Re: Should Fedora switch to full kernel preemption (CONFIG_PREEMPT=y)?

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On 5/24/23 08:44, Zdenek Kabelac wrote:
> Dne 20. 05. 23 v 22:43 Demi Marie Obenour napsal(a):
>> I noticed that by default, Qubes OS has voluntary kernel preemption
>> as opposed to full preemption.  I found that enabling full preemption
>> (preempt=full on kernel command line) makes the system significantly
>> more responsive under heavy I/O load.  In particular, if I build a
>> kernel in a Qubes OS VM, it significantly degrades responsiveness
>> without preempt=full.  With preempt=full, the system remains
>> responsive.  The storage stack used is LVM thin provisioning on LUKS,
>> and I have observed significant CPU usage in dom0 kernel threads with
>> names that indicate they are related to dm-thin and dm-crypt.
>>
>> The kernel config used by the Qubes kernel package I use (6.1.28) is
>> based on Fedora 37’s config, and Marek Marczykowski-Górecki (CCd)
>> indicated that the same arguments apply to Fedora.  Therefore, I am
>> asking if Fedora should use full kernel preemption by default.
> 
> 
> Hi Demi
> 
> 
> Could you please provide   'dmsetup table'   - so we could see how doe your 
> device stack looks like ?

The output of 'dmsetup table' contains all qube names so I would prefer to not
post it publicly.  The device stack is NVMe -> crypt -> linear -> thin pool -> thin.

> Aren't you disabling work-queues on the table line for crypt targets ?

The only optional parameter passed to dm_crypt is allow_discards,
so presumably no.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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