From: David Hildenbrand <david@redhat.com>
To: Qian Cai <cai@lca.pw>, Michal Hocko <mhocko@kernel.org>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
Andrew Morton <akpm@linux-foundation.org>,
Oscar Salvador <osalvador@suse.de>,
Pavel Tatashin <pasha.tatashin@soleen.com>,
Dan Williams <dan.j.williams@intel.com>,
Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v1] mm/memory_hotplug: Don't take the cpu_hotplug_lock
Date: Thu, 26 Sep 2019 15:14:46 +0200 [thread overview]
Message-ID: <79a554e9-1f6c-d538-dcd6-486a095d1fa3@redhat.com> (raw)
In-Reply-To: <1569502946.5576.237.camel@lca.pw>
On 26.09.19 15:02, Qian Cai wrote:
> On Thu, 2019-09-26 at 13:52 +0200, Michal Hocko wrote:
>> On Thu 26-09-19 07:19:27, Qian Cai wrote:
>>>
>>>
>>>> On Sep 26, 2019, at 3:26 AM, Michal Hocko <mhocko@kernel.org> wrote:
>>>>
>>>> OK, this is using for_each_online_cpu but why is this a problem? Have
>>>> you checked what the code actually does? Let's say that online_pages is
>>>> racing with cpu hotplug. A new CPU appears/disappears from the online
>>>> mask while we are iterating it, right? Let's start with cpu offlining
>>>> case. We have two choices, either the cpu is still visible and we update
>>>> its local node configuration even though it will disappear shortly which
>>>> is ok because we are not touching any data that disappears (it's all
>>>> per-cpu). Case when the cpu is no longer there is not really
>>>> interesting. For the online case we might miss a cpu but that should be
>>>> tolerateable because that is not any different from triggering the
>>>> online independently of the memory hotplug. So there has to be a hook
>>>> from that code path as well. If there is none then this is buggy
>>>> irrespective of the locking.
>>>>
>>>> Makes sense?
>>>
>>> This sounds to me requires lots of audits and testing. Also, someone who is more
>>> familiar with CPU hotplug should review this patch.
>>
>> Thomas is on the CC list.
>>
>>> Personally, I am no fun of
>>> operating on an incorrect CPU mask to begin with, things could go wrong really
>>> quickly...
>>
>> Do you have any specific arguments? Just think of cpu and memory
>> hotplugs being independent operations. There is nothing really
>> inherently binding them together. If the cpu_online_mask really needs a
>> special treatment here then I would like to hear about that. Handwaving
>> doesn't really helps us.
>
> That is why I said it needs CPU hotplug experts to confirm that things including
> if CPU masks are tolerate to this kind of "abuse", or in-depth analysis of each
> calls sites that access CPU masks in both online_pages() and offline_pages() as
> well as ideally, more testing data in those areas.
>
> However, many kernel commits were merged with the expectations that people are
> going to deal with the aftermath, so I am not going to insist.
>
I am going to add documentation to build_all_zonelists() regarding
locking and the details we discussed.
Of course, I'll do more testing, and as Michal suggested, we should let
this "mature" in linux-next for some time.
--
Thanks,
David / dhildenb
next prev parent reply other threads:[~2019-09-26 13:14 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-24 14:36 [PATCH v1] mm/memory_hotplug: Don't take the cpu_hotplug_lock David Hildenbrand
2019-09-24 14:48 ` Michal Hocko
2019-09-24 15:01 ` David Hildenbrand
2019-09-24 15:03 ` Qian Cai
2019-09-24 15:11 ` Michal Hocko
2019-09-24 18:54 ` Qian Cai
2019-09-25 7:02 ` David Hildenbrand
2019-09-25 16:01 ` Qian Cai
2019-09-25 17:48 ` Michal Hocko
2019-09-25 18:20 ` Qian Cai
2019-09-25 19:48 ` David Hildenbrand
2019-09-25 20:32 ` Qian Cai
2019-09-26 7:26 ` David Hildenbrand
2019-09-26 7:38 ` Michal Hocko
2019-09-26 7:26 ` Michal Hocko
2019-09-26 11:19 ` Qian Cai
2019-09-26 11:52 ` Michal Hocko
2019-09-26 13:02 ` Qian Cai
2019-09-26 13:14 ` David Hildenbrand [this message]
2019-09-25 10:03 ` Michal Hocko
2019-09-24 15:23 ` David Hildenbrand
2019-10-02 21:37 ` Qian Cai
2019-10-04 7:42 ` David Hildenbrand
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