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

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Date:   Thu, 4 Nov 2021 12:34:56 -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 11/4/21 12:21 PM, Juergen Gross wrote:
> On 04.11.21 16:55, Boris Ostrovsky wrote:
>>
>> 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.
>
> This shows that before it was just by chance that the PoD cache wasn't
> exhausted.


True.


>
>> So at least I think we should consider bumping log level down from info.
>
> Which level would you prefer? warn?
>

Notice? Although that won't make much difference as WARN is the default level.


I suppose we can't turn scrubbing off at this point?


> And if so, would you mind doing this while committing (I have one day
> off tomorrow)?


Yes, of course.


-boris