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

[Alistair Popple <apopple@nvidia.com>]

On Friday, 29 April 2022 1:27:36 PM AEST ying.huang@intel.com wrote:
> On Thu, 2022-04-28 at 19:58 -0700, Wei Xu wrote:
> > On Thu, Apr 28, 2022 at 7:21 PM ying.huang@intel.com
> > <ying.huang@intel.com> wrote:
> > > 
> > > On Fri, 2022-04-29 at 11:27 +1000, Alistair Popple wrote:
> > > > On Friday, 29 April 2022 3:14:29 AM AEST Yang Shi wrote:
> > > > > On Wed, Apr 27, 2022 at 9:11 PM Wei Xu <weixugc@google.com> wrote:
> > > > > > 
> > > > > > On Wed, Apr 27, 2022 at 5:56 PM ying.huang@intel.com
> > > > > > <ying.huang@intel.com> wrote:
> > > > > > > 
> > > > > > > On Wed, 2022-04-27 at 11:27 -0700, Wei Xu wrote:
> > > > > > > > On Tue, Apr 26, 2022 at 10:06 PM Aneesh Kumar K V
> > > > > > > > <aneesh.kumar@linux.ibm.com> wrote:
> > > > > > > > > 
> > > > > > > > > On 4/25/22 10:26 PM, Wei Xu wrote:
> > > > > > > > > > On Sat, Apr 23, 2022 at 8:02 PM ying.huang@intel.com
> > > > > > > > > > <ying.huang@intel.com> 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.
> > > > > > > 
> > > > > > > This works for mempolicy in struct vm_area_struct, but not for that in
> > > > > > > struct task_struct.  Mutiple threads in a process may have different
> > > > > > > mempolicy.
> > > > > > 
> > > > > > From vm_area_struct, we can get to mm_struct and then to the owner
> > > > > > task_struct, which has the process mempolicy.
> > > > > > 
> > > > > > It is indeed a problem when a page is shared by different threads or
> > > > > > different processes that have different thread default mempolicy
> > > > > > values.
> > > > > 
> > > > > Sorry for chiming in late, this is a known issue when we were working
> > > > > on demotion. Yes, it is hard to handle the shared pages and multi
> > > > > threads since mempolicy is applied to each thread so each thread may
> > > > > have different mempolicy. And I don't think this case is rare. And not
> > > > > only mempolicy but also may cpuset settings cause the similar problem,
> > > > > different threads may have different cpuset settings for cgroupv1.
> > > > > 
> > > > > If this is really a problem for real life workloads, we may consider
> > > > > tackling it for exclusively owned pages first. Thanks to David's
> > > > > patches, now we have dedicated flags to tell exclusively owned pages.
> > > > 
> > > > One of the problems with demotion when I last looked is it does almost exactly
> > > > the opposite of what we want on systems like POWER9 where GPU memory is a
> > > > CPU-less memory node.
> > > > 
> > > > On those systems users tend to use MPOL_BIND or MPOL_PREFERRED to allocate
> > > > memory on the GPU node. Under memory pressure demotion should migrate GPU
> > > > allocations to the CPU node and finally other slow memory nodes or swap.
> > > > 
> > > > Currently though demotion considers the GPU node slow memory (because it is
> > > > CPU-less) so will demote CPU memory to GPU memory which is a limited resource.
> > > > And when trying to allocate GPU memory with MPOL_BIND/PREFERRED it will swap
> > > > everything to disk rather than demote to CPU memory (which would be preferred).
> > > > 
> > > > I'm still looking at this series but as I understand it it will help somewhat
> > > > because we could make GPU memory the top-tier so nothing gets demoted to it.
> > > 
> > > Yes.  If we have a way to put GPU memory in top-tier (tier 0) and
> > > CPU+DRAM in tier 1.  Your requirement can be satisfied.  One way is to
> > > override the auto-generated demotion order via some user space tool.
> > > Another way is to change the GPU driver (I guess where the GPU memory is
> > > enumerated and onlined?) to change the tier of GPU memory node.

Yes, although I think in this case it would be firmware that determines memory
tiers (similar to ACPI HMAT which I saw discussed somewhere here). I agree it's
a system level property though that in an ideal world shouldn't need overriding
from userspace. However being able to override it with a user space tool could
be useful.

> > > > However I wouldn't want to see demotion skipped entirely when a memory policy
> > > > such as MPOL_BIND is specified. For example most memory on a GPU node will have
> > > > some kind of policy specified and IMHO it would be better to demote to another
> > > > node in the mempolicy nodemask rather than going straight to swap, particularly
> > > > as GPU memory capacity tends to be limited in comparison to CPU memory
> > > > capacity.
> > > > > 
> > > 
> > > Can you use MPOL_PREFERRED?  Even if we enforce MPOL_BIND as much as
> > > possible, we will not stop demoting from GPU to DRAM with
> > > MPOL_PREFERRED.  And in addition to demotion, allocation fallbacking can
> > > be used too to avoid allocation latency caused by demotion.

I think so. It's been a little while since I last looked at this but I was
under the impression MPOL_PREFERRED didn't do direct reclaim (and therefore
wouldn't trigger demotion so once GPU memory was full became effectively a
no-op). However looking at the source I don't think that's the case now - if
I'm understanding correctly MPOL_PREFERRED will do reclaim/demotion.

The other problem with MPOL_PREFERRED is it doesn't allow the fallback nodes to
be specified. I was hoping the new MPOL_PREFERRED_MANY and
set_mempolicy_home_node() would help here but currently that does disable
reclaim (and therefore demotion) in the first pass.

However that problem is tangential to this series and I can look at that
separately. My main aim here given you were looking at requirements was just
to raise this as a slightly different use case (one where the CPU isn't the top
tier).

Thanks for looking into all this.

 - Alistair

> > I expect that MPOL_BIND can be used to either prevent demotion or
> > select a particular demotion node/nodemask. It all depends on the
> > mempolicy nodemask specified by MPOL_BIND.
> 
> Yes.  I think so too.
> 
> Best Regards,
> Huang, Ying
> 
> > > This is another example of a system with 3 tiers if PMEM is installed in
> > > this machine too.
> > > 
> > > Best Regards,
> > > Huang, Ying
> > > 
> > > > > > On the other hand, it can already support most interesting use cases
> > > > > > for demotion (e.g. selecting the demotion node, mbind to prevent
> > > > > > demotion) by respecting cpuset and vma mempolicies.
> > > > > > 
> > > > > > > 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.
> > > > > > > > > > 
> > > > > > > > > > 
> > > > > > > > > -aneesh
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