linux-kernel - Re: [memcg] 45208c9105: aim7.jobs-per-min -14.0% regression

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Date:   Thu, 9 Sep 2021 17:43:40 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     kernel test robot <oliver.sang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        0day robot <lkp@...el.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Hillf Danton <hdanton@...a.com>,
        Huang Ying <ying.huang@...el.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        "Michal Koutn??" <mkoutny@...e.com>,
        Muchun Song <songmuchun@...edance.com>,
        Roman Gushchin <guro@...com>, Tejun Heo <tj@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        Xing Zhengjun <zhengjun.xing@...ux.intel.com>,
        Linux MM <linux-mm@...ck.org>, mm-commits@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [memcg] 45208c9105: aim7.jobs-per-min -14.0% regression

On Mon, Sep 6, 2021 at 8:30 PM Feng Tang <feng.tang@...el.com> wrote:
>
> Hi Shakeel,
>
> On Sun, Sep 05, 2021 at 03:15:46PM -0700, Shakeel Butt wrote:
> > On Sun, Sep 5, 2021 at 5:27 AM kernel test robot <oliver.sang@...el.com> wrote:
> [...]
> > > =========================================================================================
> > > compiler/cpufreq_governor/disk/fs/kconfig/load/rootfs/tbox_group/test/testcase/ucode:
> > >   gcc-9/performance/1BRD_48G/xfs/x86_64-rhel-8.3/3000/debian-10.4-x86_64-20200603.cgz/lkp-icl-2sp2/disk_rr/aim7/0xd000280
> > >
> > > commit:
> > >   3c28c7680e ("memcg: switch lruvec stats to rstat")
> > >   45208c9105 ("memcg: infrastructure to flush memcg stats")
> >
> > I am looking into this. I was hoping we have resolution for [1] as
> > these patches touch similar data structures.
> >
> > [1] https://lore.kernel.org/all/20210811031734.GA5193@xsang-OptiPlex-9020/T/#u
>
> I tried 2 debug methods for that 36.4% vm-scalability regression:
>
> 1. Disable the HW cache prefetcher, no effect on this case
> 2. relayout and add padding to 'struct cgroup_subsys_state', reduce
>    the regression to 3.1%
>

Thanks Feng but it seems like the issue for this commit is different.
Rearranging the layout didn't help. Actually the cause of slowdown is
the call to queue_work() inside __mod_memcg_lruvec_state().

At the moment, queue_work() is called after 32 updates. I changed it
to 128 and the slowdown of will-it-scale:page_fault[1|2|3] halved
(from around 10% to 5%). I am unable to run reaim or
will-it-scale:fallocate2 as I was getting weird errors.

Feng, is it possible for you to run these benchmarks with the change
(basically changing MEMCG_CHARGE_BATCH to 128 in the if condition
before queue_work() inside __mod_memcg_lruvec_state())?

For the formal patch/fix, I will write down a better explanation on
what should be the batch size.

thanks,
Shakeel