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 10:51:32 -0800
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Michal Hocko <mhocko@...e.com>,
        Muchun Song <songmuchun@...edance.com>
Cc:     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>,
        Roman Gushchin <guro@...com>, 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>,
        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 12/7/20 7:02 AM, 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.
> 
> 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.
> 

Hi,

Are there any documentation changes that go with these patches?
Or are none needed?

If the patches change the output in /proc/* or /sys/* then I expect
there would need to be some doc changes.

And is there any chance of confusing userspace s/w (binary or scripts)
with these changes?

thanks.
-- 
~Randy