               continue;
>>
>> -        t = acquire_slab(s, n, page, object == NULL, &objects);
>> +        if (kmem_cache_debug(s))
>> +            t = acquire_slab_debug(s, n, page, object == NULL, &objects);
>> +        else
>> +            t = acquire_slab(s, n, page, object == NULL, &objects);
>> +
>>           if (!t)
>>               break;
>>
>> @@ -2284,7 +2346,17 @@ static inline void *new_slab_objects(struct kmem_cache *s, gfp_t flags,
>>            * muck around with it freely without cmpxchg
>>            */
>>           freelist = page->freelist;
>> -        page->freelist = NULL;
>> +        page->freelist = kmem_cache_debug(s) ?
>> +                get_freepointer(s, freelist) : NULL;
>> +
>> +        if (kmem_cache_debug(s)) {
>> +            struct kmem_cache_node *n;
>> +
>> +            n = get_node(s, page_to_nid(page));
>> +            spin_lock(&n->list_lock);
>> +            add_partial(n, page, DEACTIVATE_TO_HEAD);
>> +            spin_unlock(&n->list_lock);
>> +        }

This needs to account for slabs full after one object, otherwise it bugs out on the
partial list.

>>
>>           stat(s, ALLOC_SLAB);
>>           c->page = page;
>> @@ -2446,7 +2518,7 @@ new_slab:
>>               !alloc_debug_processing(s, page, freelist, addr))
>>           goto new_slab;    /* Slab failed checks. Next slab needed */
>>
>> -    deactivate_slab(s, page, get_freepointer(s, freelist));
>> +    /* No need to deactivate, no cpu slab */
>>       c->page = NULL;
>>       c->freelist = NULL;
>>       return freelist;
>>
>

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