From: Michal Hocko <mhocko@kernel.org>
To: David Hildenbrand <david@redhat.com>
Cc: Baoquan He <bhe@redhat.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
akpm@linux-foundation.org, aarcange@redhat.com
Subject: Re: Memory hotplug softlock issue
Date: Wed, 14 Nov 2018 10:01:34 +0100 [thread overview]
Message-ID: <20181114090134.GG23419@dhcp22.suse.cz> (raw)
In-Reply-To: <5a6c6d6b-ebcd-8bfa-d6e0-4312bfe86586@redhat.com>
On Wed 14-11-18 09:18:09, David Hildenbrand wrote:
> On 14.11.18 08:09, Baoquan He wrote:
> > Hi,
> >
> > Tested memory hotplug on a bare metal system, hot removing always
> > trigger a lock. Usually need hot plug/unplug several times, then the hot
> > removing will hang there at the last block. Surely with memory pressure
> > added by executing "stress -m 200".
> >
> > Will attach the log partly. Any idea or suggestion, appreciated.
> >
> > Thanks
> > Baoquan
> >
>
> Code seems to be waiting for the mem_hotplug_lock in read.
> We hold mem_hotplug_lock in write whenever we online/offline/add/remove
> memory. There are two ways to trigger offlining of memory:
>
> 1. Offlining via "cat offline > /sys/devices/system/memory/memory0/state"
>
> This always properly took the mem_hotplug_lock. Nothing changed
>
> 2. Offlining via "cat 0 > /sys/devices/system/memory/memory0/online"
>
> This didn't take the mem_hotplug_lock and I fixed that for this release.
This discrepancy should go.
> So if you were testing with 1., you should have seen the same error
> before this release (unless there is something else now broken in this
> release).
>
>
> The real question is, however, why offlining of the last block doesn't
> succeed. In __offline_pages() we basically have an endless loop (while
> holding the mem_hotplug_lock in write). Now I consider this piece of
> code very problematic (we should automatically fail after X
> attempts/after X seconds, we should not ignore -ENOMEM), and we've had
> other BUGs whereby we would run into an endless loop here (e.g. related
> to hugepages I guess).
We used to have number of retries previous and it was too fragile. If
you need a timeout then you can easily do that from userspace. Just do
timeout $TIME echo 0 > $MEM_PATH/online
I have seen an issue when the migration cannot make a forward progress
because of a glibc page with a reference count bumping up and down. Most
probable explanation is the faultaround code. I am working on this and
will post a patch soon. In any case the migration should converge and if
it doesn't do then there is a bug lurking somewhere.
Failing on ENOMEM is a questionable thing. I haven't seen that happening
wildly but if it is a case then I wouldn't be opposed.
> You mentioned memory pressure, if our host is under memory pressure we
> can easily trigger running into an endless loop there, because we
> basically ignore -ENOMEM e.g. when we cannot get a page to migrate some
> memory to be offlined. I assume this is the case here.
> do_migrate_range() could be the bad boy if it keeps failing forever and
> we keep retrying.
My hotplug debugging patches [1] should help to tell us.
[1] http://lkml.kernel.org/r/20181107101830.17405-1-mhocko@kernel.org
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2018-11-14 9:01 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-14 7:09 Memory hotplug softlock issue Baoquan He
2018-11-14 7:16 ` Baoquan He
2018-11-14 8:18 ` David Hildenbrand
2018-11-14 9:00 ` Baoquan He
2018-11-14 9:25 ` David Hildenbrand
2018-11-14 9:41 ` Michal Hocko
2018-11-14 9:48 ` David Hildenbrand
2018-11-14 10:04 ` Michal Hocko
2018-11-14 9:01 ` Michal Hocko [this message]
2018-11-14 9:22 ` David Hildenbrand
2018-11-14 9:37 ` Michal Hocko
2018-11-14 9:39 ` David Hildenbrand
2018-11-14 14:52 ` Baoquan He
2018-11-14 15:00 ` Michal Hocko
2018-11-15 5:10 ` Baoquan He
2018-11-15 7:30 ` Michal Hocko
2018-11-15 7:53 ` Baoquan He
2018-11-15 8:30 ` Michal Hocko
2018-11-15 9:42 ` David Hildenbrand
2018-11-15 9:52 ` Baoquan He
2018-11-15 9:53 ` David Hildenbrand
2018-11-15 13:12 ` Baoquan He
2018-11-15 13:19 ` Michal Hocko
2018-11-15 13:23 ` Baoquan He
2018-11-15 14:25 ` Michal Hocko
2018-11-15 13:38 ` Baoquan He
2018-11-15 14:32 ` Michal Hocko
2018-11-15 14:34 ` Baoquan He
2018-11-16 1:24 ` Baoquan He
2018-11-16 9:14 ` Michal Hocko
2018-11-17 4:22 ` Baoquan He
[not found] ` <20181119105202.GE18471@MiWiFi-R3L-srv>
2018-11-19 12:40 ` Michal Hocko
2018-11-19 12:51 ` Michal Hocko
2018-11-19 14:10 ` Michal Hocko
2018-11-19 16:36 ` Vlastimil Babka
2018-11-19 16:46 ` Michal Hocko
2018-11-19 16:46 ` Vlastimil Babka
2018-11-19 16:48 ` Vlastimil Babka
2018-11-19 17:01 ` Michal Hocko
2018-11-19 17:33 ` Michal Hocko
2018-11-19 20:34 ` Hugh Dickins
2018-11-19 20:59 ` Michal Hocko
2018-11-20 1:56 ` Baoquan He
2018-11-20 5:44 ` Hugh Dickins
2018-11-20 13:38 ` Vlastimil Babka
2018-11-20 13:58 ` Baoquan He
2018-11-20 14:05 ` Michal Hocko
2018-11-20 14:12 ` Baoquan He
2018-11-21 1:21 ` Hugh Dickins
2018-11-21 1:08 ` Hugh Dickins
2018-11-21 3:20 ` Hugh Dickins
2018-11-21 17:31 ` Michal Hocko
2018-11-22 1:53 ` Hugh Dickins
2018-11-14 10:00 ` Michal Hocko
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