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

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Date:   Wed, 22 Nov 2023 19:35:56 +0800
From:   Chengming Zhou <chengming.zhou@...ux.dev>
To:     Vlastimil Babka <vbabka@...e.cz>, 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 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.

Should I change back to reuse the slab->__unused (mapcount) field?

Or should we check "s->flags & __CMPXCHG_DOUBLE" in slab_set/clear_node_partial()
to avoid using the atomic version?

Thanks!

> ----8<----
> From 000030c1ff055ef6a2ca624d0142f08f3ef19d51 Mon Sep 17 00:00:00 2001
> From: Vlastimil Babka <vbabka@...e.cz>
> Date: Wed, 22 Nov 2023 10:32:41 +0100
> Subject: [PATCH] mm/slub: try to fix hangs without cmpxchg64/128
> 
> If we don't have cmpxchg64/128 and resort to slab_lock()/slab_unlock()
> which uses PG_locked, we can get RMW with the newly introduced
> slab_set/clear_node_partial() operation that modify PG_workingset so all
> the operations have to be atomic now.
> 
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> ---
>  mm/slub.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index bcb5b2c4e213..f2cdb81ab02e 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -522,7 +522,7 @@ 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
> @@ -2127,12 +2127,12 @@ 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));
> +	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));
> +	clear_bit(PG_workingset, folio_flags(slab_folio(slab), 0));
>  }
>  
>  /*