linux-kernel - Re: [PATCH 1/2] mm/slub: Introduce two counters for the partial objects

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Date:   Thu, 2 Jul 2020 14:59:07 +0300
From:   Pekka Enberg <penberg@...il.com>
To:     Xunlei Pang <xlpang@...ux.alibaba.com>
Cc:     Christoph Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Wen Yang <wenyang@...ux.alibaba.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Roman Gushchin <guro@...com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm/slub: Introduce two counters for the partial objects

On Thu, Jul 2, 2020 at 11:32 AM Xunlei Pang <xlpang@...ux.alibaba.com> wrote:
> The node list_lock in count_partial() spend long time iterating
> in case of large amount of partial page lists, which can cause
> thunder herd effect to the list_lock contention, e.g. it cause
> business response-time jitters when accessing "/proc/slabinfo"
> in our production environments.

Would you have any numbers to share to quantify this jitter? I have no
objections to this approach, but I think the original design
deliberately made reading "/proc/slabinfo" more expensive to avoid
atomic operations in the allocation/deallocation paths. It would be
good to understand what is the gain of this approach before we switch
to it. Maybe even run some slab-related benchmark (not sure if there's
something better than hackbench these days) to see if the overhead of
this approach shows up.

> This patch introduces two counters to maintain the actual number
> of partial objects dynamically instead of iterating the partial
> page lists with list_lock held.
>
> New counters of kmem_cache_node are: pfree_objects, ptotal_objects.
> The main operations are under list_lock in slow path, its performance
> impact is minimal.
>
> Co-developed-by: Wen Yang <wenyang@...ux.alibaba.com>
> Signed-off-by: Xunlei Pang <xlpang@...ux.alibaba.com>
> ---
>  mm/slab.h |  2 ++
>  mm/slub.c | 38 +++++++++++++++++++++++++++++++++++++-
>  2 files changed, 39 insertions(+), 1 deletion(-)
>
> diff --git a/mm/slab.h b/mm/slab.h
> index 7e94700..5935749 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -616,6 +616,8 @@ struct kmem_cache_node {
>  #ifdef CONFIG_SLUB
>         unsigned long nr_partial;
>         struct list_head partial;
> +       atomic_long_t pfree_objects; /* partial free objects */
> +       atomic_long_t ptotal_objects; /* partial total objects */

You could rename these to "nr_partial_free_objs" and
"nr_partial_total_objs" for readability.

- Pekka