From: David Hildenbrand <david@redhat.com>
To: Michal Hocko <mhocko@kernel.org>
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 10:22:31 +0100 [thread overview]
Message-ID: <4449a0a2-be72-02bb-9f02-ed2484b160f8@redhat.com> (raw)
In-Reply-To: <20181114090134.GG23419@dhcp22.suse.cz>
>>
>> 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 agree that number of retries is not a good measure.
But as far as I can see this happens from the kernel via an ACPI event.
E.g. failing to offline a block after X seconds would still make sense.
(if something takes 120seconds to offline 128MB/2G there is something
very bad going on, we could set the default limit to e.g. 30seconds),
however ...
>
> 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.
... I also agree that this should converge. And if we detect a serious
issue that we can't handle/where we can't converge (e.g. -ENOMEM) we
should abort.
>
> 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.
I've seen quite some issues while playing with virtio-mem, but didn't
have the time to look into the details. Still on my long list of things
to look into.
>
> My hotplug debugging patches [1] should help to tell us.
>
> [1] http://lkml.kernel.org/r/20181107101830.17405-1-mhocko@kernel.org
>
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2018-11-14 9:22 UTC|newest]
Thread overview: 55+ 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
2018-11-14 9:22 ` David Hildenbrand [this message]
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
2018-11-19 10:52 ` Baoquan He
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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