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

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Message-ID: <CAGsJ_4yBkry-rw75AciT8OiYWrw+=D0okcxiyXzzNrz=QJxiBA@mail.gmail.com>
Date: Tue, 29 Oct 2024 04:42:07 +0800
From: Barry Song <21cnbao@...il.com>
To: Usama Arif <usamaarif642@...il.com>
Cc: Yosry Ahmed <yosryahmed@...gle.com>, Nhat Pham <nphamcs@...il.com>, 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>, 
	Johannes Weiner <hannes@...xchg.org>, David Hildenbrand <david@...hat.com>, 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>, joshua.hahnjy@...il.com
Subject: Re: [PATCH RFC] mm: count zeromap read and set for swapout and swapin

On Tue, Oct 29, 2024 at 4:00 AM Usama Arif <usamaarif642@...il.com> wrote:
>
>
>
> On 28/10/2024 19:54, Barry Song wrote:
> > On Tue, Oct 29, 2024 at 1:20 AM Usama Arif <usamaarif642@...il.com> wrote:
> >>
> >>
> >>
> >> On 28/10/2024 17:08, Yosry Ahmed wrote:
> >>> On Mon, Oct 28, 2024 at 10:00 AM Usama Arif <usamaarif642@...il.com> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On 28/10/2024 16:33, Nhat Pham wrote:
> >>>>> On Mon, Oct 28, 2024 at 5:23 AM Usama Arif <usamaarif642@...il.com> wrote:
> >>>>>>
> >>>>>> I wonder if instead of having counters, it might be better to keep track
> >>>>>> of the number of zeropages currently stored in zeromap, similar to how
> >>>>>> zswap_same_filled_pages did it. It will be more complicated then this
> >>>>>> patch, but would give more insight of the current state of the system.
> >>>>>>
> >>>>>> Joshua (in CC) was going to have a look at that.
> >>>>>
> >>>>> I don't think one can substitute for the other.
> >>>>
> >>>> Yes agreed, they have separate uses and provide different information, but
> >>>> maybe wasteful to have both types of counters? They are counters so maybe
> >>>> dont consume too much resources but I think we should still think about
> >>>> it..
> >>>
> >>> Not for or against here, but I would say that statement is debatable
> >>> at best for memcg stats :)
> >>>
> >>> Each new counter consumes 2 longs per-memcg per-CPU (see
> >>> memcg_vmstats_percpu), about 16 bytes, which is not a lot but it can
> >>> quickly add up with a large number of CPUs/memcgs/stats.
> >>>
> >>> Also, when flushing the stats we iterate all of them to propagate
> >>> updates from per-CPU counters. This is already a slowpath so adding
> >>> one stat is not a big deal, but again because we iterate all stats on
> >>> multiple CPUs (and sometimes on each node as well), the overall flush
> >>> latency becomes a concern sometimes.
> >>>
> >>> All of that is not to say we shouldn't add more memcg stats, but we
> >>> have to be mindful of the resources.
> >>
> >> Yes agreed! Plus the cost of incrementing similar counters (which ofcourse is
> >> also not much).
> >>
> >> Not trying to block this patch in anyway. Just think its a good point
> >> to discuss here if we are ok with both types of counters. If its too wasteful
> >> then which one we should have.
> >
> > Hi Usama,
> > my point is that with all the below three counters:
> > 1. PSWPIN/PSWPOUT
> > 2. ZSWPIN/ZSWPOUT
> > 3. SWAPIN_SKIP/SWAPOUT_SKIP or (ZEROSWPIN, ZEROSWPOUT what ever)
> >
> > Shouldn't we have been able to determine the portion of zeromap
> > swap indirectly?
> >
>
> Hmm, I might be wrong, but I would have thought no?
>
> What if you swapout a zero folio, but then discard it?
> zeromap_swpout would be incremented, but zeromap_swapin would not.

I understand. It looks like we have two issues to tackle:
1. We shouldn't let zeromap swap in or out anything that vanishes into
a black hole
2. We want to find out how much I/O/memory has been saved due to zeromap so far

>From my perspective, issue 1 requires a "fix", while issue 2 is more
of an optimization.

I consider issue 1 to be more critical because, after observing a phone
running for some time, I've been able to roughly estimate the portion
zeromap can
help save using only PSWPOUT, ZSWPOUT, and SWAPOUT_SKIP, even without a
SWPIN counter. However, I agree that issue 2 still holds significant value
as a separate patch.

Thanks
Barry