Re: [PATCH v3] [mm-unstable] mm: Fix memcg reclaim on memory tiered systems

[Michal Hocko]

On Thu 08-12-22 01:00:40, Mina Almasry wrote:
> On Thu, Dec 8, 2022 at 12:09 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >
> > On Wed 07-12-22 13:43:55, Mina Almasry wrote:
> > > On Wed, Dec 7, 2022 at 3:12 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
> > [...]
> > > > Anyway a proper nr_reclaimed tracking should be rather straightforward
> > > > but I do not expect to make a big difference in practice
> > > >
> > > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > > index 026199c047e0..1b7f2d8cb128 100644
> > > > --- a/mm/vmscan.c
> > > > +++ b/mm/vmscan.c
> > > > @@ -1633,7 +1633,7 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
> > > >         LIST_HEAD(ret_folios);
> > > >         LIST_HEAD(free_folios);
> > > >         LIST_HEAD(demote_folios);
> > > > -       unsigned int nr_reclaimed = 0;
> > > > +       unsigned int nr_reclaimed = 0, nr_demoted = 0;
> > > >         unsigned int pgactivate = 0;
> > > >         bool do_demote_pass;
> > > >         struct swap_iocb *plug = NULL;
> > > > @@ -2065,8 +2065,17 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
> > > >         }
> > > >         /* 'folio_list' is always empty here */
> > > >
> > > > -       /* Migrate folios selected for demotion */
> > > > -       nr_reclaimed += demote_folio_list(&demote_folios, pgdat);
> > > > +       /*
> > > > +        * Migrate folios selected for demotion.
> > > > +        * Do not consider demoted pages to be reclaimed for the memcg reclaim
> > > > +        * because no charges are really freed during the migration. Global
> > > > +        * reclaim aims at releasing memory from nodes/zones so consider
> > > > +        * demotion to reclaim memory.
> > > > +        */
> > > > +       nr_demoted += demote_folio_list(&demote_folios, pgdat);
> > > > +       if (!cgroup_reclaim(sc))
> > > > +               nr_reclaimed += nr_demoted;
> > > > +
> > > >         /* Folios that could not be demoted are still in @demote_folios */
> > > >         if (!list_empty(&demote_folios)) {
> > > >                 /* Folios which weren't demoted go back on @folio_list for retry: */
> > > >
> > > > [...]
> > >
> > > Thank you again, but this patch breaks the memory.reclaim nodes arg
> > > for me. This is my test case. I run it on a machine with 2 memory
> > > tiers.
> > >
> > > Memory tier 1= nodes 0-2
> > > Memory tier 2= node 3
> > >
> > >     mkdir -p /sys/fs/cgroup/unified/test
> > >     cd /sys/fs/cgroup/unified/test
> > >     echo $$ > cgroup.procs
> > >     head -c 500m /dev/random > /tmp/testfile
> > >     echo $$ > /sys/fs/cgroup/unified/cgroup.procs
> > >     echo "1m nodes=0-2" > memory.reclaim
> > >
> > > In my opinion the expected behavior is for the kernel to demote 1mb of
> > > memory from nodes 0-2 to node 3.
> > >
> > > Actual behavior on the tip of mm-unstable is as expected.
> > >
> > > Actual behavior with your patch cherry-picked to mm-unstable is that
> > > the kernel demotes all 500mb of memory from nodes 0-2 to node 3, and
> > > returns -EAGAIN to the user. This may be the correct behavior you're
> > > intending, but it completely breaks the use case I implemented the
> > > nodes= arg for and listed on the commit message of that change.
> >
> > Yes, strictly speaking the behavior is correct albeit unexpected. You
> > have told the kernel to _reclaim_ that much memory but demotion are
> > simply aging handling rather than a reclaim if the demotion target has a
> > lot of memory free.
> 
> Yes, by the strict definition of reclaim, you're completely correct.
> But in reality earlier I proposed a patch to the kernel that disables
> demotion in proactive reclaim. That would have been a correct change
> by the strict definition of reclaim, but Johannes informed me that
> meta already has a dependency on proactive reclaim triggering demotion
> and directed me to add a nodes= arg to memory.reclaim to trigger
> demotion instead, to satisfy both use cases. Seems both us and meta
> are using this interface to trigger both reclaim and demotion, despite
> the strict definition of the word?

Well, demotion is a part of aging and that is a part of the reclaim so I
believe we want both and demotion mostly an implementation detail. If
you want to have a very precise control then the nodemask should drive
you there.

[...]
> > I am worried this will popping up again and again. I thought your nodes
> > subset approach could deal with this but I have overlooked one important
> > thing in your patch. The user provided nodemask controls where to
> > reclaim from but it doesn't constrain demotion targets. Is this
> > intentional? Would it actually make more sense to control demotion by
> > addint demotion nodes into the nodemask?
> >
> 
> IMO, yes it is intentional, and no I would not recommend adding
> demotion nodes (I assume you mean adding both demote_from_nodes and
> demote_to_nodes as arg).

What I really mean is to add demotion nodes to the nodemask along with
the set of nodes you want to reclaim from. To me that sounds like a
more natural interface allowing for all sorts of usecases:
- free up demotion targets (only specify demotion nodes in the mask)
- control where to demote (e.g. select specific demotion target(s))
- do not demote at all (skip demotion nodes from the node mask)

> My opinion is based on 2 reasons:
> 
> 1. We control proactive reclaim by looking for nodes/tiers approaching
> pressure and triggering reclaim/demotion from said nodes/tiers. So we
> know the node/tier we would like to reclaim from, but not necessarily
> have a requirement on where the memory should go. I think it should be
> up to the kernel.
> 2. Currently I think most tiered machines will have 2 memory tiers,
> but I think the code is designed to support N memory tiers. What
> happens if N=3 and the user asks you to demote from the top tier nodes
> to the bottom tier nodes (skipping the middle tier)? The user in this
> case is explicitly asking to break the aging pipeline. From my short
> while on the mailing list I see great interest in respecting the aging
> pipeline, so it seems to me the demotion target should be decided by
> the kernel based on the aging pipeline, and the user should not be
> able to override it? I don't know. Maybe there is a valid use case for
> that somewhere.

I agree that the agining should be preserved as much as possible unless
there is an explicit requirement to do otherwise which might be
something application specific.

It is really hard to assume all the usecases at this stage but we should
keep in mind that the semantic of the interface will get cast into stone
once it is released. As of now I do see a great confusion point in the
nodemask semantic which pretends to allow some fine control while it is
largerly hard to predict because it makes some assumptions about the
reclaim while it has a very limited control of the demotion.

-- 
Michal Hocko
SUSE Labs