linux-kernel - Re: [PATCH RFC v3 2/8] slab: add opt-in caching layer of percpu sheaves

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Message-ID: <81ffcfee-8f18-4392-a9ce-ff3f60f7b5b1@suse.cz>
Date: Thu, 3 Apr 2025 16:11:22 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Harry Yoo <harry.yoo@...cle.com>
Cc: Suren Baghdasaryan <surenb@...gle.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, Christoph Lameter
 <cl@...ux.com>, David Rientjes <rientjes@...gle.com>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Uladzislau Rezki <urezki@...il.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
 maple-tree@...ts.infradead.org
Subject: Re: [PATCH RFC v3 2/8] slab: add opt-in caching layer of percpu
 sheaves

On 4/3/25 10:31, Harry Yoo wrote:
>> +/*
>> + * Bulk free objects to the percpu sheaves.
>> + * Unlike free_to_pcs() this includes the calls to all necessary hooks
>> + * and the fallback to freeing to slab pages.
>> + */
>> +static void free_to_pcs_bulk(struct kmem_cache *s, size_t size, void **p)
>> +{
> 
> [...snip...]
> 
>> +next_batch:
>> +	if (!localtry_trylock(&s->cpu_sheaves->lock))
>> +		goto fallback;
>> +
>> +	pcs = this_cpu_ptr(s->cpu_sheaves);
>> +
>> +	if (unlikely(pcs->main->size == s->sheaf_capacity)) {
>> +
>> +		struct slab_sheaf *empty;
>> +
>> +		if (!pcs->spare) {
>> +			empty = barn_get_empty_sheaf(pcs->barn);
>> +			if (empty) {
>> +				pcs->spare = pcs->main;
>> +				pcs->main = empty;
>> +				goto do_free;
>> +			}
>> +			goto no_empty;
> 
> Maybe a silly question, but if neither of alloc_from_pcs_bulk() or
> free_to_pcs_bulk() allocates empty sheaves (and sometimes put empty or full
> sheaves in the barn), you should expect usually sheaves not to be in the barn
> when using bulk interfces?

Hm maybe, but with patch 5/8 it becomes cheap to check? And there might be
caches mixing both bulk and individual allocs?
But maybe I should at least add the free sheaf alloc with GFP_NOWAIT attempt
to bulk free? Can't recall if I missed it intentionally or forgot.

>> -static void
>> -init_kmem_cache_node(struct kmem_cache_node *n)
>> +static bool
>> +init_kmem_cache_node(struct kmem_cache_node *n, struct node_barn *barn)
>>  {
> 
> Why is the return type bool, when it always succeeds?

I guess leftover from earlier versions. Will fix.

>> @@ -5421,20 +6295,27 @@ static int init_kmem_cache_nodes(struct kmem_cache *s)
>>  
>>  	for_each_node_mask(node, slab_nodes) {
>>  		struct kmem_cache_node *n;
>> +		struct node_barn *barn = NULL;
>>  
>>  		if (slab_state == DOWN) {
>>  			early_kmem_cache_node_alloc(node);
>>  			continue;
>>  		}
>> +
>> +		if (s->cpu_sheaves) {
>> +			barn = kmalloc_node(sizeof(*barn), GFP_KERNEL, node);
>> +
>> +			if (!barn)
>> +				return 0;
>> +		}
>> +
>>  		n = kmem_cache_alloc_node(kmem_cache_node,
>>  						GFP_KERNEL, node);
>> -
>> -		if (!n) {
>> -			free_kmem_cache_nodes(s);
>> +		if (!n)
>>  			return 0;
>> -		}
> 
> Looks like it's leaking the barn
> if the allocation of kmem_cache_node fails?

Oops right, will add kfree(barn) before return 0;

> 
>> -		init_kmem_cache_node(n);
>> +		init_kmem_cache_node(n, barn);
>> +
>>  		s->node[node] = n;
>>  	}
>>  	return 1;
>> @@ -6005,12 +6891,24 @@ static int slab_mem_going_online_callback(void *arg)
>>  	 */
>>  	mutex_lock(&slab_mutex);
>>  	list_for_each_entry(s, &slab_caches, list) {
>> +		struct node_barn *barn = NULL;
>> +
>>  		/*
>>  		 * The structure may already exist if the node was previously
>>  		 * onlined and offlined.
>>  		 */
>>  		if (get_node(s, nid))
>>  			continue;
>> +
>> +		if (s->cpu_sheaves) {
>> +			barn = kmalloc_node(sizeof(*barn), GFP_KERNEL, nid);
>> +
>> +			if (!barn) {
>> +				ret = -ENOMEM;
>> +				goto out;
>> +			}
>> +		}
>> +
> 
> Ditto.
> 
> Otherwise looks good to me :)

Thanks a lot!