Re: [PATCH v2 0/5] mm: demotion: Introduce new node state N_DEMOTION_TARGETS

[Wei Xu]

On Tue, Apr 26, 2022 at 10:06 PM Aneesh Kumar K V
<aneesh.kumar@xxxxxxxxxxxxx> wrote:
>
> On 4/25/22 10:26 PM, Wei Xu wrote:
> > On Sat, Apr 23, 2022 at 8:02 PM ying.huang@xxxxxxxxx
> > <ying.huang@xxxxxxxxx> wrote:
> >>
>
> ....
>
> >> 2. For machines with PMEM installed in only 1 of 2 sockets, for example,
> >>
> >> Node 0 & 2 are cpu + dram nodes and node 1 are slow
> >> memory node near node 0,
> >>
> >> available: 3 nodes (0-2)
> >> node 0 cpus: 0 1
> >> node 0 size: n MB
> >> node 0 free: n MB
> >> node 1 cpus:
> >> node 1 size: n MB
> >> node 1 free: n MB
> >> node 2 cpus: 2 3
> >> node 2 size: n MB
> >> node 2 free: n MB
> >> node distances:
> >> node   0   1   2
> >>    0:  10  40  20
> >>    1:  40  10  80
> >>    2:  20  80  10
> >>
> >> We have 2 choices,
> >>
> >> a)
> >> node    demotion targets
> >> 0       1
> >> 2       1
> >>
> >> b)
> >> node    demotion targets
> >> 0       1
> >> 2       X
> >>
> >> a) is good to take advantage of PMEM.  b) is good to reduce cross-socket
> >> traffic.  Both are OK as defualt configuration.  But some users may
> >> prefer the other one.  So we need a user space ABI to override the
> >> default configuration.
> >
> > I think 2(a) should be the system-wide configuration and 2(b) can be
> > achieved with NUMA mempolicy (which needs to be added to demotion).
> >
> > In general, we can view the demotion order in a way similar to
> > allocation fallback order (after all, if we don't demote or demotion
> > lags behind, the allocations will go to these demotion target nodes
> > according to the allocation fallback order anyway).  If we initialize
> > the demotion order in that way (i.e. every node can demote to any node
> > in the next tier, and the priority of the target nodes is sorted for
> > each source node), we don't need per-node demotion order override from
> > the userspace.  What we need is to specify what nodes should be in
> > each tier and support NUMA mempolicy in demotion.
> >
>
> I have been wondering how we would handle this. For ex: If an
> application has specified an MPOL_BIND policy and restricted the
> allocation to be from Node0 and Node1, should we demote pages allocated
> by that application
> to Node10? The other alternative for that demotion is swapping. So from
> the page point of view, we either demote to a slow memory or pageout to
> swap. But then if we demote we are also breaking the MPOL_BIND rule.

IMHO, the MPOL_BIND policy should be respected and demotion should be
skipped in such cases.  Such MPOL_BIND policies can be an important
tool for applications to override and control their memory placement
when transparent memory tiering is enabled.  If the application
doesn't want swapping, there are other ways to achieve that (e.g.
mlock, disabling swap globally, setting memcg parameters, etc).

> The above says we would need some kind of mem policy interaction, but
> what I am not sure about is how to find the memory policy in the
> demotion path.

This is indeed an important and challenging problem.  One possible
approach is to retrieve the allowed demotion nodemask from
page_referenced() similar to vm_flags.

>
> > Cross-socket demotion should not be too big a problem in practice
> > because we can optimize the code to do the demotion from the local CPU
> > node (i.e. local writes to the target node and remote read from the
> > source node).  The bigger issue is cross-socket memory access onto the
> > demoted pages from the applications, which is why NUMA mempolicy is
> > important here.
> >
> >
> -aneesh