linux-kernel - Re: [v2 RFC PATCH 0/9] Another Approach to Use PMEM as NUMA Node

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Date:   Wed, 17 Apr 2019 09:13:47 -0600
From:   Keith Busch <keith.busch@...el.com>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Michal Hocko <mhocko@...nel.org>, mgorman@...hsingularity.net,
        riel@...riel.com, hannes@...xchg.org, akpm@...ux-foundation.org,
        dan.j.williams@...el.com, fengguang.wu@...el.com, fan.du@...el.com,
        ying.huang@...el.com, ziy@...dia.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v2 RFC PATCH 0/9] Another Approach to Use PMEM as NUMA Node

On Tue, Apr 16, 2019 at 04:17:44PM -0700, Yang Shi wrote:
> On 4/16/19 4:04 PM, Dave Hansen wrote:
> > On 4/16/19 2:59 PM, Yang Shi wrote:
> > > On 4/16/19 2:22 PM, Dave Hansen wrote:
> > > > Keith Busch had a set of patches to let you specify the demotion order
> > > > via sysfs for fun.  The rules we came up with were:
> > > > 1. Pages keep no history of where they have been
> > > > 2. Each node can only demote to one other node
> > > Does this mean any remote node? Or just DRAM to PMEM, but remote PMEM
> > > might be ok?
> > In Keith's code, I don't think we differentiated.  We let any node
> > demote to any other node you want, as long as it follows the cycle rule.
> 
> I recall Keith's code let the userspace define the target node.

Right, you have to opt-in in my original proposal since it may be a
bit presumptuous of the kernel to decide how a node's memory is going
to be used. User applications have other intentions for it.

It wouldn't be too difficult to make HMAT to create a reasonable initial
migration graph too, and that can also make that an opt-in user choice.

> Anyway, we may need add one rule: not migrate-on-reclaim from PMEM
> node.  Demoting from  PMEM to DRAM sounds pointless.

I really don't think we should be making such hard rules on PMEM. It
makes more sense to consider performance and locality for migration
rules than on a persistence attribute.