From: Vlastimil Babka <vbabka@suse.cz>
To: David Hildenbrand <david@redhat.com>, osalvador@suse.de
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
linux-hyperv@vger.kernel.org, xen-devel@lists.xenproject.org,
linux-acpi@vger.kernel.org,
Andrew Morton <akpm@linux-foundation.org>,
Alexander Duyck <alexander.h.duyck@linux.intel.com>,
Dave Hansen <dave.hansen@intel.com>,
Haiyang Zhang <haiyangz@microsoft.com>,
"K. Y. Srinivasan" <kys@microsoft.com>,
Mel Gorman <mgorman@techsingularity.net>,
Michael Ellerman <mpe@ellerman.id.au>,
Michal Hocko <mhocko@kernel.org>, Mike Rapoport <rppt@kernel.org>,
Scott Cheloha <cheloha@linux.ibm.com>,
Stephen Hemminger <sthemmin@microsoft.com>,
Wei Liu <wei.liu@kernel.org>,
Wei Yang <richard.weiyang@linux.alibaba.com>
Subject: Re: [PATCH RFC 0/4] mm: place pages to the freelist tail when onling and undoing isolation
Date: Wed, 23 Sep 2020 16:31:25 +0200 [thread overview]
Message-ID: <67928cbd-950a-3279-bf9b-29b04c87728b@suse.cz> (raw)
In-Reply-To: <DAC9E747-BDDF-41B6-A89B-604880DD7543@redhat.com>
On 9/16/20 9:31 PM, David Hildenbrand wrote:
>
>
>> Am 16.09.2020 um 20:50 schrieb osalvador@suse.de:
>>
>> On 2020-09-16 20:34, David Hildenbrand wrote:
>>> When adding separate memory blocks via add_memory*() and onlining them
>>> immediately, the metadata (especially the memmap) of the next block will be
>>> placed onto one of the just added+onlined block. This creates a chain
>>> of unmovable allocations: If the last memory block cannot get
>>> offlined+removed() so will all dependant ones. We directly have unmovable
>>> allocations all over the place.
>>> This can be observed quite easily using virtio-mem, however, it can also
>>> be observed when using DIMMs. The freshly onlined pages will usually be
>>> placed to the head of the freelists, meaning they will be allocated next,
>>> turning the just-added memory usually immediately un-removable. The
>>> fresh pages are cold, prefering to allocate others (that might be hot)
>>> also feels to be the natural thing to do.
>>> It also applies to the hyper-v balloon xen-balloon, and ppc64 dlpar: when
>>> adding separate, successive memory blocks, each memory block will have
>>> unmovable allocations on them - for example gigantic pages will fail to
>>> allocate.
>>> While the ZONE_NORMAL doesn't provide any guarantees that memory can get
>>> offlined+removed again (any kind of fragmentation with unmovable
>>> allocations is possible), there are many scenarios (hotplugging a lot of
>>> memory, running workload, hotunplug some memory/as much as possible) where
>>> we can offline+remove quite a lot with this patchset.
>>
>> Hi David,
>>
>
> Hi Oscar.
>
>> I did not read through the patchset yet, so sorry if the question is nonsense, but is this not trying to fix the same issue the vmemmap patches did? [1]
>
> Not nonesense at all. It only helps to some degree, though. It solves the dependencies due to the memmap. However, it‘s not completely ideal, especially for single memory blocks.
>
> With single memory blocks (virtio-mem, xen-balloon, hv balloon, ppc dlpar) you still have unmovable (vmemmap chunks) all over the physical address space. Consider the gigantic page example after hotplug. You directly fragmented all hotplugged memory.
>
> Of course, there might be (less extreme) dependencies due page tables for the identity mapping, extended struct pages and similar.
>
> Having that said, there are other benefits when preferring other memory over just hotplugged memory. Think about adding+onlining memory during boot (dimms under QEMU, virtio-mem), once the system is up you will have most (all) of that memory completely untouched.
>
> So while vmemmap on hotplugged memory would tackle some part of the issue, there are cases where this approach is better, and there are even benefits when combining both.
I see the point, but I don't think the head/tail mechanism is great for this. It
might sort of work, but with other interfering activity there are no guarantees
and it relies on a subtle implementation detail. There are better mechanisms
possible I think, such as preparing a larger MIGRATE_UNMOVABLE area in the
existing memory before we allocate those long-term management structures. Or
onlining a bunch of blocks as zone_movable first and only later convert to
zone_normal in a controlled way when existing normal zone becomes depeted?
I guess it's an issue that the e.g. 128M block onlines are so disconnected from
each other it's hard to employ a strategy that works best for e.g. a whole bunch
of GB onlined at once. But I noticed some effort towards new API, so maybe that
will be solved there too?
> Thanks!
>
> David
>
>>
>> I was about to give it a new respin now that thw hwpoison stuff has been settled.
>>
>> [1] https://patchwork.kernel.org/cover/11059175/
>>
>
next prev parent reply other threads:[~2020-09-23 14:31 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-16 18:34 [PATCH RFC 0/4] mm: place pages to the freelist tail when onling and undoing isolation David Hildenbrand
2020-09-16 18:34 ` [PATCH RFC 1/4] mm/page_alloc: convert "report" flag of __free_one_page() to a proper flag David Hildenbrand
2020-09-16 21:44 ` Alexander Duyck
2020-09-18 1:53 ` Wei Yang
2020-09-18 7:23 ` David Hildenbrand
2020-09-24 10:19 ` Vlastimil Babka
2020-09-25 10:34 ` Oscar Salvador
2020-09-16 18:34 ` [PATCH RFC 2/4] mm/page_alloc: place pages to tail in __putback_isolated_page() David Hildenbrand
2020-09-16 21:50 ` Alexander Duyck
2020-09-18 2:07 ` Wei Yang
2020-09-18 7:27 ` David Hildenbrand
2020-09-21 1:57 ` Wei Yang
2020-09-18 2:16 ` Wei Yang
2020-09-18 7:29 ` David Hildenbrand
2020-09-24 10:37 ` Vlastimil Babka
2020-09-25 8:10 ` David Hildenbrand
2020-09-25 13:19 ` Oscar Salvador
2020-09-25 13:30 ` David Hildenbrand
2020-09-16 18:34 ` [PATCH RFC 3/4] mm/page_alloc: always move pages to the tail of the freelist in unset_migratetype_isolate() David Hildenbrand
2020-09-18 2:29 ` Wei Yang
2020-09-18 7:30 ` David Hildenbrand
2020-09-24 11:13 ` Vlastimil Babka
2020-09-25 2:45 ` Wei Yang
2020-09-25 8:05 ` David Hildenbrand
2020-09-25 8:39 ` Vlastimil Babka
2020-09-25 13:57 ` Oscar Salvador
2020-09-16 18:34 ` [PATCH RFC 4/4] mm/page_alloc: place pages to tail in __free_pages_core() David Hildenbrand
2020-09-24 13:44 ` Vlastimil Babka
2020-09-28 7:58 ` Oscar Salvador
2020-09-28 8:36 ` David Hildenbrand
2020-09-28 12:53 ` Oscar Salvador
2020-09-16 18:50 ` [PATCH RFC 0/4] mm: place pages to the freelist tail when onling and undoing isolation osalvador
2020-09-16 19:31 ` David Hildenbrand
2020-09-18 2:30 ` Wei Yang
2020-09-18 7:32 ` David Hildenbrand
2020-09-23 14:31 ` Vlastimil Babka [this message]
2020-09-23 15:26 ` David Hildenbrand
2020-09-24 9:40 ` Mel Gorman
2020-09-24 9:54 ` David Hildenbrand
2020-09-24 13:59 ` Vlastimil Babka
2020-09-24 14:29 ` David Hildenbrand
2020-09-24 1:57 ` Wei Yang
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