Re: Let's talk about the elephant in the room - the Linux kernel's inability to gracefully handle low memory pressure

[Hillf Danton <hdanton@sina.com>]

On Sun, 4 Aug 2019 09:23:17 +0000 "Artem S. Tashkinov" <aros@gmx.com> wrote:
> Hello,
> 
> There's this bug which has been bugging many people for many years
> already and which is reproducible in less than a few minutes under the
> latest and greatest kernel, 5.2.6. All the kernel parameters are set to
> defaults.

Thanks for report!
> 
> Steps to reproduce:
> 
> 1) Boot with mem=4G
> 2) Disable swap to make everything faster (sudo swapoff -a)
> 3) Launch a web browser, e.g. Chrome/Chromium or/and Firefox
> 4) Start opening tabs in either of them and watch your free RAM decrease

We saw another corner-case cpu hog report under memory pressure also
with swap disabled. In that report the xfs filesystem was an factor
with CONFIG_MEMCG enabled. Anything special, say like

 kernel:watchdog: BUG: soft lockup - CPU#7 stuck for 22s! [leaker1:7193]
or
 [ 3225.313209] Xorg: page allocation failure: order:4, mode:0x40dc0(GFP_KERNEL|__GFP_COMP|__GFP_ZERO), nodemask=(null),cpuset=/,mems_allowed=0

in your kernel log?
> 
> Once you hit a situation when opening a new tab requires more RAM than
> is currently available, the system will stall hard. You will barely  be
> able to move the mouse pointer. Your disk LED will be flashing
> incessantly (I'm not entirely sure why). You will not be able to run new
> applications or close currently running ones.

A cpu hog may come on top of memory hog in some scenario.
> 
> This little crisis may continue for minutes or even longer. I think
> that's not how the system should behave in this situation. I believe
> something must be done about that to avoid this stall.

Yes, Sir.
> 
> I'm almost sure some sysctl parameters could be changed to avoid this
> situation but something tells me this could be done for everyone and
> made default because some non tech-savvy users will just give up on
> Linux if they ever get in a situation like this and they won't be keen
> or even be able to Google for solutions.

I am not willing to repeat that it is hard to produce a pill for all
patients, but the info you post will help solve the crisis sooner.

Hillf