linux-kernel - Re: [PATCH v4 1/9] slab: add opt-in caching layer of percpu sheaves

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Message-ID: <4fa1f4fb-4148-45e8-930c-1175ce212cab@suse.cz>
Date: Mon, 28 Apr 2025 09:01:48 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: "Christoph Lameter (Ampere)" <cl@...two.org>
Cc: Suren Baghdasaryan <surenb@...gle.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 David Rientjes <rientjes@...gle.com>,
 Roman Gushchin <roman.gushchin@...ux.dev>, Harry Yoo <harry.yoo@...cle.com>,
 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 v4 1/9] slab: add opt-in caching layer of percpu sheaves

On 4/25/25 19:31, Christoph Lameter (Ampere) wrote:
> On Fri, 25 Apr 2025, Vlastimil Babka wrote:
> 
>> @@ -4195,7 +4793,11 @@ static __fastpath_inline void *slab_alloc_node(struct kmem_cache *s, struct list
>>  	if (unlikely(object))
>>  		goto out;
>>
>> -	object = __slab_alloc_node(s, gfpflags, node, addr, orig_size);
>> +	if (s->cpu_sheaves && node == NUMA_NO_NODE)
>> +		object = alloc_from_pcs(s, gfpflags);
> 
> The node to use is determined in __slab_alloc_node() only based on the
> memory policy etc. NUMA_NO_NODE allocations can be redirected by memory
> policies and this check disables it.

To handle that, alloc_from_pcs() contains this:

#ifdef CONFIG_NUMA
        if (static_branch_unlikely(&strict_numa)) {
                if (current->mempolicy)
                        return NULL;
        }
#endif

And so there will be a fallback. It doesn't (currently) try to evaluate if
the local node is compatible as this is before taking the local lock (and
thus preventing migration).

>> @@ -4653,7 +5483,10 @@ void slab_free(struct kmem_cache *s, struct slab *slab, void *object,
>>  	memcg_slab_free_hook(s, slab, &object, 1);
>>  	alloc_tagging_slab_free_hook(s, slab, &object, 1);
>>
>> -	if (likely(slab_free_hook(s, object, slab_want_init_on_free(s), false)))
>> +	if (unlikely(!slab_free_hook(s, object, slab_want_init_on_free(s), false)))
>> +		return;
>> +
>> +	if (!s->cpu_sheaves || !free_to_pcs(s, object))
>>  		do_slab_free(s, slab, object, object, 1, addr);
>>  }
> 
> We free to pcs even if the object is remote?
>