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From:   Pekka Enberg <penberg@gmail.com>
Date:   Tue, 7 Jul 2020 18:23:46 +0300
Message-ID: <CAOJsxLFnY=4v6UQigyiZKTMTQXKakVOKf6KA+bCkMe-XVY6sqA@mail.gmail.com>
Subject: Re: [PATCH 1/2] mm/slub: Introduce two counters for the partial objects
To:     Xunlei Pang <xlpang@linux.alibaba.com>
Cc:     Christoph Lameter <cl@linux.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Wen Yang <wenyang@linux.alibaba.com>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        Roman Gushchin <guro@fb.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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Hi!

(Sorry for the delay, I missed your response.)

On Fri, Jul 3, 2020 at 12:38 PM xunlei <xlpang@linux.alibaba.com> wrote:
>
> On 2020/7/2 PM 7:59, Pekka Enberg wrote:
> > On Thu, Jul 2, 2020 at 11:32 AM Xunlei Pang <xlpang@linux.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
>
> We have HSF RT(High-speed Service Framework Response-Time) monitors, the
> RT figures fluctuated randomly, then we deployed a tool detecting "irq
> off" and "preempt off" to dump the culprit's calltrace, capturing the
> list_lock cost up to 100ms with irq off issued by "ss", this also caused
> network timeouts.

Thanks for the follow up. This sounds like a good enough motivation
for this patch, but please include it in the changelog.

> > 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.
>
> I thought that before, but most atomic operations are serialized by the
> list_lock. Another possible way is to hold list_lock in __slab_free(),
> then these two counters can be changed from atomic to long.
>
> I also have no idea what's the standard SLUB benchmark for the
> regression test, any specific suggestion?

I don't know what people use these days. When I did benchmarking in
the past, hackbench and netperf were known to be slab-allocation
intensive macro-benchmarks. Christoph also had some SLUB
micro-benchmarks, but I don't think we ever merged them into the tree.

- Pekka