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

[Wei Xu]

On Wed, Apr 27, 2022 at 12:11 AM ying.huang@xxxxxxxxx
<ying.huang@xxxxxxxxx> wrote:
>
> On Mon, 2022-04-25 at 09:56 -0700, Wei Xu wrote:
> > On Sat, Apr 23, 2022 at 8:02 PM ying.huang@xxxxxxxxx
> > <ying.huang@xxxxxxxxx> wrote:
> > >
> > > 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.
> > >
> > > 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).
>
> Unfortunately, some NUMA mempolicy information isn't available at
> demotion time, for example, mempolicy enforced via set_mempolicy() is
> for thread. But I think that cpusets can work for 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.
>
> This sounds interesting. Tier sounds like a natural and general concept
> for these memory types. It's attracting to use it for user space
> interface too. For example, we may use that for mem_cgroup limits of a
> specific memory type (tier).
>
> And if we take a look at the N_DEMOTION_TARGETS again from the "tier"
> point of view. The nodes are divided to 2 classes via
> N_DEMOTION_TARGETS.
>
> - The nodes without N_DEMOTION_TARGETS are top tier (or tier 0).
>
> - The nodes with N_DEMOTION_TARGETS are non-top tier (or tier 1, 2, 3,
> ...)
>

Yes, this is one of the main reasons why we (Google) want this interface.

> So, another possibility is to fit N_DEMOTION_TARGETS and its overriding
> into "tier" concept too.  !N_DEMOTION_TARGETS == TIER0.
>
> - All nodes start with TIER0
>
> - TIER0 can be cleared for some nodes via e.g. kmem driver
>
> TIER0 node list can be read or overriden by the user space via the
> following interface,
>
>   /sys/devices/system/node/tier0
>
> In the future, if we want to customize more tiers, we can add tier1,
> tier2, tier3, .....  For now, we can add just tier0.  That is, the
> interface is extensible in the future compared with
> .../node/demote_targets.
>

This more explicit tier definition interface works, too.

> This isn't as flexible as the writable per-node demotion targets.  But
> it may be enough for most requirements?

I would think so. Besides, it doesn't really conflict with the
per-node demotion target interface if we really want to introduce the
latter.

> Best Regards,
> Huang, Ying
>
> > 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.
> >
> > > 3. For machines with HBM (High Bandwidth Memory), as in
> > >
> > > https://lore.kernel.org/lkml/39cbe02a-d309-443d-54c9-678a0799342d@xxxxxxxxx/
> > >
> > > > [1] local DDR = 10, remote DDR = 20, local HBM = 31, remote HBM = 41
> > >
> > > Although HBM has better performance than DDR, in ACPI SLIT, their
> > > distance to CPU is longer.  We need to provide a way to fix this.  The
> > > user space ABI is one way.  The desired result will be to use local DDR
> > > as demotion targets of local HBM.
> > >
> > > Best Regards,
> > > Huang, Ying
> > >
>
>
>