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

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Message-ID: <ZVOXWx8XNJJNC23A@memverge.com>
Date:   Tue, 14 Nov 2023 10:50:51 -0500
From:   Gregory Price <gregory.price@...verge.com>
To:     Michal Hocko <mhocko@...e.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>,
        "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 Tue, Nov 14, 2023 at 10:43:13AM +0100, Michal Hocko wrote:
> On Fri 10-11-23 22:42:39, Gregory Price wrote:
> [...]
> > If I can ask, do you think it would be out of line to propose a major
> > refactor to mempolicy to enable external task's the ability to change a
> > running task's mempolicy *as well as* a cgroup-wide mempolicy component?
> 
> No, I actually think this is a reasonable idea. pidfd_setmempolicy is a
> generally useful extension. The mempolicy code is heavily current task
> based and there might be some challenges but I believe this will a)
> improve the code base and b) allow more usecases.

Just read up on the pidfd_set_mempolicy lore, and yes I'm seeing all the
same problems (I know there was discussion of vma policies, but i think
that can be a topic for later).  Have some thoughts on this, but will
take some time to work through a few refactoring tickets first.

> 
> That being said, I still believe that a cgroup based interface is a much
> better choice over a global one. Cpusets seem to be a good fit as the
> controller does control memory placement wrt NUMA interfaces.

I think cpusets is a non-starter due to the global spinlock required when
reading informaiton from it:

https://elixir.bootlin.com/linux/latest/source/kernel/cgroup/cpuset.c#L391

Unless the proposal is to place the weights as a global cgroups value,
in which case I think it would be better placed in default_mempolicy :]