linux-kernel - Re: [PATCH v1] drivers/base/memory.c: Don't store end_section

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Message-ID: <20190731143714.GX9330@dhcp22.suse.cz>
Date:   Wed, 31 Jul 2019 16:37:14 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH v1] drivers/base/memory.c: Don't store end_section_nr in
 memory blocks

On Wed 31-07-19 16:21:46, David Hildenbrand wrote:
[...]
> > Thinking about it some more, I believe that we can reasonably provide
> > both APIs controlable by a command line parameter for backwards
> > compatibility. It is the hotplug code to control sysfs APIs.  E.g.
> > create one sysfs entry per add_memory_resource for the new semantic.
> 
> Yeah, but the real question is: who needs it. I can only think about
> some DIMM scenarios (some, not all). I would be interested in more use
> cases. Of course, to provide and maintain two APIs we need a good reason.

Well, my 3TB machine that has 7 movable nodes could really go with less
than
$ find /sys/devices/system/memory -name "memory*" | wc -l
1729

when it doesn't really make any sense to offline less than a
hotremovable entity which is the whole node effectivelly. I have seen
reports where a similarly large machine chocked on boot just because of
too many udev events...

In other words allowing smaller granularity is a nice toy but real
usecases usually work with the whole hotplugable entity (e.g. the whole
ACPI container).

> (one sysfs per add_memory_resource() won't cover all DIMMs completely as
> far as I remember - I might be wrong, I remember there could be a
> sequence of add_memory(). Also, some DIMMs might actually overlap with
> memory indicated during boot - complicated stuff)

Which is something we have to live with anyway due to nodes interleaving.
So nothing really new.
-- 
Michal Hocko
SUSE Labs