linux-kernel - Re: [PATCHv4 10/13] node: Add memory caching attributes

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Date:   Tue, 12 Feb 2019 10:31:20 -0700
From:   Keith Busch <keith.busch@...el.com>
To:     Jonathan Cameron <jonathan.cameron@...wei.com>
Cc:     Brice Goglin <Brice.Goglin@...ia.fr>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rafael Wysocki <rafael@...nel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCHv4 10/13] node: Add memory caching attributes

On Tue, Feb 12, 2019 at 08:49:03AM +0000, Jonathan Cameron wrote:
> On Mon, 11 Feb 2019 08:23:04 -0700
> Keith Busch <keith.busch@...el.com> wrote:
> 
> > On Sun, Feb 10, 2019 at 09:19:58AM -0800, Jonathan Cameron wrote:
> > > On Sat, 9 Feb 2019 09:20:53 +0100
> > > Brice Goglin <Brice.Goglin@...ia.fr> wrote:
> > >   
> > > > Hello Keith
> > > > 
> > > > Could we ever have a single side cache in front of two NUMA nodes ? I
> > > > don't see a way to find that out in the current implementation. Would we
> > > > have an "id" and/or "nodemap" bitmask in the sidecache structure ?  
> > > 
> > > This is certainly a possible thing for hardware to do.
> > >
> > > ACPI IIRC doesn't provide any means of representing that - your best
> > > option is to represent it as two different entries, one for each of the
> > > memory nodes.  Interesting question of whether you would then claim
> > > they were half as big each, or the full size.  Of course, there are
> > > other possible ways to get this info beyond HMAT, so perhaps the interface
> > > should allow it to be exposed if available?  
> > 
> > HMAT doesn't do this, but I want this interface abstracted enough from
> > HMAT to express whatever is necessary.
> > 
> > The CPU cache is the closest existing exported attributes to this,
> > and they provide "shared_cpu_list". To that end, I can export a
> > "shared_node_list", though previous reviews strongly disliked multi-value
> > sysfs entries. :(
> > 
> > Would shared-node symlinks capture the need, and more acceptable?
> 
> My inclination is that it's better to follow an existing pattern than
> invent a new one that breaks people's expectations.
> 
> However, don't feel that strongly about it as long as the interface
> is functional and intuitive.

Okay, considering I'd have a difficult time testing such an interface
since it doesn't apply to HMAT, and I've received only conflicting
feedback on list attributes, I would prefer to leave this feature out
of this series for now. I'm certainly not against adding it later.