linux-kernel - Re: [PATCH v5 6/9] slub: Delay freezing of partial slabs

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Date:   Wed, 22 Nov 2023 14:19:41 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Chengming Zhou <chengming.zhou@...ux.dev>,
        Mark Brown <broonie@...nel.org>
Cc:     cl@...ux.com, 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>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v5 6/9] slub: Delay freezing of partial slabs

On 11/22/23 12:54, Chengming Zhou wrote:
> On 2023/11/22 19:40, Vlastimil Babka wrote:
>> On 11/22/23 12:35, Chengming Zhou wrote:
>>> On 2023/11/22 17:37, Vlastimil Babka wrote:
>>>> On 11/20/23 19:49, Mark Brown wrote:
>>>>> On Thu, Nov 02, 2023 at 03:23:27AM +0000, chengming.zhou@...ux.dev wrote:
>>>>>> From: Chengming Zhou <zhouchengming@...edance.com>
>>>>>>
>>>>>> Now we will freeze slabs when moving them out of node partial list to
>>>>>> cpu partial list, this method needs two cmpxchg_double operations:
>>>>>>
>>>>>> 1. freeze slab (acquire_slab()) under the node list_lock
>>>>>> 2. get_freelist() when pick used in ___slab_alloc()
>>>>>
>>>>> Recently -next has been failing to boot on a Raspberry Pi 3 with an arm
>>>>> multi_v7_defconfig and a NFS rootfs, a bisect appears to point to this
>>>>> patch (in -next as c8d312e039030edab25836a326bcaeb2a3d4db14) as having
>>>>> introduced the issue.  I've included the full bisect log below.
>>>>>
>>>>> When we see problems we see RCU stalls while logging in, for example:
>>>>
>>>> Can you try this, please?
>>>>
>>>
>>> Great! I manually disabled __CMPXCHG_DOUBLE to reproduce the problem,
>>> and this patch can solve the machine hang problem.
>>>
>>> BTW, I also did the performance testcase on the machine with 128 CPUs.
>>>
>>> stress-ng --rawpkt 128 --rawpkt-ops 100000000
>>>
>>> base    patched
>>> 2.22s   2.35s
>>> 2.21s   3.14s
>>> 2.19s   4.75s
>>>
>>> Found this atomic version performance numbers are not stable.
>> 
>> That's weirdly too bad. Is that measured also with __CMPXCHG_DOUBLE
>> disabled, or just the patch? The PG_workingset flag change should be
> 
> The performance test is just the patch.
> 
>> uncontended as we are doing it under list_lock, and with __CMPXCHG_DOUBLE
>> there should be no interfering PG_locked interference.
>> 
> 
> Yes, I don't know. Maybe it's related with my kernel config, making the
> atomic operation much expensive? Will look again..

I doubt it can explain going from 2.19s to 4.75s, must have been some
interference on the machine?

> And I also tested the atomic-optional version like below, found the
> performance numbers are much stable.

This gets rather ugly and fragile so I'd maybe rather go back to the
__unused field approach :/

> diff --git a/mm/slub.c b/mm/slub.c
> index a307d319e82c..e11d34d51a14 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -531,7 +531,7 @@ static __always_inline void slab_unlock(struct slab *slab)
>         struct page *page = slab_page(slab);
> 
>         VM_BUG_ON_PAGE(PageTail(page), page);
> -       __bit_spin_unlock(PG_locked, &page->flags);
> +       bit_spin_unlock(PG_locked, &page->flags);
>  }
> 
>  static inline bool
> @@ -2136,12 +2136,18 @@ static inline bool slab_test_node_partial(const struct slab *slab)
> 
>  static inline void slab_set_node_partial(struct slab *slab)
>  {
> -       __set_bit(PG_workingset, folio_flags(slab_folio(slab), 0));
> +       if (slab->slab_cache->flags & __CMPXCHG_DOUBLE)
> +               __set_bit(PG_workingset, folio_flags(slab_folio(slab), 0));
> +       else
> +               set_bit(PG_workingset, folio_flags(slab_folio(slab), 0));
>  }
> 
>  static inline void slab_clear_node_partial(struct slab *slab)
>  {
> -       __clear_bit(PG_workingset, folio_flags(slab_folio(slab), 0));
> +       if (slab->slab_cache->flags & __CMPXCHG_DOUBLE)
> +               __clear_bit(PG_workingset, folio_flags(slab_folio(slab), 0));
> +       else
> +               clear_bit(PG_workingset, folio_flags(slab_folio(slab), 0));
>  }