Re: [PATCH v3 0/2] slub: introduce count_partial_free_approx()

From: Vlastimil Babka
Date: Mon Apr 22 2024 - 03:56:38 EST


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@xxxxxxxxxx/
> [2] https://lore.kernel.org/linux-mm/20240417185938.5237-2-jianfeng.w.wang@xxxxxxxxxx/
>
> 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(-)
>