linux-kernel - Re: [RFC PATCH 1/5] mm, slab/slub: introduce kmalloc-reclaimable caches

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Date:   Mon, 28 May 2018 10:03:48 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Christopher Lameter <cl@...ux.com>
Cc:     linux-mm@...ck.org, Roman Gushchin <guro@...com>,
        Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Vijayanand Jitta <vjitta@...eaurora.org>
Subject: Re: [RFC PATCH 1/5] mm, slab/slub: introduce kmalloc-reclaimable
 caches

On 05/25/2018 05:51 PM, Christopher Lameter wrote:
> On Thu, 24 May 2018, Vlastimil Babka wrote:
> 
>> diff --git a/include/linux/slab.h b/include/linux/slab.h
>> index 9ebe659bd4a5..5bff0571b360 100644
>> --- a/include/linux/slab.h
>> +++ b/include/linux/slab.h
>> @@ -296,11 +296,16 @@ static inline void __check_heap_object(const void *ptr, unsigned long n,
>>                                 (KMALLOC_MIN_SIZE) : 16)
>>
>>  #ifndef CONFIG_SLOB
>> -extern struct kmem_cache *kmalloc_caches[KMALLOC_SHIFT_HIGH + 1];
>> +extern struct kmem_cache *kmalloc_caches[2][KMALLOC_SHIFT_HIGH + 1];
>>  #ifdef CONFIG_ZONE_DMA
>>  extern struct kmem_cache *kmalloc_dma_caches[KMALLOC_SHIFT_HIGH + 1];
>>  #endif
> 
> In the existing code we used a different array name for the DMA caches.
> This is a similar situation.
> 
> I would suggest to use
> 
> kmalloc_reclaimable_caches[]
> 
> or make it consistent by folding the DMA caches into the array too (but
> then note the issues below).
> 
>> @@ -536,12 +541,13 @@ static __always_inline void *kmalloc(size_t size, gfp_t flags)
>>  #ifndef CONFIG_SLOB
>>  		if (!(flags & GFP_DMA)) {
>>  			unsigned int index = kmalloc_index(size);
>> +			unsigned int recl = kmalloc_reclaimable(flags);
> 
> This is a hotpath reserved for regular allocations. The reclaimable slabs
> need to be handled like the DMA slabs.  So check for GFP_DMA plus the
> reclaimable flags.

Yeah I thought that by doing reclaimable via array index manipulation
and not a branch, there would be no noticeable overhead. And GFP_DMA
should go away eventually. I will see if I can convert GFP_DMA to
another index, and completely remove the branch quoted above.

>> @@ -588,12 +594,13 @@ static __always_inline void *kmalloc_node(size_t size, gfp_t flags, int node)
>>  	if (__builtin_constant_p(size) &&
>>  		size <= KMALLOC_MAX_CACHE_SIZE && !(flags & GFP_DMA)) {
>>  		unsigned int i = kmalloc_index(size);
>> +		unsigned int recl = kmalloc_reclaimable(flags);
>>
> 
> 
> Same situation here and additional times below.
>