linux-kernel - Re: [RFC PATCH v3 0/7] slub: Delay freezing of CPU partial slabs

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Message-ID: <82797556-eb7b-4867-86a1-859d11598fc9@linux.dev>
Date:   Tue, 31 Oct 2023 12:57:23 +0800
From:   Chengming Zhou <chengming.zhou@...ux.dev>
To:     Christoph Lameter <cl@...ux.com>
Cc:     vbabka@...e.cz, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org,
        roman.gushchin@...ux.dev, 42.hyeyoo@...il.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Chengming Zhou <zhouchengming@...edance.com>
Subject: Re: [RFC PATCH v3 0/7] slub: Delay freezing of CPU partial slabs

On 2023/10/31 11:47, Christoph Lameter wrote:
> On Tue, 31 Oct 2023, Chengming Zhou wrote:
> 
>> The current scheme (which this series implemented) is:
>>
>> - node partial slabs: PG_Workingset (set or clear with per-node list_lock protection)
>> - cpu partial slabs: !PG_Workingset
> 
> And then the frozen flag needs to be set. Otherwise slab_free() would conclude it is on a partial list?
> 

- cpu partial slabs: !PG_Workingset && !frozen

Here comes the optimization that "frozen" is not set for the cpu partial slabs,
slab_free() will grab node list_lock then check by !PG_Workingset that it's not
on a node partial list.

>> - cpu slabs: !PG_Workingset && frozen (set or clear using cmpxchg together with freelist)
> 
> 
> 
>> - full slabs: !PG_Workingset
> 
> And frozen is clear? Otherwise it is the same as a cpu partial slab.
> 

Right, - full slabs: !PG_Workingset && !frozen

Yes, it's the same as a cpu partial slab from only these two flags, but
slab_free() also know whether it was full or not.

>> As Vlastimil noted, it's possible to drop "frozen" bit for cpu slabs, but
>> we keep it for performance, since we don't need to grab node list_lock to
>> check whether PG_Workingset is set or not if the "frozen" bit is set.
>>
>> Thanks!
>>