Re: [PATCH v7 05/12] mm: multigenerational LRU: minimal implementation

["Huang, Ying" <ying.huang@intel.com>]

Yu Zhao <yuzhao@google.com> writes:

> On Wed, Feb 23, 2022 at 5:59 PM Huang, Ying <ying.huang@intel.com> wrote:
>>
>> Yu Zhao <yuzhao@google.com> writes:
>>
>> > On Wed, Feb 23, 2022 at 1:28 AM Huang, Ying <ying.huang@intel.com> wrote:
>> >>
>> >> Hi, Yu,
>> >>
>> >> Yu Zhao <yuzhao@google.com> writes:
>> >>
>> >> > To avoid confusions, the terms "promotion" and "demotion" will be
>> >> > applied to the multigenerational LRU, as a new convention; the terms
>> >> > "activation" and "deactivation" will be applied to the active/inactive
>> >> > LRU, as usual.
>> >>
>> >> In the memory tiering related commits and patchset, for example as follows,
>> >>
>> >> commit 668e4147d8850df32ca41e28f52c146025ca45c6
>> >> Author: Yang Shi <yang.shi@linux.alibaba.com>
>> >> Date:   Thu Sep 2 14:59:19 2021 -0700
>> >>
>> >>     mm/vmscan: add page demotion counter
>> >>
>> >> https://lore.kernel.org/linux-mm/20220221084529.1052339-1-ying.huang@intel.com/
>> >>
>> >> "demote" and "promote" is used for migrating pages between different
>> >> types of memory.  Is it better for us to avoid overloading these words
>> >> too much to avoid the possible confusion?
>> >
>> > Given that LRU and migration are usually different contexts, I think
>> > we'd be fine, unless we want a third pair of terms.
>>
>> This is true before memory tiering is introduced.  In systems with
>> multiple types memory (called memory tiering), LRU is used to identify
>> pages to be migrated to the slow memory node.  Please take a look at
>> can_demote(), which is called in shrink_page_list().
>
> This sounds clearly two contexts to me. Promotion/demotion (move
> between generations) while pages are on LRU; or promotion/demotion
> (migration between nodes) after pages are taken off LRU.
>
> Note that promotion/demotion are not used in function names. They are
> used to describe how MGLRU works, in comparison with the
> active/inactive LRU. Memory tiering is not within this context.

Because we have used pgdemote_* in /proc/vmstat, "demotion_enabled" in
/sys/kernel/mm/numa, and will use pgpromote_* in /proc/vmstat.  It seems
better to avoid to use promote/demote directly for MGLRU in ABI.  A
possible solution is to use "mglru" and "promote/demote" together (such
as "mglru_promote_*" when it is needed?

>> >> > +static int get_swappiness(struct mem_cgroup *memcg)
>> >> > +{
>> >> > +     return mem_cgroup_get_nr_swap_pages(memcg) >= MIN_LRU_BATCH ?
>> >> > +            mem_cgroup_swappiness(memcg) : 0;
>> >> > +}
>> >>
>> >> After we introduced demotion support in Linux kernel.  The anonymous
>> >> pages in the fast memory node could be demoted to the slow memory node
>> >> via the page reclaiming mechanism as in the following commit.  Can you
>> >> consider that too?
>> >
>> > Sure. How do I check whether there is still space on the slow node?
>>
>> You can always check the watermark of the slow node.  But now, we
>> actually don't check that (as in demote_page_list()), instead we will
>> wake up kswapd of the slow node.  The intended behavior is something
>> like,
>>
>>   DRAM -> PMEM -> disk
>
> I'll look into this later -- for now, it's a low priority because
> there isn't much demand. I'll bump it up if anybody is interested in
> giving it a try. Meanwhile, please feel free to cook up something if
> you are interested.

When we introduce a new feature, we shouldn't break an existing one.
That is, not introducing regression.  I think that it is a rule?

If my understanding were correct, MGLRU will ignore to scan anonymous
page list even if there's demotion target for the node.  This breaks the
demotion feature in the upstream kernel.  Right?

It's a new feature to check whether there is still space on the slow
node.  We can look at that later.

Best Regards,
Huang, Ying