linux-kernel - Re: [PATCH v5 01/14] slab: add opt-in caching layer of percpu sheaves

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Message-ID: <bbdb72e1-ada9-44e2-9ec1-53fd6ed92e84@suse.cz>
Date: Sun, 14 Sep 2025 22:24:19 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Hillf Danton <hdanton@...a.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 01/14] slab: add opt-in caching layer of percpu sheaves

On 9/14/25 04:22, Hillf Danton wrote:
> On Wed, 23 Jul 2025 15:34:34 +0200 Vlastimil Babka wrote:
>> +
>> +/*
>> + * Caller needs to make sure migration is disabled in order to fully flush
>> + * single cpu's sheaves
>> + *
> This misguides, see below for a workqueue case.
> 
>> + * must not be called from an irq
>> + *
>> + * flushing operations are rare so let's keep it simple and flush to slabs
>> + * directly, skipping the barn
>> + */
>> +static void pcs_flush_all(struct kmem_cache *s)
>> +{
>> +	struct slub_percpu_sheaves *pcs;
>> +	struct slab_sheaf *spare;
>> +
>> +	local_lock(&s->cpu_sheaves->lock);
>> +	pcs = this_cpu_ptr(s->cpu_sheaves);
>> +
>> +	spare = pcs->spare;
>> +	pcs->spare = NULL;
>> +
>> +	local_unlock(&s->cpu_sheaves->lock);
>> +
>> +	if (spare) {
>> +		sheaf_flush_unused(s, spare);
>> +		free_empty_sheaf(s, spare);
>> +	}
>> +
>> +	sheaf_flush_main(s);
>> +}
>> +
>> @@ -3326,30 +3755,18 @@ struct slub_flush_work {
>>  static void flush_cpu_slab(struct work_struct *w)
>>  {
>>  	struct kmem_cache *s;
>> -	struct kmem_cache_cpu *c;
>>  	struct slub_flush_work *sfw;
>>  
>>  	sfw = container_of(w, struct slub_flush_work, work);
>>  
>>  	s = sfw->s;
>> -	c = this_cpu_ptr(s->cpu_slab);
>>  
>> -	if (c->slab)
>> -		flush_slab(s, c);
>> +	if (s->cpu_sheaves)
>> +		pcs_flush_all(s);
>>  
> Migration is not disabled.

Can you elaborate how it's not? There's a comment above the function saying
"Called from CPU work handler with migration disabled." and we have relied
on this before sheaves. queue_work_on() says it will run on the specific
cpu. AFAIK the workqueue workers are bound which is effectively disabled
migration (we hold the cpu hotplug lock).

>> -	put_partials(s);
>> +	flush_this_cpu_slab(s);
>>  }