linux-kernel - Re: [PATCH v5 2/7] mm: memcontrol: convert NR_ANON

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Date:   Wed, 23 Dec 2020 14:08:00 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>, rafael@...nel.org,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Hugh Dickins <hughd@...gle.com>, Roman Gushchin <guro@...com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Feng Tang <feng.tang@...el.com>, Neil Brown <neilb@...e.de>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Randy Dunlap <rdunlap@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Cgroups <cgroups@...r.kernel.org>
Subject: Re: [PATCH v5 2/7] mm: memcontrol: convert NR_ANON_THPS account to pages

On Wed, Dec 16, 2020 at 7:45 PM Muchun Song <songmuchun@...edance.com> wrote:
>
> Currently we use struct per_cpu_nodestat to cache the vmstat
> counters, which leads to inaccurate statistics expecially THP

*especially

> vmstat counters. In the systems with hundreads of processors

*hundreds

> it can be GBs of memory. For example, for a 96 CPUs system,
> the threshold is the maximum number of 125. And the per cpu
> counters can cache 23.4375 GB in total.
>
> The THP page is already a form of batched addition (it will
> add 512 worth of memory in one go) so skipping the batching
> seems like sensible. Although every THP stats update overflows
> the per-cpu counter, resorting to atomic global updates. But
> it can make the statistics more accuracy for the THP vmstat
> counters.
>
> So we convert the NR_ANON_THPS account to pages. This patch
> is consistent with 8f182270dfec ("mm/swap.c: flush lru pvecs
> on compound page arrival"). Doing this also can make the unit
> of vmstat counters more unified. Finally, the unit of the vmstat
> counters are pages, kB and bytes. The B/KB suffix can tell us
> that the unit is bytes or kB. The rest which is without suffix
> are pages.
>
> Signed-off-by: Muchun Song <songmuchun@...edance.com>

I agree with the motivation behind this patch but I would like to see
some performance numbers in the commit message. We might agree to pay
the price but at least we will know what exactly that cost is.