linux-kernel - Re: [RFC PATCH v3 0/4] Node Weights and Weighted Interleave

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Message-ID: <20231103141636.000007e4@Huawei.com>
Date:   Fri, 3 Nov 2023 14:16:36 +0000
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     "Huang, Ying" <ying.huang@...el.com>,
        Gregory Price <gregory.price@...verge.com>
CC:     Michal Hocko <mhocko@...e.com>,
        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>, <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 Fri, 03 Nov 2023 15:45:13 +0800
"Huang, Ying" <ying.huang@...el.com> wrote:

> Gregory Price <gregory.price@...verge.com> writes:
> 
> > On Thu, Nov 02, 2023 at 10:47:33AM +0100, Michal Hocko wrote:  
> >> On Wed 01-11-23 12:58:55, Gregory Price wrote:  
> >> > Basically consider: `numactl --interleave=all ...`
> >> > 
> >> > If `--weights=...`: when a node hotplug event occurs, there is no
> >> > recourse for adding a weight for the new node (it will default to 1).  
> >> 
> >> Correct and this is what I was asking about in an earlier email. How
> >> much do we really need to consider this setup. Is this something nice to
> >> have or does the nature of the technology requires to be fully dynamic
> >> and expect new nodes coming up at any moment?
> >>    
> >
> > Dynamic Capacity is expected to cause a numa node to change size (in
> > number of memory blocks) rather than cause numa nodes to come and go, so
> > maybe handling the full node hotplug is a bit of an overreach.  
> 
> Will node max bandwidth change with the number of memory blocks?

Typically no as even a single memory extent would probably be interleaved
across all the actual memory devices (think DIMMS for simplicity) within
a CXL device. I guess a device 'could' do some scaling based on capacity
provided to a particular host but feels like they should be separate controls.
I don't recall there being anything in the specification to suggest the
need to recheck the CDAT info for updates when DC add / remove events happen.

Mind you, who knows in future :)  We'll point out in relevant forums that
doing so would be very hard to handle cleanly in Linux.

Jonathan