From: Michal Hocko <mhocko@kernel.org>
To: David Hildenbrand <david@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
Michael Ellerman <mpe@ellerman.id.au>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
"Rafael J. Wysocki" <rafael@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Leonardo Bras <leonardo@linux.ibm.com>,
Nathan Lynch <nathanl@linux.ibm.com>,
Allison Randal <allison@lohutok.net>,
Nathan Fontenot <nfont@linux.vnet.ibm.com>,
Thomas Gleixner <tglx@linutronix.de>,
Dan Williams <dan.j.williams@intel.com>,
Stephen Rothwell <sfr@canb.auug.org.au>,
Anshuman Khandual <anshuman.khandual@arm.com>,
lantianyu1986@gmail.com, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH RFC v1] mm: is_mem_section_removable() overhaul
Date: Fri, 17 Jan 2020 15:52:33 +0100 [thread overview]
Message-ID: <20200117145233.GB19428@dhcp22.suse.cz> (raw)
In-Reply-To: <c82a0dd7-a99b-6def-83d4-a19fbdd405d9@redhat.com>
On Fri 17-01-20 14:08:06, David Hildenbrand wrote:
> On 17.01.20 12:33, Michal Hocko wrote:
> > On Fri 17-01-20 11:57:59, David Hildenbrand wrote:
> >> Let's refactor that code. We want to check if we can offline memory
> >> blocks. Add a new function is_mem_section_offlineable() for that and
> >> make it call is_mem_section_offlineable() for each contained section.
> >> Within is_mem_section_offlineable(), add some more sanity checks and
> >> directly bail out if the section contains holes or if it spans multiple
> >> zones.
> >
> > I didn't read the patch (yet) but I am wondering. If we want to touch
> > this code, can we simply always return true there? I mean whoever
> > depends on this check is racy and the failure can happen even after
> > the sysfs says good to go, right? The check is essentially as expensive
> > as calling the offlining code itself. So the only usecase I can think of
> > is a dumb driver to crawl over blocks and check which is removable and
> > try to hotremove it. But just trying to offline one block after another
> > is essentially going to achieve the same.
>
> Some thoughts:
>
> 1. It allows you to check if memory is likely to be offlineable without
> doing expensive locking and trying to isolate pages (meaning:
> zone->lock, mem_hotplug_lock. but also, calling drain_all_pages()
> when isolating)
>
> 2. There are use cases that want to identify a memory block/DIMM to
> unplug. One example is PPC DLPAR code (see this patch). Going over all
> memory block trying to offline them is an expensive operation.
>
> 3. powerpc-utils (https://github.com/ibm-power-utilities/powerpc-utils)
> makes use of /sys/.../removable to speed up the search AFAIK.
Well, while I do see those points I am not really sure they are worth
having a broken (by-definition) interface.
> 4. lsmem displays/groups by "removable".
Is anybody really using that?
> 5. If "removable=false" then it usually really is not offlineable.
> Of course, there could also be races (free the last unmovable page),
> but it means "don't even try". OTOH, "removable=true" is more racy,
> and gives less guarantees. ("looks okay, feel free to try")
Yeah, but you could be already pessimistic and try movable zones before
other kernel zones.
> > Or does anybody see any reasonable usecase that would break if we did
> > that unconditional behavior?
>
> If we would return always "true", then the whole reason the
> interface originally was introduced would be "broken" (meaning, less
> performant as you would try to offline any memory block).
I would argue that the whole interface is broken ;). Not the first time
in the kernel development history and not the last time either. What I
am trying to say here is that unless there are _real_ usecases depending
on knowing that something surely is _not_ offlineable then I would just
try to drop the functionality while preserving the interface and see
what happens.
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2020-01-17 14:52 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-17 10:57 [PATCH RFC v1] mm: is_mem_section_removable() overhaul David Hildenbrand
2020-01-17 11:33 ` Michal Hocko
2020-01-17 13:08 ` David Hildenbrand
2020-01-17 14:52 ` Michal Hocko [this message]
2020-01-17 14:58 ` David Hildenbrand
2020-01-17 15:29 ` Michal Hocko
2020-01-17 15:54 ` Dan Williams
2020-01-17 16:10 ` David Hildenbrand
2020-01-17 16:57 ` Dan Williams
2020-01-20 7:48 ` Michal Hocko
2020-01-20 9:14 ` David Hildenbrand
2020-01-20 9:20 ` David Hildenbrand
2020-01-21 12:07 ` Michal Hocko
2020-01-22 10:39 ` David Hildenbrand
2020-01-22 10:42 ` Michal Hocko
2020-01-22 10:54 ` David Hildenbrand
2020-01-22 11:58 ` David Hildenbrand
2020-01-22 16:46 ` Michal Hocko
2020-01-22 18:15 ` David Hildenbrand
2020-01-22 18:38 ` Michal Hocko
2020-01-22 18:46 ` David Hildenbrand
2020-01-22 19:09 ` Michal Hocko
2020-01-22 20:51 ` Dan Williams
2020-01-22 19:01 ` Michal Hocko
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20200117145233.GB19428@dhcp22.suse.cz \
--to=mhocko@kernel.org \
--cc=akpm@linux-foundation.org \
--cc=allison@lohutok.net \
--cc=anshuman.khandual@arm.com \
--cc=benh@kernel.crashing.org \
--cc=dan.j.williams@intel.com \
--cc=david@redhat.com \
--cc=gregkh@linuxfoundation.org \
--cc=lantianyu1986@gmail.com \
--cc=leonardo@linux.ibm.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=linuxppc-dev@lists.ozlabs.org \
--cc=mpe@ellerman.id.au \
--cc=nathanl@linux.ibm.com \
--cc=nfont@linux.vnet.ibm.com \
--cc=paulus@samba.org \
--cc=rafael@kernel.org \
--cc=sfr@canb.auug.org.au \
--cc=tglx@linutronix.de \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).