Re: [PATCH v2] mm/vmscan: fix high cpu usage of kswapd if there are no reclaimable pages

[hejianet <hejianet@gmail.com>]

Hi Johannes

I have another concern:
kswapd -> balance_pgdat -> age_active_anon
This code path will do some background works to age anon list, will this
patch have some impact on it if the retry time is > 16 and kswapd is
not waken up?

B.R.
Jia

On 28/02/2017 1:06 AM, Johannes Weiner wrote:
> On Mon, Feb 27, 2017 at 09:50:24AM +0100, Michal Hocko wrote:
>> On Fri 24-02-17 11:51:05, Johannes Weiner wrote:
>> [...]
>>> >From 29fefdca148e28830e0934d4e6cceb95ed2ee36e Mon Sep 17 00:00:00 2001
>>> From: Johannes Weiner <hannes@cmpxchg.org>
>>> Date: Fri, 24 Feb 2017 10:56:32 -0500
>>> Subject: [PATCH] mm: vmscan: disable kswapd on unreclaimable nodes
>>>
>>> Jia He reports a problem with kswapd spinning at 100% CPU when
>>> requesting more hugepages than memory available in the system:
>>>
>>> $ echo 4000 >/proc/sys/vm/nr_hugepages
>>>
>>> top - 13:42:59 up  3:37,  1 user,  load average: 1.09, 1.03, 1.01
>>> Tasks:   1 total,   1 running,   0 sleeping,   0 stopped,   0 zombie
>>> %Cpu(s):  0.0 us, 12.5 sy,  0.0 ni, 85.5 id,  2.0 wa,  0.0 hi,  0.0 si,  0.0 st
>>> KiB Mem:  31371520 total, 30915136 used,   456384 free,      320 buffers
>>> KiB Swap:  6284224 total,   115712 used,  6168512 free.    48192 cached Mem
>>>
>>>   PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
>>>    76 root      20   0       0      0      0 R 100.0 0.000 217:17.29 kswapd3
>>>
>>> At that time, there are no reclaimable pages left in the node, but as
>>> kswapd fails to restore the high watermarks it refuses to go to sleep.
>>>
>>> Kswapd needs to back away from nodes that fail to balance. Up until
>>> 1d82de618ddd ("mm, vmscan: make kswapd reclaim in terms of nodes")
>>> kswapd had such a mechanism. It considered zones whose theoretically
>>> reclaimable pages it had reclaimed six times over as unreclaimable and
>>> backed away from them. This guard was erroneously removed as the patch
>>> changed the definition of a balanced node.
>>>
>>> However, simply restoring this code wouldn't help in the case reported
>>> here: there *are* no reclaimable pages that could be scanned until the
>>> threshold is met. Kswapd would stay awake anyway.
>>>
>>> Introduce a new and much simpler way of backing off. If kswapd runs
>>> through MAX_RECLAIM_RETRIES (16) cycles without reclaiming a single
>>> page, make it back off from the node. This is the same number of shots
>>> direct reclaim takes before declaring OOM. Kswapd will go to sleep on
>>> that node until a direct reclaimer manages to reclaim some pages, thus
>>> proving the node reclaimable again.
>>
>> Yes this looks, nice&simple. I would just be worried about [1] a bit.
>> Maybe that is worth a separate patch though.
>>
>> [1] http://lkml.kernel.org/r/20170223111609.hlncnvokhq3quxwz@dhcp22.suse.cz
>
> I think I'd prefer the simplicity of keeping this contained inside
> vmscan.c, as an interaction between direct reclaimers and kswapd, as
> well as leaving the wakeup tied to actually seeing reclaimable pages
> rather than merely producing free pages (e.g. should we also add a
> kick to a large munmap() for example?).
>
> OOM kills come with such high latencies that I cannot imagine a
> slightly quicker kswapd restart would matter in practice.
>
>>> Reported-by: Jia He <hejianet@gmail.com>
>>> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
>>
>> Acked-by: Michal Hocko <mhocko@suse.com>
>
> Thanks!
>
>> I would have just one more suggestion. Please move MAX_RECLAIM_RETRIES
>> to mm/internal.h. This is MM internal thing and there is no need to make
>> it visible.
>
> Good point, I'll move it.
>