linux-kernel - Re: [External] Re: [RESEND PATCH v2 00/12] Convert all vmstat counters to pages or bytes

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Date:   Mon, 7 Dec 2020 11:51:41 -0800
From:   Roman Gushchin <guro@...com>
To:     Michal Hocko <mhocko@...e.com>
CC:     Muchun Song <songmuchun@...edance.com>,
        Greg KH <gregkh@...uxfoundation.org>, <rafael@...nel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Hugh Dickins <hughd@...gle.com>, Will Deacon <will@...nel.org>,
        Mike Rapoport <rppt@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, <esyr@...hat.com>,
        <peterx@...hat.com>, <krisman@...labora.com>,
        Suren Baghdasaryan <surenb@...gle.com>, <avagin@...nvz.org>,
        Marco Elver <elver@...gle.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Cgroups <cgroups@...r.kernel.org>
Subject: Re: [External] Re: [RESEND PATCH v2 00/12] Convert all vmstat
 counters to pages or bytes

On Mon, Dec 07, 2020 at 04:02:54PM +0100, Michal Hocko wrote:
> On Mon 07-12-20 22:52:30, Muchun Song wrote:
> > On Mon, Dec 7, 2020 at 9:00 PM Michal Hocko <mhocko@...e.com> wrote:
> > >
> > > On Sun 06-12-20 18:14:39, Muchun Song wrote:
> > > > Hi,
> > > >
> > > > This patch series is aimed to convert all THP vmstat counters to pages
> > > > and some KiB vmstat counters to bytes.
> > > >
> > > > The unit of some vmstat counters are pages, some are bytes, some are
> > > > HPAGE_PMD_NR, and some are KiB. When we want to expose these vmstat
> > > > counters to the userspace, we have to know the unit of the vmstat counters
> > > > is which one. It makes the code complex. Because there are too many choices,
> > > > the probability of making a mistake will be greater.
> > > >
> > > > For example, the below is some bug fix:
> > > >   - 7de2e9f195b9 ("mm: memcontrol: correct the NR_ANON_THPS counter of hierarchical memcg")
> > > >   - not committed(it is the first commit in this series) ("mm: memcontrol: fix NR_ANON_THPS account")
> > > >
> > > > This patch series can make the code simple (161 insertions(+), 187 deletions(-)).
> > > > And make the unit of the vmstat counters are either pages or bytes. Fewer choices
> > > > means lower probability of making mistakes :).
> > > >
> > > > This was inspired by Johannes and Roman. Thanks to them.
> > >
> > > It would be really great if you could summarize the current and after
> > > the patch state so that exceptions are clear and easier to review. The
> > 
> > Agree. Will do in the next version. Thanks.
> > 
> > 
> > > existing situation is rather convoluted but we have at least units part
> > > of the name so it is not too hard to notice that. Reducing exeptions
> > > sounds nice but I am not really sure it is such an improvement it is
> > > worth a lot of code churn. Especially when it comes to KB vs B. Counting
> > 
> > There are two vmstat counters (NR_KERNEL_STACK_KB and
> > NR_KERNEL_SCS_KB) whose units are KB. If we do this, all
> > vmstat counter units are either pages or bytes in the end. When
> > we expose those counters to userspace, it can be easy. You can
> > reference to:
> > 
> >     [RESEND PATCH v2 11/12] mm: memcontrol: make the slab calculation consistent
> > 
> > From this point of view, I think that it is worth doing this. Right?
> 
> Well, unless I am missing something, we have two counters in bytes, two
> in kB, both clearly distinguishable by the B/KB suffix. Changing KB to B
> will certainly reduce the different classes of units, no question about
> that, but I am not really sure this is worth all the code churn. Maybe
> others will think otherwise.

Even if it was me who suggested it, I do agree. It's nice to have a smaller number
of units, but if it creates a lot of hassle, then it makes not much sense.
I think we need to look at the final version of patches and decide if it
worth it or not.

> 
> As I've said the THP accounting change makes more sense to me because it
> allows future changes which are already undergoing so there is more
> merit in those.

+1
And this part is absolutely trivial.