Message-ID: <7e04c8d3-b92e-400d-a1dd-f5902749f6d4@redhat.com>
Date: Tue, 5 Nov 2024 12:09:07 +0100
From: David Hildenbrand <david@...hat.com>
To: Barry Song <21cnbao@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Usama Arif <usamaarif642@...il.com>, 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 05.11.24 10:15, 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%. 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.
> 
> 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 :-)

Thanks for sharing these numbers. Just for the records, I think the 
optimization is quite powerful. What I am not convinced is that the 
missing stat is a big deal.

If I upgrade my car and it suddenly consumes 5% less fuel/energy I (a) 
either won't notice at all or (b) am happy and don't care why; I won't 
start digging through the updated manual, looking for some hidden gages 
that might explain why :)

But I might be missing something important, and I do understand why it 
can be helpful to have it black-on-white how much this optimization 
gives you. So feel free to move forward with this as you like, having it 
in 6.12 sounds very reasonable to me.

-- 
Cheers,

David / dhildenb

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