Re: [PATCH v2 0/1] pseries/hotplug: Change the default behaviour of cede_offline

[Nathan Lynch <nathanl@linux.ibm.com>]

Gautham R Shenoy <ego@linux.vnet.ibm.com> writes:
> On Fri, Oct 25, 2019 at 06:03:26PM -0500, Nathan Lynch wrote:
>> "Gautham R. Shenoy" <ego@linux.vnet.ibm.com> writes:
>> > This is the v2 of the fix to change the default behaviour of
>> > cede_offline.
>> 
>> OK, but why keep the cede offline behavior at all? Can we remove it? I
>> think doing so would allow us to remove all the code that temporarily
>> onlines threads for partition migration.
>
> May be I am missing something. But don't we want all the CPUs to come
> online and execute the H_JOIN hcall before performing partition
> migration? How will this change whether the offlined CPUs are in
> H_CEDE or rtas-stop-self?

The platform considers threads in H_CEDE to be active. It considers
threads that have performed stop-self to be inactive until they have
been restarted. The Thread Join Option section of the PAPR says active
threads must perform the H_JOIN. I have confirmed with hypervisor
development that this implies that the OS needn't involve inactive
threads in the join/suspend sequence.

It isn't quite explicit in the log for 120496ac2d2d ("powerpc: Bring all
threads online prior to migration/hibernation"), but it stands to reason
that using cede for offline is the reason that the code to online all
threads for join/suspend was introduced in the first place.