linux-kernel - Re: [PATCH v3 0/2] slub: introduce count_partial_free

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Date: Mon, 22 Apr 2024 09:56:20 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Jianfeng Wang <jianfeng.w.wang@...cle.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Cc: cl@...ux.com, akpm@...ux-foundation.org, penberg@...nel.org,
 rientjes@...gle.com
Subject: Re: [PATCH v3 0/2] slub: introduce count_partial_free_approx()

On 4/19/24 7:56 PM, Jianfeng Wang wrote:
> This patch fixes a known issue in get_slabinfo() which relies on
> count_partial() to get the exact count of free objects in a
> kmem_cache_node's partial list. For some slubs, their partial lists
> can be extremely long. Currently, count_partial() traverses a partial
> list to get the exact count of objects. This process may take a long
> time, during which slab allocations are blocked and IRQs are disabled.
> In production, even NMI watchdog can be triggered due to this matter.
> 
> The proposed fix is to limit the number of slabs to scan and output an
> approximated count for a long partial list. The v1 patch counts N slabs
> from the list's head and then uses it to estimate the total object
> count in the list. As suggested by Vlastimil, the v2 patch uses an
> alternative, i.e., counting N/2 from the list's head and tail, produces
> a more accurate approximation after the partial list is sorted by
> kmem_cache_shrink(). In this version, the implementation is moved to a
> new function count_partial_free_approx(). count_partial() is still used
> in sysfs for users who still want the exact object count.

Added to slab/for-next, thanks!

> 
> ---
> Changes since v2 [2]
>  - Introduce count_partial_free_approx() and keep count_partial()
>  - Use count_partial_free_approx() in get_slabinfo() and slab_out_of_memory()
> 
> Changes since v1 [1]
>  - Update the approximation method by counting from the list's head and tail
>  - Cap the approximation by the total object count
>  - Update the commit message to add benchmark results and explain the choice
> 
> [1] https://lore.kernel.org/linux-mm/20240411164023.99368-1-jianfeng.w.wang@oracle.com/
> [2] https://lore.kernel.org/linux-mm/20240417185938.5237-2-jianfeng.w.wang@oracle.com/
> 
> Thanks,
> --Jianfeng
> 
> Jianfeng Wang (2):
>   slub: introduce count_partial_free_approx()
>   slub: use count_partial_free_approx() in slab_out_of_memory()
> 
>  mm/slub.c | 41 +++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 39 insertions(+), 2 deletions(-)
>