Re: [RFC] mm/slub: Reduce memory consumption in extreme scenarios

["chenjun (AM)" <chenjun102@huawei.com>]

Hi,

Thanks for reply.

在 2023/3/7 22:20, Hyeonggon Yoo 写道:
> On Tue, Mar 07, 2023 at 08:28:11AM +0000, Chen Jun wrote:
>> If call kmalloc_node with NO __GFP_THISNODE and node[A] with no memory.
>> Slub will alloc a slub page which is not belong to A, and put the page
>> to kmem_cache_node[page_to_nid(page)]. The page can not be reused
>> at next calling, because NULL will be get from get_partical().
>> That make kmalloc_node consume more memory.
> 
> Hello,
> 
> elaborating a little bit:
> 
> "When kmalloc_node() is called without __GFP_THISNODE and the target node
> lacks sufficient memory, SLUB allocates a folio from a different node other
> than the requested node, instead of taking a partial slab from it.
> 
> However, since the allocated folio does not belong to the requested node,
> it is deactivated and added to the partial slab list of the node it
> belongs to.
> 
> This behavior can result in excessive memory usage when the requested
> node has insufficient memory, as SLUB will repeatedly allocate folios from
> other nodes without reusing the previously allocated ones.
> 
> To prevent memory wastage, take a partial slab from a different node when
> the requested node has no partial slab and __GFP_THISNODE is not explicitly
> specified."
> 

Thanks, This is more clear than what I described.

>> On qemu with 4 numas and each numa has 1G memory, Write a test ko
>> to call kmalloc_node(196, 0xd20c0, 3) for 5 * 1024 * 1024 times.
>>
>> cat /proc/slabinfo shows:
>> kmalloc-256       4302317 15151808    256   32    2 : tunables..
>>
>> the total objects is much more then active objects.
>>
>> After this patch, cat /prac/slubinfo shows:
>> kmalloc-256       5244950 5245088    256   32    2 : tunables..
>>
>> Signed-off-by: Chen Jun <chenjun102@huawei.com>
>> ---
>>   mm/slub.c | 17 ++++++++++++++---
>>   1 file changed, 14 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 39327e98fce3..c0090a5de54e 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -2384,7 +2384,7 @@ static void *get_partial(struct kmem_cache *s, int node, struct partial_context
>>   		searchnode = numa_mem_id();
>>   
>>   	object = get_partial_node(s, get_node(s, searchnode), pc);
>> -	if (object || node != NUMA_NO_NODE)
>> +	if (object || (node != NUMA_NO_NODE && (pc->flags & __GFP_THISNODE)))
>>   		return object;
> 
> I think the problem here is to avoid taking a partial slab from
> different node than the requested node even if __GFP_THISNODE is not set.
> (and then allocating new slab instead)
> 
> Thus this hunk makes sense to me,
> even if SLUB currently do not implement __GFP_THISNODE semantics.
> 
>>   	return get_any_partial(s, pc);
>> @@ -3069,6 +3069,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
>>   	struct slab *slab;
>>   	unsigned long flags;
>>   	struct partial_context pc;
>> +	int try_thisndoe = 0;
>>
>>   
>>   	stat(s, ALLOC_SLOWPATH);
>>   
>> @@ -3181,8 +3182,12 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
>>   	}
>>   
>>   new_objects:
>> -
>>   	pc.flags = gfpflags;
>> +
>> +	/* Try to get page from specific node even if __GFP_THISNODE is not set */
>> +	if (node != NUMA_NO_NODE && !(gfpflags & __GFP_THISNODE) && try_thisnode)
>> +			pc.flags |= __GFP_THISNODE;
>> +
>>   	pc.slab = &slab;
>>   	pc.orig_size = orig_size;
>>   	freelist = get_partial(s, node, &pc);
>> @@ -3190,10 +3195,16 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
>>   		goto check_new_slab;
>>   
>>   	slub_put_cpu_ptr(s->cpu_slab);
>> -	slab = new_slab(s, gfpflags, node);
>> +	slab = new_slab(s, pc.flags, node);
>>   	c = slub_get_cpu_ptr(s->cpu_slab);
>>   
>>   	if (unlikely(!slab)) {
>> +		/* Try to get page from any other node */
>> +		if (node != NUMA_NO_NODE && !(gfpflags & __GFP_THISNODE) && try_thisnode) {
>> +			try_thisnode = 0;
>> +			goto new_objects;
>> +		}
>> +
>>   		slab_out_of_memory(s, gfpflags, node);
>>   		return NULL;
> 
> But these hunks do not make sense to me.
> Why force __GFP_THISNODE even when the caller did not specify it?
> 
> (Apart from the fact that try_thisnode is defined as try_thisndoe,
>   and try_thisnode is never set to nonzero value.)

My mistake， It should be:
int try_thisnode = 0;

> 
> IMHO the first hunk is enough to solve the problem.

I think, we should try to alloc a page on the target node before getting 
one from other nodes' partial.

If the caller does not specify __GFP_THISNODE, we add __GFP_THISNODE to 
try to get the slab only on the target node. If it fails, use the 
original GFP FLAG to allow fallback.

> 
> Thanks,
> Hyeonggon
> 
>>   	}
>> -- 
>> 2.17.1
>>
>>
> 

Thanks,
Chen Jun