Date:   Wed, 1 Nov 2023 14:45:50 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Gregory Price <gregory.price@...verge.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Gregory Price <gourry.memverge@...il.com>,
        linux-kernel@...r.kernel.org, linux-cxl@...r.kernel.org,
        linux-mm@...ck.org, ying.huang@...el.com,
        akpm@...ux-foundation.org, aneesh.kumar@...ux.ibm.com,
        weixugc@...gle.com, apopple@...dia.com, tim.c.chen@...el.com,
        dave.hansen@...el.com, shy828301@...il.com,
        gregkh@...uxfoundation.org, rafael@...nel.org
Subject: Re: [RFC PATCH v3 0/4] Node Weights and Weighted Interleave

On Tue 31-10-23 00:27:04, Gregory Price wrote:
> On Tue, Oct 31, 2023 at 04:56:27PM +0100, Michal Hocko wrote:
> 
> > > This hopefully also explains why it's a global setting. The usecase is
> > > different from conventional NUMA interleaving, which is used as a
> > > locality measure: spread shared data evenly between compute
> > > nodes. This one isn't about locality - the CXL tier doesn't have local
> > > compute. Instead, the optimal spread is based on hardware parameters,
> > > which is a global property rather than a per-workload one.
> > 
> > Well, I am not convinced about that TBH. Sure it is probably a good fit
> > for this specific CXL usecase but it just doesn't fit into many others I
> > can think of - e.g. proportional use of those tiers based on the
> > workload - you get what you pay for.
> > 
> > Is there any specific reason for not having a new interleave interface
> > which defines weights for the nodemask? Is this because the policy
> > itself is very dynamic or is this more driven by simplicity of use?
> > 
> 
> I had originally implemented it this way while experimenting with new
> mempolicies.
> 
> https://lore.kernel.org/linux-cxl/20231003002156.740595-5-gregory.price@memverge.com/
> 
> The downside of doing it in mempolicy is...
> 1) mempolicy is not sysfs friendly, and to make it sysfs friendly is a
>    non-trivial task.  It is very "current-task" centric.

True. Cpusets is the way to make it less process centric but that comes
with its own constains (namely which NUMA policies are supported).
 
> 2) Barring a change to mempolicy to be sysfs friendly, the options for
>    implementing weights in the mempolicy are either a) new flag and
>    setting every weight individually in many syscalls, or b) a new
>    syscall (set_mempolicy2), which is what I demonstrated in the RFC.

Yes, that would likely require a new syscall.
 
> 3) mempolicy is also subject to cgroup nodemasks, and as a result you
>    end up with a rats nest of interactions between mempolicy nodemasks
>    changing as a result of cgroup migrations, nodes potentially coming
>    and going (hotplug under CXL), and others I'm probably forgetting.

Is this really any different from what you are proposing though?

>    Basically:  If a node leaves the nodemask, should you retain the
>    weight, or should you reset it? If a new node comes into the node
>    mask... what weight should you set? I did not have answers to these
>    questions.

I am not really sure I follow you. Are you talking about cpuset
nodemask changes or memory hotplug here.

> It was recommended to explore placing it in tiers instead, so I took a
> crack at it here: 
> 
> https://lore.kernel.org/linux-mm/20231009204259.875232-1-gregory.price@memverge.com/
> 
> This had similar issue with the idea of hotplug nodes: if you give a
> tier a weight, and one or more of the nodes goes away/comes back... what
> should you do with the weight?  Split it up among the remaining nodes?
> Rebalance? Etc.

How is this any different from node becoming depleted? You cannot
really expect that you get memory you are asking for and you can easily
end up getting memory from a different node instead.
 
> The result of this discussion lead us to simply say "What if we place
> the weights directly in the node".  And that lead us to this RFC.

Maybe I am missing something really crucial here but I do not see how
this fundamentally changes anything.

Memory hotremove (or mere node memory depletion) is not really a thing
because interleaving is a best effort operation so you have to live with
memory not being strictly distributed per your preferences.

Memory hotadd will be easier to manage because you just update a single
place after node is hotadded rather than gazillions partial policies.
But, that requires that interleave policy nodemask is assuming future
nodes going online and put them to the mask.

> I am not against implementing it in mempolicy (as proof: my first RFC).
> I am simply searching for the acceptable way to implement it.
> 
> One of the benefits of having it set as a global setting is that weights
> can be automatically generated from HMAT/HMEM information (ACPI tables)
> and programs already using MPOL_INTERLEAVE will have a direct benefit.

Right. This is understood. My main concern is whether this is outweights
the limitations of having a _global_ policy _only_. Historically a single
global policy usually led to finding ways how to make that more scoped
(usually through cgroups).
 
> I have been considering whether MPOL_WEIGHTED_INTERLEAVE should be added
> along side this patch so that MPOL_INTERLEAVE is left entirely alone.
> 
> Happy to discuss more,
> ~Gregory

-- 
Michal Hocko
SUSE Labs

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