Message-Id: <20240320165619.71478-1-sj@kernel.org>
Date: Wed, 20 Mar 2024 09:56:19 -0700
From: SeongJae Park <sj@...nel.org>
To: Honggyu Kim <honggyu.kim@...com>
Cc: SeongJae Park <sj@...nel.org>,
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Subject: Re: [RFC PATCH v2 0/7] DAMON based 2-tier memory management for CXL memory

Hi Honggyu,

On Wed, 20 Mar 2024 16:07:48 +0900 Honggyu Kim <honggyu.kim@...com> wrote:

> Hi SeongJae,
> 
> On Mon, 18 Mar 2024 12:07:21 -0700 SeongJae Park <sj@...nel.org> wrote:
> > On Mon, 18 Mar 2024 22:27:45 +0900 Honggyu Kim <honggyu.kim@...com> wrote:
> > 
> > > Hi SeongJae,
> > > 
> > > On Sun, 17 Mar 2024 08:31:44 -0700 SeongJae Park <sj@...nel.org> wrote:
> > > > Hi Honggyu,
> > > > 
> > > > On Sun, 17 Mar 2024 17:36:29 +0900 Honggyu Kim <honggyu.kim@...com> wrote:
> > > > 
> > > > > Hi SeongJae,
> > > > > 
> > > > > Thanks for the confirmation.  I have a few comments on young filter so
> > > > > please read the inline comments again.
> > > > > 
> > > > > On Wed, 12 Mar 2024 08:53:00 -0800 SeongJae Park <sj@...nel.org> wrote:
> > > > > > Hi Honggyu,
> > > > > > 
> > > > > > > > -----Original Message-----
> > > > > > > > From: SeongJae Park <sj@...nel.org>
> > > > > > > > Sent: Tuesday, March 12, 2024 3:33 AM
> > > > > > > > To: Honggyu Kim <honggyu.kim@...com>
> > > > > > > > Cc: SeongJae Park <sj@...nel.org>; kernel_team <kernel_team@...ynix.com>
> > > > > > > > Subject: RE: Re: [RFC PATCH v2 0/7] DAMON based 2-tier memory management for CXL memory
> > > > > > > >
> > > > > > > > Hi Honggyu,
> > > > > > > >
> > > > > > > > On Mon, 11 Mar 2024 12:51:12 +0000 "honggyu.kim@...com" <honggyu.kim@...com> wrote:
> > > > > > > >
> > > > > > > > > Hi SeongJae,
> > > > > > > > >
> > > > > > > > > I've tested it again and found that "young" filter has to be set
> > > > > > > > > differently as follows.
> > > > > > > > > - demote action: set "young" filter with "matching" true
> > > > > > > > > - promote action: set "young" filter with "matching" false
> > > 
> > > Thinking it again, I feel like "matching" true or false looks quite
> > > vague to me as a general user.
> > > 
> > > Instead, I would like to have more meaningful names for "matching" as
> > > follows.
> > > 
> > > - matching "true" can be either (filter) "out" or "skip".
> > > - matching "false" can be either (filter) "in" or "apply".
> > 
> > I agree the naming could be done much better.  And thank you for the nice
> > suggestions.  I have a few concerns, though.
> 
> I don't think my suggestion is best.  I just would like to have more
> discussion about it.

I also understand my naming sense is far from good :)  I'm grateful to have
this constructive discussion!

> 
> > Firstly, increasing the number of behavioral concepts.  DAMOS filter feature
> > has only single behavior: excluding some types of memory from DAMOS action
> > target.  The "matching" is to provide a flexible way for further specifying the
> > target to exclude in a bit detail.  Without it, we would need non-variant for
> > each filter type.  Comapred to the current terms, the new terms feel like
> > implying there are two types of behaviors.  I think one behavior is easier to
> > understand than two behaviors, and better match what internal code is doing.
> > 
> > Secondly, ambiguity in "in" and "apply".  To me, the terms sound like _adding_
> > something more than _excluding_.
> 
> I understood that young filter "matching" "false" means apply action
> only to young pages.  Do I misunderstood something here?  If not,

Technically speaking, having a DAMOS filter with 'matching' parameter as
'false' for 'young pages' type means you want DAMOS to "exclude pages that not
young from the scheme's action target".  That's the only thing it truly does,
and what it tries to guarantee.  Whether the action will be applied to young
pages or not depends on more factors including additional filters and DAMOS
parameter.  IOW, that's not what the simple setting promises.

Of course, I know you are assuming there is only the single filter.  Hence,
effectively you're correct.  And the sentence may be a better wording for end
users.  However, it tooke me a bit time to understand your assumption and
concluding whether your sentence is correct or not, since I had to think about
the assumptions.

I'd say this also reminds me the first concern that I raised on the previous
mail.  IOW, I feel this sentence is introducing one more behavior and making it
bit taking longer time to digest, for developers who should judge it based on
the source code.  I'd suggest use only one behavioral term, "exclude", since it
is what the code really does, unless it is wording for end users.

> "apply" means _adding_ or _including_ only the matched pages for action.
> It looks like you thought about _excluding_ non matched pages here.

Yes.  I'd prefer using only single term, _excluding_.  It fits with the code,
and require one word less that "adding" or "including", since "adding" or
"including" require one more word, "only".

Also, even with "only", the fact that there could be more filters makes me
unsure what is the consequence of having it.  That is, if we have a filter that
includes only pages of type A, but if there could be yet another filter that
includes only pages of type B, would the consequence is the action being
applied to pages of type A and B?  Or, type A or type B?

In my opinion, exclusion based approach is simpler for understanding the
consequence of such combinational usage.

> 
> > I think that might confuse people in some
> > cases.  Actually, I have used the term "filter-out" and "filter-in" on
> > this  and several threads.  When saying about "filter-in" scenario, I had to
> > emphasize the fact that it is not adding something but excluding others.
> 
> Excluding others also means including matched pages.  I think we better
> focus on what to do only for the matched pages.

I agree that is true for the end-users in many cases.  But I think that depends
on the case, and at least this specific case (kernel ABI level discussion about
DAMOS filters), I don't really feel that's better.

> 
> > I now think that was not a good approach.
> > 
> > Finally, "apply" sounds a bit deterministic.  I think it could be a bit
> > confusing in some cases such as when using multiple filters in a combined way.
> > For example, if we have two filters for 1) "apply" a memcg and 2) skip anon
> > pages, the given DAMOS action will not be applied to anon pages of the memcg.
> > I think this might be a bit confusing.
> 
> No objection on this.  If so, I think "in" sounds better than "apply".

Thanks for understanding.  I think allowlists or denylists might also been
better names.

> 
> > > 
> > > Internally, the type of "matching" can be boolean, but it'd be better
> > > for general users have another ways to set it such as "out"/"in" or
> > > "skip"/"apply" via sysfs interface.  I prefer "skip" and "apply" looks
> > > more intuitive, but I don't have strong objection on "out" and "in" as
> > > well.
> > 
> > Unfortunately, DAMON sysfs interface is an ABI that we want to keep stable.  Of
> > course we could make some changes on it if really required.  But I'm unsure if
> > the problem of current naming and benefit of the sugegsted change are big
> > enough to outweighs the stability risk and additional efforts.
> 
> I don't ask to change the interface, but just provide another way for
> the setting.  For example, the current "matching" accepts either 1,
> true, or Y but internally keeps as "true" as a boolean type.
> 
>   $ cd /sys/kernel/mm/damon/admin/kdamonds/0/contexts/0/schemes/0/filters/0
> 
>   $ echo 1 | tee matching && cat matching
>   1
>   Y
> 
>   $ echo true | tee matching && cat matching
>   true
>   Y
> 
>   $ echo Y | tee matching && cat matching
>   Y
>   Y
> 
> I'm asking if it's okay making "matching" receive "out" or "skip" as
> follows.
> 
>   $ echo out | tee matching && cat matching
>   out
>   Y
> 
>   $ echo skip | tee matching && cat matching
>   skip
>   Y

I have no strong concern about this.  But also not seeing significant benefit
of this change.  This will definitely not regress user experience.  But it will
require introducing more kernel code, though the amount will be fairly small.
And this new interface will be something that we need to keep maintain, so
adding a tiny bit of maintenance burden.  I'd prefer improving the documents or
user-space tool and keep the kernel code simple.

IMHO, end users shouldn't deal directly with DAMOS filters at all, and kernel
ABI document should be clear enough to avoid confusion.  But, if someone uses
kernel ABI on production without reading the document, I'd say it might better
to crash or OOPS to give clear warning and lessons.

> 
> > Also, DAMON sysfs interface is arguably not for _very_ general users.  DAMON
> > user-space tool is the one for _more_ general users.  To quote DAMON usage
> > document,
> > 
> >     - *DAMON user space tool.*
> >       `This <https://github.com/awslabs/damo>`_ is for privileged people such as
> >       system administrators who want a just-working human-friendly interface.
> >       [...]
> >     - *sysfs interface.*
> >       :ref:`This <sysfs_interface>` is for privileged user space programmers who
> >       want more optimized use of DAMON. [...]
> >  
> > If the concept is that confused, I think we could improve the documentation, or
> > the user space tool.  But for DAMON sysfs interface, I think we need more
> > discussions for getting clear pros/cons that justifies the risk and the effort.
> 
> If my suggestion is not what you want in sysfs interface, then "damo"
> can receive these more meaningful names and translate to "true" or
> "false" when writing to sysfs.

Yes, I agree.  We could further hide filter concept at all.  For example, we
could let damo user call "migrate" DAMOS action plus "non-young" filter as
"promote" action.  Or, have a dedicated command for tiered-memory management.
Similar to the gen_config.py of HMSDK
(https://github.com/skhynix/hmsdk/blob/main/tools/gen_config.py).  But this
would be something to further discuss on different threads.

> 
> > > 
> > > I also feel the filter name "young" is more for developers not for
> > > general users.  I think this can be changed to "accessed" filter
> > > instead.
> > 
> > In my humble opinion, "accessed" might be confusing with the term that being
> > used by DAMON, specifically, the concept of "nr_accesses".  I also thought
> > about using more specific term such as "pg-accessed" or something else, but I
> > felt it is still unclear or making it too verbose.
> > 
> > I agree "young" sounds more for developers.  But, again, DAMON sysfs is for not
> > _very_ general users.
> 
> I worried the developer term is also going to be used for "damo" user
> space tool as "young" filter.  But if you think it's good enough, then I
> will follow the decision as I also think "accessed" is not the best term
> for this.

The line is not very clear, but I think the line for "damo" should be different
from that for DAMON sysfs interface.

[...]
> > > > > > > > DAMOS filter is basically for filtering "out" memory regions that matches to
> > > > > > > > the condition.
> > > 
> > > Right.  In other tools, I see filters are more used as filtering "in"
> > > rather than filtering "out".  I feel this makes me more confused.
> > 
> > I understand that the word, "filtering", can be used for both, and therefore
> > can be confused.  I was also spending no small times at naming since I was
> > thinking about both coffee filters and color filters (of photoshop or glasses).
> > But it turned out that I'm more familiar with coffee filters, and might be same
> > for DAMON community, since the community is having beer/coffee/tea chat series
> > ;) (https://lore.kernel.org/damon/20220810225102.124459-1-sj@kernel.org/)
> 
> Yeah, I thought about filter for including pages for given config as
> follows.
> 
>     \    /
>      \  /     only matched items pass this filter.
>       ||
> 
> But the current DAMOS filter is about excluding pages for given config
> as follows just like a strainer.
>       ___
>      /###\
>     |#####|   matched items are excluded via this filter.
>      \###/
>       ---
> 
> I think I won't get confused after keeping this difference in mind.

My mind model was describing it as "excluding" coffee beans, but I'd say these
are just different perspectives, not a thing about right or wrong.  I'm
grateful to learn one more perspective that is different from mine :)

> 
> > That said, I think we may be able to make this better documented, or add a
> > layer of abstraction on DAMON user-space tool.
> > 
[...]
> > To summarize my opinion again,
> > 
> > 1. I agree the concept and names of DAMOS filters are confusing and not very
> >    intuitive.
> > 2. However, it's unclear if the problem and the benefit from the suggested new
> >    names are huge enough to take the risk and effort on changing ABI.
> > 3. We could improve documentation and/or user-space tool.
> 
> I think improving "damo" can be a good solution.

Looking forward to the discussion on it! :)

> 
> > Thank you again for the suggestion and confirmations to my questions.
> 
> Likewise, thank you for the explanation in details.

My great pleasure, and thank you for patiently keeping this grateful
discussion!


Thanks,
SJ

[...]

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