lkml.org 
[lkml]   [2017]   [Feb]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH v2] mm/vmscan: fix high cpu usage of kswapd if there are no reclaimable pages
From
Date
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.
>

\
 
 \ /
  Last update: 2017-02-28 02:54    [W:0.338 / U:0.172 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site