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

[Wei Xu]

On Tue, Apr 5, 2022 at 7:50 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> 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)?
>

I am fine with either approach: a separate interface similar to
memory.reclaim, but dedicated to demotion, or multiplexing
memory.reclaim for demotion with a flag.

My understanding is that with the "demote pages during reclaim"
support, b) is the expected behavior, or more precisely, pages that
cannot be demoted may be freed or swapped out.  This is reasonable.
Demotion-only can also be supported via some arguments to the
interface and changes to demotion code in the kernel.  After all, this
interface is being designed to be extensible based on the discussions
so far.

> > 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