Re: [PATCH resend] memcg: introduce per-memcg reclaim interface

[Huang, Ying]

Wei Xu <weixugc@xxxxxxxxxx> writes:

> On Tue, Apr 5, 2022 at 5:49 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>>
>> Wei Xu <weixugc@xxxxxxxxxx> writes:
>>
>> > On Sat, Apr 2, 2022 at 1:13 AM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>> >>
>> >> Wei Xu <weixugc@xxxxxxxxxx> writes:
>> >>
>> >> > On Fri, Apr 1, 2022 at 6:54 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>> >> >>
>> >> >> On Thu 31-03-22 08:41:51, Yosry Ahmed wrote:
>> >> >> > From: Shakeel Butt <shakeelb@xxxxxxxxxx>
>> >> >> >
>> >>
>> >> [snip]
>> >>
>> >> >> > Possible Extensions:
>> >> >> > --------------------
>> >> >> >
>> >> >> > - This interface can be extended with an additional parameter or flags
>> >> >> >   to allow specifying one or more types of memory to reclaim from (e.g.
>> >> >> >   file, anon, ..).
>> >> >> >
>> >> >> > - The interface can also be extended with a node mask to reclaim from
>> >> >> >   specific nodes. This has use cases for reclaim-based demotion in memory
>> >> >> >   tiering systens.
>> >> >> >
>> >> >> > - A similar per-node interface can also be added to support proactive
>> >> >> >   reclaim and reclaim-based demotion in systems without memcg.
>> >> >> >
>> >> >> > For now, let's keep things simple by adding the basic functionality.
>> >> >>
>> >> >> Yes, I am for the simplicity and this really looks like a bare minumum
>> >> >> interface. But it is not really clear who do you want to add flags on
>> >> >> top of it?
>> >> >>
>> >> >> I am not really sure we really need a node aware interface for memcg.
>> >> >> The global reclaim interface will likely need a different node because
>> >> >> we do not want to make this CONFIG_MEMCG constrained.
>> >> >
>> >> > A nodemask argument for memory.reclaim can be useful for memory
>> >> > tiering between NUMA nodes with different performance.  Similar to
>> >> > proactive reclaim, it can allow a userspace daemon to drive
>> >> > memcg-based proactive demotion via the reclaim-based demotion
>> >> > mechanism in the kernel.
>> >>
>> >> I am not sure whether nodemask is a good way for demoting pages between
>> >> different types of memory.  For example, for a system with DRAM and
>> >> PMEM, if specifying DRAM node in nodemask means demoting to PMEM, what
>> >> is the meaning of specifying PMEM node? reclaiming to disk?
>> >>
>> >> In general, I have no objection to the idea in general.  But we should
>> >> have a clear and consistent interface.  Per my understanding the default
>> >> memcg interface is for memory, regardless of memory types.  The memory
>> >> reclaiming means reduce the memory usage, regardless of memory types.
>> >> We need to either extending the semantics of memory reclaiming (to
>> >> include memory demoting too), or add another interface for memory
>> >> demoting.
>> >
>> > Good point.  With the "demote pages during reclaim" patch series,
>> > reclaim is already extended to demote pages as well.  For example,
>> > can_reclaim_anon_pages() returns true if demotion is allowed and
>> > shrink_page_list() can demote pages instead of reclaiming pages.
>>
>> These are in-kernel implementation, not the ABI.  So we still have
>> the opportunity to define the ABI now.
>>
>> > Currently, demotion is disabled for memcg reclaim, which I think can
>> > be relaxed and also necessary for memcg-based proactive demotion.  I'd
>> > like to suggest that we extend the semantics of memory.reclaim to
>> > cover memory demotion as well.  A flag can be used to enable/disable
>> > the demotion behavior.
>>
>> If so,
>>
>> # echo A > memory.reclaim
>>
>> means
>>
>> a) "A" bytes memory are freed from the memcg, regardless demoting is
>>    used or not.
>>
>> or
>>
>> b) "A" bytes memory are reclaimed from the memcg, some of them may be
>>    freed, some of them may be just demoted from DRAM to PMEM.  The total
>>    number is "A".
>>
>> For me, a) looks more reasonable.
>>
>
> We can use a DEMOTE flag to control the demotion behavior for
> memory.reclaim.  If the flag is not set (the default), then
> no_demotion of scan_control can be set to 1, similar to
> reclaim_pages().

If we have to use a flag to control the behavior, I think it's better to
have a separate interface (e.g. memory.demote).  But do we really need b)?

> The question is then whether we want to rename memory.reclaim to
> something more general.  I think this name is fine if reclaim-based
> demotion is an accepted concept.

Best Regards,
Huang, Ying