linux-kernel - Re: [PATCH 1/3] mm/slub: directly load freelist from cpu partial slab in the likely case

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Date: Tue, 23 Jan 2024 09:24:31 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Chengming Zhou <zhouchengming@...edance.com>,
 "Christoph Lameter (Ampere)" <cl@...ux.com>
Cc: Hyeonggon Yoo <42.hyeyoo@...il.com>, Joonsoo Kim
 <iamjoonsoo.kim@....com>, Pekka Enberg <penberg@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] mm/slub: directly load freelist from cpu partial slab
 in the likely case

On 1/23/24 03:51, Chengming Zhou wrote:
> On 2024/1/23 01:13, Vlastimil Babka wrote:
>> On 1/19/24 04:53, Chengming Zhou wrote:
>>> On 2024/1/19 06:14, Christoph Lameter (Ampere) wrote:
>>>> On Thu, 18 Jan 2024, Chengming Zhou wrote:
>>>>
>>>>> So get_freelist() has two cases to handle: cpu slab and cpu partial list slab.
>>>>> The latter is NOT frozen, so need to remove "VM_BUG_ON(!new.frozen)" from it.
>>>>
>>>> Right so keep the check if it is the former?
>>>>
>>>
>>> Ok, I get it. Maybe like this:
>> 
>> I think that's just too ugly for a VM_BUG_ON(). I'd just remove the check
>> and be done with that.
> 
> Ok with me.
> 
>> 
>> I have a somewhat different point. You reused get_freelist() but in fact
>> it's more like freeze_slab(), but that one uses slab_update_freelist() and
>> we are under the local_lock so we want the cheaper __slab_update_freelist(),
>> which get_freelist() has and I guess that's why you reused that one.
> 
> Right, we already have the lock_lock, so reuse get_freelist().
> 
>> 
>> However get_freelist() also assumes it can return NULL if the freelist is
>> empty. If that's possible to happen on the percpu partial list, we should
>> not "goto load_freelist;" but rather create a new label above that, above
>> the "if (!freelist) {" block that handles the case.
>> 
>> If that's not possible to happen (needs careful audit) and we have guarantee
> 
> Yes, it's not possible for now.
> 
>> that slabs on percpu partial list must have non-empty freelist, then we
>> probably instead want a new __freeze_slab() variant that is like
>> freeze_slab(), but uses __slab_update_freelist() and probably also has
>> VM_BUG_ON(!freelist) before returning it?
>> 
> 
> Instead of introducing another new function, how about still reusing get_freelist()
> and VM_BUG_ON(!freelist) after calling it? I feel this is simpler.

Could you measure if introducing new function that sets new.frozen = 1; has
any performance benefit? If not, we can reuse get_freelist() as you say.
Thanks!

> Thanks!