Re: [RFC PATCH V1] mm: Disable demotion from proactive reclaim

[Johannes Weiner]

On Wed, Nov 23, 2022 at 01:20:57PM -0800, Mina Almasry wrote:
> On Wed, Nov 23, 2022 at 10:00 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
> >
> > Hello Mina,
> >
> > On Tue, Nov 22, 2022 at 12:38:45PM -0800, Mina Almasry wrote:
> > > Since commit 3f1509c57b1b ("Revert "mm/vmscan: never demote for memcg
> > > reclaim""), the proactive reclaim interface memory.reclaim does both
> > > reclaim and demotion. This is likely fine for us for latency critical
> > > jobs where we would want to disable proactive reclaim entirely, and is
> > > also fine for latency tolerant jobs where we would like to both
> > > proactively reclaim and demote.
> > >
> > > However, for some latency tiers in the middle we would like to demote but
> > > not reclaim. This is because reclaim and demotion incur different latency
> > > costs to the jobs in the cgroup. Demoted memory would still be addressable
> > > by the userspace at a higher latency, but reclaimed memory would need to
> > > incur a pagefault.
> > >
> > > To address this, I propose having reclaim-only and demotion-only
> > > mechanisms in the kernel. There are a couple possible
> > > interfaces to carry this out I considered:
> > >
> > > 1. Disable demotion in the memory.reclaim interface and add a new
> > >    demotion interface (memory.demote).
> > > 2. Extend memory.reclaim with a "demote=<int>" flag to configure the demotion
> > >    behavior in the kernel like so:
> > >       - demote=0 would disable demotion from this call.
> > >       - demote=1 would allow the kernel to demote if it desires.
> > >       - demote=2 would only demote if possible but not attempt any
> > >         other form of reclaim.
> >
> > Unfortunately, our proactive reclaim stack currently relies on
> > memory.reclaim doing both. It may not stay like that, but I'm a bit
> > wary of changing user-visible semantics post-facto.
> >
> > In patch 2, you're adding a node interface to memory.demote. Can you
> > add this to memory.reclaim instead? This would allow you to control
> > demotion and reclaim independently as you please: if you call it on a
> > node with demotion targets, it will demote; if you call it on a node
> > without one, it'll reclaim. And current users will remain unaffected.
> 
> Hello Johannes, thanks for taking a look!
> 
> I can certainly add the "nodes=" arg to memory.reclaim and you're
> right, that would help in bridging the gap. However, if I understand
> the underlying code correctly, with only the nodes= arg the kernel
> will indeed attempt demotion first, but the kernel will also merrily
> fall back to reclaiming if it can't demote the full amount. I had
> hoped to have the flexibility to protect latency sensitive jobs from
> reclaim entirely while attempting to do demotion.

The fallback to reclaim actually strikes me as wrong.

Think of reclaim as 'demoting' the pages to the storage tier. If we
have a RAM -> CXL -> storage hierarchy, we should demote from RAM to
CXL and from CXL to storage. If we reclaim a page from RAM, it means
we 'demote' it directly from RAM to storage, bypassing potentially a
huge amount of pages colder than it in CXL. That doesn't seem right.

If demotion fails, IMO it shouldn't satisfy the reclaim request by
breaking the layering. Rather it should deflect that pressure to the
lower layers to make room. This makes sure we maintain an aging
pipeline that honors the memory tier hierarchy.

So I'm hesitant to design cgroup controls around the current behavior.

> The above is just one angle of the issue. Another angle (which Yosry
> would care most about I think) is that at Google we call
> memory.reclaim mainly when memory.current is too close to memory.max
> and we expect the memory usage of the cgroup to drop as a result of a
> success memory.reclaim call. I suspect once we take in commit
> 3f1509c57b1b ("Revert "mm/vmscan: never demote for memcg reclaim""),
> we would run into that regression, but I defer to Yosry here, he may
> have a solution for that in mind already.

IMO it should both demote and reclaim. Simliar to how memory.reclaim
on a non-tiered memory system would both deactivate active pages and
reclaim inactive pages.