linux-kernel - Re: [RFC PATCH v4 0/3] memcg weighted interleave mempolicy control

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Date:   Sun, 12 Nov 2023 21:22:20 -0500
From:   Gregory Price <gregory.price@...verge.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     "tj@...nel.org" <tj@...nel.org>, John Groves <john@...alactic.com>,
        Gregory Price <gourry.memverge@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-cxl@...r.kernel.org" <linux-cxl@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "ying.huang@...el.com" <ying.huang@...el.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
        "lizefan.x@...edance.com" <lizefan.x@...edance.com>,
        "hannes@...xchg.org" <hannes@...xchg.org>,
        "corbet@....net" <corbet@....net>,
        "roman.gushchin@...ux.dev" <roman.gushchin@...ux.dev>,
        "shakeelb@...gle.com" <shakeelb@...gle.com>,
        "muchun.song@...ux.dev" <muchun.song@...ux.dev>,
        "jgroves@...ron.com" <jgroves@...ron.com>
Subject: Re: [RFC PATCH v4 0/3] memcg weighted interleave mempolicy control

On Sat, Nov 11, 2023 at 03:54:55PM -0800, Dan Williams wrote:
> tj@...nel.org wrote:
> > Hello,
> > 
> > On Fri, Nov 10, 2023 at 10:42:39PM -0500, Gregory Price wrote:
> > > On Fri, Nov 10, 2023 at 05:05:50PM -1000, tj@...nel.org wrote:
> 
> > Here, even if CXL actually becomes popular, how many are going to use memory
> > hotplug and need to dynamically rebalance memory in actively running
> > workloads? What's the scenario? Are there going to be an army of data center
> > technicians going around plugging and unplugging CXL devices depending on
> > system memory usage?
> 
> While I have personal skepticism that all of the infrastructure in the
> CXL specification is going to become popular, one mechanism that seems
> poised to cross that threshold is "dynamic capacity". So it is not the
> case that techs are running around hot-adjusting physical memory. A host
> will have a cable hop to a shared memory pool in the rack where it can
> be dynamically provisioned across hosts.
> 
> However, even then the bounds of what is dynamic is going to be
> constrained to a fixed address space with likely predictable performance
> characteristics for that address range. That potentially allows for a
> system wide memory interleave policy to be viable. That might be the
> place to start and mirrors, at a coarser granularity, what hardware
> interleaving can do.
> 
> [..]

Funny enough, this is exactly why I skipped cgroups and went directly to 
implementing the weights as an attribute of numa nodes. It cuts out a
middle-man and lets you apply weights globally.

BUT the policy is still ultimately opt-in, so you don't really get a
global effect, just a global control.  Just given that lesson, yeah
it's better to reduce the scope to mempolicy first.

Getting to global interleave weights from there... more complicated.

The simplees way I can think of to test system-wide weighted interleave
is to have the init task create a default mempolicy and have all tasks
inherit it.  That feels like a big, dumb hammer - but it might work.

Comparatively, implementing a mempolicy in the root cgroup and having
tasks use that directly "feels" better, though lessons form this patch
- interating cgroup parent trees on allocations feels not great.

Barring that, if a cgroup.mempolicy and a default mempolicy for init
aren't realistic, I don't see a good path to fruition for a global
interleave approach that doesn't require nastier allocator changes.

In the meantime, unless there's other pro-cgroups voices, I'm going to
pivot back to my initial approach of doing it in mempolicy, though I
may explore extending mempolicy into procfs at the same time.

~Gregory