Re: [PATCH] xen/manage: Always freeze/thaw processes when suspend/resuming

[Ross Lagerwall <ross.lagerwall@citrix.com>]

On 08/22/2014 12:00 PM, David Vrabel wrote:
> On 18/08/14 10:41, Ross Lagerwall wrote:
>> Always freeze processes when suspending and thaw processes when resuming
>> to prevent a race noticeable with HVM guests.
>>
>> This prevents a deadlock where the khubd kthread (which is designed to
>> be freezable) acquires a usb device lock and then tries to allocate
>> memory which requires the disk which hasn't been resumed yet.
>> Meanwhile, the xenwatch thread deadlocks waiting for the usb device
>> lock.
>>
>> Freezing processes fixes this because the khubd thread is only thawed
>> after the xenwatch thread finishes resuming all the devices.
>
> I think this is the safest fix for this issue but I'm a bit concerned
> about how much extra time the freeze_processes() call will add to the
> suspend.
>
> Can you measure how long it takes for a typical VM?
>

With 2 vCPUs and 1024MB RAM, 8ms for freeze_processes() and 6ms for 
thaw_processes().

With 4 vCPUs and 4096MB RAM, 8ms for freeze_processes() and 5ms for 
thaw_processes().

With 4 vCPUs and 4096MB RAM running stress -c 4, 12ms for 
freeze_processes() and 66ms for thaw_processes().

It doesn't seem to take too long...

Cheers
-- 
Ross Lagerwall