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

[Davidlohr Bueso]

On Mon, 25 Apr 2022, Aneesh Kumar K V wrote:

> On 4/25/22 11:40 AM, ying.huang@xxxxxxxxx wrote:
> > On Mon, 2022-04-25 at 09:20 +0530, Aneesh Kumar K.V wrote:
> > > "ying.huang@xxxxxxxxx" <ying.huang@xxxxxxxxx> writes:
> > >
> > > > Hi, All,
> > > >
> > > > On Fri, 2022-04-22 at 16:30 +0530, Jagdish Gediya wrote:
> > > >
> > > > [snip]
> > > >
> > > > > I think it is necessary to either have per node demotion targets
> > > > > configuration or the user space interface supported by this patch
> > > > > series. As we don't have clear consensus on how the user interface
> > > > > should look like, we can defer the per node demotion target set
> > > > > interface to future until the real need arises.
> > > > >
> > > > > Current patch series sets N_DEMOTION_TARGET from dax device kmem
> > > > > driver, it may be possible that some memory node desired as demotion
> > > > > target is not detected in the system from dax-device kmem probe path.
> > > > >
> > > > > It is also possible that some of the dax-devices are not preferred as
> > > > > demotion target e.g. HBM, for such devices, node shouldn't be set to
> > > > > N_DEMOTION_TARGETS. In future, Support should be added to distinguish
> > > > > such dax-devices and not mark them as N_DEMOTION_TARGETS from the
> > > > > kernel, but for now this user space interface will be useful to avoid
> > > > > such devices as demotion targets.
> > > > >
> > > > > We can add read only interface to view per node demotion targets
> > > > > from /sys/devices/system/node/nodeX/demotion_targets, remove
> > > > > duplicated /sys/kernel/mm/numa/demotion_target interface and instead
> > > > > make /sys/devices/system/node/demotion_targets writable.
> > > > >
> > > > > Huang, Wei, Yang,
> > > > > What do you suggest?
> > > >
> > > > We cannot remove a kernel ABI in practice.  So we need to make it right
> > > > at the first time.  Let's try to collect some information for the kernel
> > > > ABI definitation.
> > > >
> > > > The below is just a starting point, please add your requirements.
> > > >
> > > > 1. Jagdish has some machines with DRAM only NUMA nodes, but they don't
> > > > want to use that as the demotion targets.  But I don't think this is a
> > > > issue in practice for now, because demote-in-reclaim is disabled by
> > > > default.
> > >
> > > It is not just that the demotion can be disabled. We should be able to
> > > use demotion on a system where we can find DRAM only NUMA nodes. That
> > > cannot be achieved by /sys/kernel/mm/numa/demotion_enabled. It needs
> > > something similar to to N_DEMOTION_TARGETS
> >
> > Can you show NUMA information of your machines with DRAM-only nodes and
> > PMEM nodes?  We can try to find the proper demotion order for the
> > system.  If you can not show it, we can defer N_DEMOTION_TARGETS until
> > the machine is available.
>
>
> Sure will find one such config. As you might have noticed this is very
> easy to have in a virtualization setup because the hypervisor can
> assign memory to a guest VM from a numa node that doesn't have CPU
> assigned to the same guest. This depends on the other guest VM
> instance config running on the system. So on any virtualization config
> that has got persistent memory attached, this can become an easy
> config to end up with.

And as hw becomes available things like CXL will also start to show
"interesting" setups. You have a mix of volatile and/or pmem nodes
with different access costs, so: CPU+DRAM, DRAM (?), volatile CXL mem,
CXL pmem, non-cxl pmem.

imo, by default, slower mem should be demotion candidates regardless of
type or socket layout (which can be a last consideration such that this
is somewhat mitigated). And afaict this is along the lines of what Jagdish's
first example refers to in patch 1/5.

> > > > 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
> > >
> > > This is achieved by
> > >
> > > [PATCH v2 1/5] mm: demotion: Set demotion list differently

Yes, I think it makes sense to do 2a.

Thanks,
Davidlohr