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

From: Muchun Song <songmuchun@bytedance.com>
To: Randy Dunlap <rdunlap@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>,
	Greg KH <gregkh@linuxfoundation.org>,
	rafael@kernel.org,  Alexey Dobriyan <adobriyan@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	 Johannes Weiner <hannes@cmpxchg.org>,
	Vladimir Davydov <vdavydov.dev@gmail.com>,
	 Hugh Dickins <hughd@google.com>, Will Deacon <will@kernel.org>,
	Roman Gushchin <guro@fb.com>,  Mike Rapoport <rppt@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	esyr@redhat.com, peterx@redhat.com,  krisman@collabora.com,
	Suren Baghdasaryan <surenb@google.com>,
	avagin@openvz.org,  Marco Elver <elver@google.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	 LKML <linux-kernel@vger.kernel.org>,
	 linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	 Linux Memory Management List <linux-mm@kvack.org>,
	Cgroups <cgroups@vger.kernel.org>
Subject: Re: [External] Re: [RESEND PATCH v2 00/12] Convert all vmstat counters to pages or bytes
Date: Tue, 8 Dec 2020 10:29:23 +0800	[thread overview]
Message-ID: <CAMZfGtUumSm5Adoz+XTzZmjxV7krGQKffuh6NaBP0FVgTFtoJg@mail.gmail.com> (raw)
In-Reply-To: <30ebae81-86e8-80db-feb6-d7c47dbaccb2@infradead.org>

On Tue, Dec 8, 2020 at 2:51 AM Randy Dunlap <rdunlap@infradead.org> wrote:
>
> 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@suse.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.

Oh, we do not change the output. It is transparent to userspace.

Thanks.

>
> And is there any chance of confusing userspace s/w (binary or scripts)
> with these changes?
>
> thanks.
> --
> ~Randy
>

-- 
Yours,
Muchun