Date:   Wed, 07 Dec 2022 09:22:25 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Mina Almasry <almasrymina@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Yosry Ahmed <yosryahmed@...gle.com>, weixugc@...gle.com,
        fvdl@...gle.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] [mm-unstable] mm: Fix memcg reclaim on memory tiered
 systems

Michal Hocko <mhocko@...e.com> writes:

> On Tue 06-12-22 08:06:51, Mina Almasry wrote:
>> On Tue, Dec 6, 2022 at 4:20 AM Michal Hocko <mhocko@...e.com> wrote:
>> >
>> > On Mon 05-12-22 18:34:05, Mina Almasry wrote:
>> > > commit 3f1509c57b1b ("Revert "mm/vmscan: never demote for memcg
>> > > reclaim"") enabled demotion in memcg reclaim, which is the right thing
>> > > to do, however, it introduced a regression in the behavior of
>> > > try_to_free_mem_cgroup_pages().
>> > >
>> > > The callers of try_to_free_mem_cgroup_pages() expect it to attempt to
>> > > reclaim - not demote - nr_pages from the cgroup. I.e. the memory usage
>> > > of the cgroup should reduce by nr_pages. The callers expect
>> > > try_to_free_mem_cgroup_pages() to also return the number of pages
>> > > reclaimed, not demoted.
>> > >
>> > > However, what try_to_free_mem_cgroup_pages() actually does is it
>> > > unconditionally counts demoted pages as reclaimed pages. So in practice
>> > > when it is called it will often demote nr_pages and return the number of
>> > > demoted pages to the caller. Demoted pages don't lower the memcg usage,
>> > > and so try_to_free_mem_cgroup_pages() is not actually doing what the
>> > > callers want it to do.
>> > >
>> > > Various things work suboptimally on memory tiered systems or don't work
>> > > at all due to this:
>> > >
>> > > - memory.high enforcement likely doesn't work (it just demotes nr_pages
>> > >   instead of lowering the memcg usage by nr_pages).
>> > > - try_charge_memcg() will keep retrying the charge while
>> > >   try_to_free_mem_cgroup_pages() is just demoting pages and not actually
>> > >   making any room for the charge.
>> >
>> > This has been brought up during the review https://lore.kernel.org/all/YoYTEDD+c4GT0xYY@dhcp22.suse.cz/
>> >
>> 
>> Ah, I did indeed miss this. Thanks for the pointer. However I don't
>> understand this bit from your email (sorry I'm probably missing
>> something):
>> 
>> "I suspect this is rather unlikely situation, though. The last tear
>> (without any fallback) should have some memory to reclaim most of
>> the time."
>> 
>> Reading the code in try_charge_memcg(), I don't see the last retry for
>> try_to_free_mem_cgroup_pages() do anything special. My concern here is
>> that try_charge_memcg() calls try_to_free_mem_cgroup_pages()
>> MAX_RECLAIM_RETRIES times. Each time that call may demote pages and
>> report back that it was able to 'reclaim' memory, but the charge keeps
>> failing because the memcg reclaim didn't actually make room for the
>> charge. What happens in this case? My understanding is that the memcg
>> oom-killer gets wrongly invoked.
>
> The memcg reclaim shrinks from all zones in the allowed zonelist. In
> general from all nodes. So unless the lower tier is outside of this
> zonelist then there is a zone to reclaim from which cannot demote.
> Correct?
>
>> > > - memory.reclaim has a wonky interface. It advertises to the user it
>> > >   reclaims the provided amount but it will actually often demote that
>> > >   amount.
>> > >
>> > > There may be more effects to this issue.
>> > >
>> > > To fix these issues I propose shrink_folio_list() to only count pages
>> > > demoted from inside of sc->nodemask to outside of sc->nodemask as
>> > > 'reclaimed'.
>> >
>> > Could you expand on why the node mask matters? From the charge point of
>> > view it should be completely uninteresting as the charge remains.
>> >
>> > I suspect we really need to change to reclaim metrics for memcg reclaim.
>> > In the memory balancing reclaim we can indeed consider demotions as a
>> > reclaim because the memory is freed in the end but for the memcg reclaim
>> > we really should be counting discharges instead. No demotion/migration will
>> > free up charges.
>> 
>> I think what you're describing is exactly what this patch aims to do.
>> I'm proposing an interface change to shrink_folio_list() such that it
>> only counts demoted pages as reclaimed iff sc->nodemask is provided by
>> the caller and the demotion removed pages from inside sc->nodemask to
>> outside sc->nodemask. In this case:
>> 
>> 1. memory balancing reclaim would pass sc->nodemask=nid to
>> shrink_folio_list() indicating that it should count pages demoted from
>> sc->nodemask as reclaimed.
>> 
>> 2. memcg reclaim would pass sc->nodemask=NULL to shrink_folio_list()
>> indicating that it is looking for reclaim across all nodes and no
>> demoted pages should count as reclaimed.
>> 
>> Sorry if the commit message was not clear. I can try making it clearer
>> in the next version but it's already very long.
>
> Either I am missing something or I simply do not understand why you are
> hooked into nodemask so much. Why cannot we have a simple rule that
> only global reclaim considers demotions as nr_reclaimed?

Yes.  This sounds reasonable to me and this simplify the logic greatly!

Best Regards,
Huang, Ying

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