Re: [PATCH] slub: do not put a slab to cpu partial list when cpu_partial is 0

[Wanpeng Li <liwanp@linux.vnet.ibm.com>]

On Thu, Jun 20, 2013 at 10:44:40AM +0900, Joonsoo Kim wrote:
>On Thu, Jun 20, 2013 at 08:26:03AM +0800, Wanpeng Li wrote:
>> On Wed, Jun 19, 2013 at 05:52:50PM +0900, Joonsoo Kim wrote:
>> >On Wed, Jun 19, 2013 at 04:00:32PM +0800, Wanpeng Li wrote:
>> >> On Wed, Jun 19, 2013 at 03:33:55PM +0900, Joonsoo Kim wrote:
>> >> >In free path, we don't check number of cpu_partial, so one slab can
>> >> >be linked in cpu partial list even if cpu_partial is 0. To prevent this,
>> >> >we should check number of cpu_partial in put_cpu_partial().
>> >> >
>> >> 
>> >> How about skip get_partial entirely? put_cpu_partial is called 
>> >> in two paths, one is during refill cpu partial lists in alloc 
>> >> slow path, the other is in free slow path. And cpu_partial is 0 
>> >> just in debug mode. 
>> >> 
>> >> - alloc slow path, there is unnecessary to call get_partial 
>> >>   since cpu partial lists won't be used in debug mode. 
>> >> - free slow patch, new.inuse won't be true in debug mode 
>> >>   which lead to put_cpu_partial won't be called.
>> >> 
>> >
>> >In debug mode, put_cpu_partial() can't be called already on both path.
>> >But, if we assign 0 to cpu_partial via sysfs, put_cpu_partial() will be called
>> >on free slow path. On alloc slow path, it can't be called, because following
>> >test in get_partial_node() is always failed.
>> >
>> >available > s->cpu_partial / 2
>> 
>> Is it always true? We can freeze slab from partial list, and 
>> s->cpu_partial is 0. 
>
>Do you mean node partial list?
>

Yup.

>At first, acquire_slab() is called for a cpu slab(not cpu partial list)
>in get_partial_node(), and then, check above test. In this time, available
>is always higher than 0, so, if we assign 0 to s->cpu_partial, we break
>the loop and we don't try to get a slab for cpu partial list.
>

Agreed. 

Reviewed-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>

>Thanks.
>
>> 
>> Regards,
>> Wanpeng Li 
>> 
>> >
>> >Thanks.
>> >
>> >> Regards,
>> >> Wanpeng Li 
>> >> 
>> >> >Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>> >> >
>> >> >diff --git a/mm/slub.c b/mm/slub.c
>> >> >index 57707f0..7033b4f 100644
>> >> >--- a/mm/slub.c
>> >> >+++ b/mm/slub.c
>> >> >@@ -1955,6 +1955,9 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
>> >> > 	int pages;
>> >> > 	int pobjects;
>> >> >
>> >> >+	if (!s->cpu_partial)
>> >> >+		return;
>> >> >+
>> >> > 	do {
>> >> > 		pages = 0;
>> >> > 		pobjects = 0;
>> >> >-- 
>> >> >1.7.9.5
>> >> >
>> >> >--
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