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

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

在 2023/3/14 22:41, Vlastimil Babka 写道:
> 
> On 3/14/23 13:34, Chen Jun wrote:
>> 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,
>> 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.
>>
>> On qemu with 4 numa nodes and each numa has 1G memory. Write a test ko
>> to call kmalloc_node(196, GFP_KERNEL, 3) for (4 * 1024 + 4) * 1024 times.
>>
>> cat /proc/slabinfo shows:
>> kmalloc-256       4200530 13519712    256   32    2 : tunables..
>>
>> after this patch,
>> cat /proc/slabinfo shows:
>> kmalloc-256       4200558 4200768    256   32    2 : tunables..
>>
>> Signed-off-by: Chen Jun <chenjun102@huawei.com>
>> ---
>>   mm/slub.c | 22 +++++++++++++++++++---
>>   1 file changed, 19 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 39327e98fce3..32e436957e03 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;
>>   
>>   	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;
>> +	bool try_thisnode = true;
>>   
>>   	stat(s, ALLOC_SLOWPATH);
>>   
>> @@ -3181,8 +3182,18 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
>>   	}
>>   
>>   new_objects:
>> -
>>   	pc.flags = gfpflags;
>> +
>> +	/*
>> +	 * when (node != NUMA_NO_NODE) && (gfpflags & __GFP_THISNODE)
>> +	 * 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.
>> +	 */
>> +	if (node != NUMA_NO_NODE && !(gfpflags & __GFP_THISNODE) && try_thisnode)
>> +			pc.flags |= __GFP_THISNODE;
> 
> Hmm I'm thinking we should also perhaps remove direct reclaim possibilities
> from the attempt 2). In your qemu test it should make no difference, as it
> fills everything with kernel memory that is not reclaimable. But in practice
> the target node might be filled with user memory, and I think it's better to
> quickly allocate on a different node than spend time in direct reclaim. So
> the following should work I think?
> 
> pc.flags = GFP_NOWAIT | __GFP_NOWARN |__GFP_THISNODE
> 

Hmm, Should it be that:

pc.flags |= GFP_NOWAIT | __GFP_NOWARN |__GFP_THISNODE
          ^
>> +
>>   	pc.slab = &slab;
>>   	pc.orig_size = orig_size;
>>   	freelist = get_partial(s, node, &pc);
>> @@ -3190,10 +3201,15 @@ 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)) {
>> +		if (node != NUMA_NO_NODE && !(gfpflags & __GFP_THISNODE) && try_thisnode) {
>> +			try_thisnode = false;
>> +			goto new_objects;
>> +		}
>> +
>>   		slab_out_of_memory(s, gfpflags, node);
>>   		return NULL;
>>   	}
> 
>