Re: shrink_inactive_list() failed to reclaim pages

[Sergey Senozhatsky <sergey.senozhatsky@gmail.com>]

Hello,

On (01/11/17 18:38), Michal Hocko wrote:
> On Thu 12-01-17 01:16:11, Cheng-yu Lee wrote:
> > Hi community,
> > 
> > I have a x86_64 Chromebook running 3.14 kernel with 8G of memory. Using
> 
> Do you see the same with the current Linus tree?
> 
> > zram with swap size set to ~12GB. When in low memory, kswapd is awaken to
> > reclaim pages, but under some circumstances the kernel can not find pages
> > to reclaim while I'm sure there're still plenty of memory which could be
> > reclaimed from background processes (For example, I run some C programs
> > which just malloc() lots of memory and get suspended in the background.
> > There's no reason they could't be swapped). The consequence is that most of
> > CPU time is spent on page reclamation. The system hangs or becomes very
> > laggy for a long period. Sometimes it even triggers a kernel panic by the
> > hung task detector like:
> > <0>[46246.676366] Kernel panic - not syncing: hung_task: blocked tasks
> > 
> > I've added kernel message to trace the problem. I found shrink_inactive_list()
> > can barely find any page to reclaim. More precisely, when the problem
> > happens, lots of page have _count > 2 in __remove_mapping(). So the
> > condition at line 662 of vmscan.c holds:
> > http://lxr.free-electrons.com/source/mm/vmscan.c#L662
> > Thus the kernel fails to reclaim those pages at line 1209
> > http://lxr.free-electrons.com/source/mm/vmscan.c#L1209
> 
> I assume that you are talking about the anonymous LRU

hm. as a side note, I think this is not the first time I see
"kswapd consumes 100% cpu" report.

https://bugzilla.kernel.org/show_bug.cgi?id=65201#c50

http://lkml.iu.edu//hypermail/linux/kernel/1601.2/03564.html

https://marc.info/?l=linux-mm&m=145442159521487

https://marc.info/?l=linux-mm&m=145443027124595

	-ss

> > It's weird that the inactive anonymous list is huge (several GB), but
> > nothing can really be freed. So I did some hack to see if moving more pages
> > from the active list helps. I commented out the "inactive_list_is_low()"
> > checking at line 2420
> > in shrink_node_memcg() so shrink_active_list() is always called.
> > http://lxr.free-electrons.com/source/mm/vmscan.c#L2420
> > It turns out that the hack helps. If moving more pages from the active
> > list, kswapd works smoothly. The whole 12G zram can be used up before
> > system enters OOM condition.
> > 
> > Any idea why the whole inactive anonymous LRU is occupied by pages which
> > can not be freed for la long time (several minutes before system dies) ?
> > Are there any parameters I can tune to help the situation ? I've tried
> > swappiness but it doesn't help.
> > 
> > An alternative is to patch the kernel to call shrink_active_list() more
> > frequently when it finds there's nothing that can be reclaimed . But I am
> > not sure if it's the right direction. Also it's not so trivial to figure
> > out where to add the call.

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