linux-kernel - Re: Linux 5.13-rc6 regression to 5.12.x: kernel OOM and panic during kernel boot in low memory Xen VM's (256MB assigned memory).

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Date:   Mon, 21 Jun 2021 23:36:48 +0200
From:   Sander Eikelenboom <linux@...elenboom.it>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Juergen Gross <jgross@...e.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        kernel test robot <oliver.sang@...el.com>
Subject: Re: Linux 5.13-rc6 regression to 5.12.x: kernel OOM and panic during
 kernel boot in low memory Xen VM's (256MB assigned memory).

On 21/06/2021 18:54, Rasmus Villemoes wrote:
> On 18/06/2021 03.06, Sander Eikelenboom wrote:
>> On 17/06/2021 21:39, Sander Eikelenboom wrote:
> 
>>
>> OK, done some experimentation and it seems with 256M assigned to the VM
>> it was almost at the edge of OOM with the 5.12 kernel as well in the
>> config I am using it.
>> With v5.12 when I assign 240M it boots, with 230M it doesn't. With 5.13
>> the tipping point seems to be around 265M and 270M, so my config was
>> already quite close to the edge.
>>
>> The "direct kernel boot" feature I'm using just seems somewhat memory
>> hungry, but using another compression algorithm for the kernel and
>> initramfs already helped in my case.
>>
>> So sorry for the noise, clearly user-error.
> 
> Hm, perhaps, but I'm still a bit nervous about that report from Oliver
> Sang/kernel test robot, which was for a VM equipped with 16G of memory.
> But despite quite a few attempts, I haven't been able to reproduce that
> locally, so unfortunately I have no idea what's going on.
> 
> Rasmus
> 

Hmm I just tried to switch all VM's to a 5.13-rc7 kernel.
Some worked since i reduced the size, but some still fail.

The difference seems the be the number of vcpu's I assign to the VM's

The ones with 1 vcpu now boot with 256MB assigned (that was what I tested before),
but the ones with 2 vcpu's assigned don't and still OOM
on the same kernel and initramfs that I pass in from the host.

Could that box from the test-robot have a massive amount of cpu-cores
and that it is some how related to that ?

--
Sander