linux-kernel - Re: [PATCH v2] mm: count zeromap read and set for swapout and swapin

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Message-ID: <CAGsJ_4xBx2WJHxEJSaJ0t=sAtXOGS7i1ZMa-xMuxerAUaXq0Eg@mail.gmail.com>
Date: Tue, 5 Nov 2024 23:57:42 +1300
From: Barry Song <21cnbao@...il.com>
To: Usama Arif <usamaarif642@...il.com>
Cc: David Hildenbrand <david@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Barry Song <v-songbaohua@...o.com>, 
	Chengming Zhou <chengming.zhou@...ux.dev>, Yosry Ahmed <yosryahmed@...gle.com>, 
	Nhat Pham <nphamcs@...il.com>, Johannes Weiner <hannes@...xchg.org>, Hugh Dickins <hughd@...gle.com>, 
	Matthew Wilcox <willy@...radead.org>, Shakeel Butt <shakeel.butt@...ux.dev>, 
	Andi Kleen <ak@...ux.intel.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>, 
	Chris Li <chrisl@...nel.org>, "Huang, Ying" <ying.huang@...el.com>, 
	Kairui Song <kasong@...cent.com>, Ryan Roberts <ryan.roberts@....com>
Subject: Re: [PATCH v2] mm: count zeromap read and set for swapout and swapin

On Tue, Nov 5, 2024 at 11:44 PM Usama Arif <usamaarif642@...il.com> wrote:
>
>
>
> On 05/11/2024 09:15, Barry Song wrote:
> > On Tue, Nov 5, 2024 at 9:23 PM David Hildenbrand <david@...hat.com> wrote:
> >>
> >> On 05.11.24 04:40, Andrew Morton wrote:
> >>> On Mon, 4 Nov 2024 13:32:55 +0100 David Hildenbrand <david@...hat.com> wrote:
> >>>
> >>>>> As mentioned above, this isn't about fixing a bug; it's simply to ensure
> >>>>> that swap-related metrics don't disappear.
> >>>>
> >>>> Documentation/process/submitting-patches.rst:
> >>>>
> >>>> "A Fixes: tag indicates that the patch fixes an issue in a previous
> >>>> commit. It is used to make it easy to determine where a bug originated,
> >>>> which can help review a bug fix."
> >>>>
> >>>> If there is no BUG, I'm afraid you are abusing that tag.
> >>>
> >>> I think the abuse is reasonable.  We have no Should-be-included-with:.
> >>
> >> A "Belongs-to:" might make sense, for this kind of stuff that is still
> >> only in an RFC. Or we update the doc to explicitly spell out this
> >> special case of using "Fixes" to sort out something into the RC.
> >>
> >> Because if this would be already in a released kernel, it would get a
> >> bit trickier: stable rules explicitly spell out "fix a real bug".
> >>
> >>>
> >>> 0ca0c24e3211 is only in 6.12-rcX so this is the time to make
> >>> userspace-visible tweaks, so the 6.12 interfaces are the same as the
> >>> 6.13+ interfaces (which is what I think is happening here?)
> >>  > > And including the Fixes in this patch might be useful to someone who is
> >>> backporting 0ca0c24e3211 into some earlier kernel for their own
> >>> purposes.
> >>
> >> Just to be clear: adding new counters would hardly be fixing existing
> >> tools that perform calculations based on existing counters. So we are
> >> already changing the "userspace-visible" portion in some way, and I have
> >> no idea what in vmstat we consider "stable".
> >>
> >> But I still don't think it's all that big of a deal except in some
> >> handcrafted scenarios hardly anybody cares about; the cover letter is
> >> also pretty clear on that.
> >
> > I may have been mistaken in the cover letter. According to the zswap data Usama
> > provided for servers, zero-filled pages accounted for about 1%.
>
> 10% not 1% (https://lore.kernel.org/all/20240612124750.2220726-1-usamaarif642@gmail.com/).
>

Sorry. My memory must have faded; my mind is confused by the 1% non-zero
same-page data and the 10% same-page data. Getting old :-)

> > So
> > really doesn't
> > matter too much, but I just checked with Hailong from our team—he has data
> > on same-page-filled usage in Android apps, which on average show 3-4%
> > same-page-filled, with over 85% being zero-filled. Some apps even reach
> > 6-7% zero-filled pages. We previously used these counters to profile
> > optimizations, but with zeromap now serving as the frontend for swap files,
> > those counters will disappear entirely from both zRAM and pswpin/pswpout
> > metrics, as folios are filtered earlier.
> >
> This is what I meant in https://lore.kernel.org/all/79deed1a-9b0e-42e0-be2f-f0c3ef5fee11@gmail.com/
> when I said it affects zram as well!
>
> I am happy with the current version of the patch, just need the change
> in Documentation/admin-guide/cgroup-v2.rst.

I will update the document and send version 3 tomorrow, incorporating both
your comments on "zero-filled" and David's suggestion regarding "move out of
memory".

>
> Thanks,
> Usama
>
> > Hailong essentially has a table that looks like the below which could be
> > collected from the existing counters:
> >
> > com.xxx.app     5% same-page-filled.    88% zero
> > com.yyy.app     6% same-page-filled.     85% zero
> > com.zzz.map   6.7 same-page-filled.       88% zero
> > ....
> >
> > Anyone on 6.12 will be unable to track zero-filled pages unless they
> > backport this patch from a newer kernel version if it doesn’t make it
> > into 6.12.
> >
> > Whether it's marked as 'Belongs-to:' or 'Fixes:', I'd prefer we aim to
> > land it in
> > 6.12 :-)
> >
> >>
> >> So I'll shut up now and let people figure out the naming first, and if a
> >> new counter is required at all :)
> >>
> >> --
> >> Cheers,
> >>
> >> David / dhildenb
> >>
> >

Thanks
Barry