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

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

在 2023/3/8 21:37, Hyeonggon Yoo 写道:
> On Wed, Mar 08, 2023 at 07:16:49AM +0000, chenjun (AM) wrote:
>> 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;
>>>> +

Any suggestions to make the change more elegant?

>>>>    	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;
> 
> I think it should be try_thisnode = 1?
> Otherwise it won't be executed at all.
> Also bool type will be more readable than int.
> 
>>
>>>
>>> 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.
> 
> You are right.
> 
> Hmm then the new behavior when
> (node != NUMA_NO_NODE) && (gfpflags & __GFP_THISNODE) is:
> 
> 1) try to get a partial slab from target node with __GFP_THISNODE
> 2) if 1) failed, try to allocate a new slab from target node with __GFP_THISNODE
> 3) if 2) failed, retry 1) and 2) without __GFP_THISNODE constraint
> 
> when node != NUMA_NO_NODE || (gfpflags & __GFP_THISNODE), the behavior
> remains unchanged.
> 
> It does not look that crazy to me, although it complicates the code
> a little bit. (Vlastimil may have some opinions?)
> 
> Now that I understand your intention, I think this behavior change also
> need to be added to the commit log.
> 

I will add it.

> Thanks,
> Hyeonggon
> 
>> 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.
> 

If there are no other questions, I will send an official patch.