linux-kernel - Re: [PATCH v4] xen/balloon: add late_initcall

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Date:   Thu, 4 Nov 2021 11:55:34 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Juergen Gross <jgross@...e.com>, xen-devel@...ts.xenproject.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Jonathan Corbet <corbet@....net>,
        Stefano Stabellini <sstabellini@...nel.org>,
        stable@...r.kernel.org,
        Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>
Subject: Re: [PATCH v4] xen/balloon: add late_initcall_sync() for initial
 ballooning done


On 11/3/21 9:55 PM, Boris Ostrovsky wrote:
>
> On 11/2/21 5:19 AM, Juergen Gross wrote:
>> When running as PVH or HVM guest with actual memory < max memory the
>> hypervisor is using "populate on demand" in order to allow the guest
>> to balloon down from its maximum memory size. For this to work
>> correctly the guest must not touch more memory pages than its target
>> memory size as otherwise the PoD cache will be exhausted and the guest
>> is crashed as a result of that.
>>
>> In extreme cases ballooning down might not be finished today before
>> the init process is started, which can consume lots of memory.
>>
>> In order to avoid random boot crashes in such cases, add a late init
>> call to wait for ballooning down having finished for PVH/HVM guests.
>>
>> Warn on console if initial ballooning fails, panic() after stalling
>> for more than 3 minutes per default. Add a module parameter for
>> changing this timeout.
>>
>> Cc: <stable@...r.kernel.org>
>> Reported-by: Marek Marczykowski-Górecki <marmarek@...isiblethingslab.com>
>> Signed-off-by: Juergen Gross <jgross@...e.com>
>
>
>
> Reviewed-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>


This appears to have noticeable effect on boot time (and boot experience in general).


I have


   memory=1024
   maxmem=8192


And my boot time (on an admittedly slow box) went from 33 to 45 seconds. And boot pauses in the middle while it is waiting for ballooning to complete.


[    5.062714] xen:balloon: Waiting for initial ballooning down having finished.
[    5.449696] random: crng init done
[   34.613050] xen:balloon: Initial ballooning down finished.


So at least I think we should consider bumping log level down from info.



-boris