[PATCH] powerpc/rtas: retry when cpu offline races with suspend/migration

[Juliet Kim]

On 6/27/19 12:01 AM, Michael Ellerman wrote:
> Juliet Kim <julietk at linux.vnet.ibm.com> writes:
>> On 6/25/19 1:51 PM, Nathan Lynch wrote:
>>> Juliet Kim <julietk at linux.vnet.ibm.com> writes:
>>>
>>>> There's some concern this could retry forever, resulting in live lock.
>>> First of all the system will make progress in other areas even if there
>>> are repeated retries; we're not indefinitely holding locks or anything
>>> like that.
>> For instance, system admin runs a script that picks and offlines CPUs in a
>> loop to keep a certain rate of onlined CPUs for energy saving. If LPM keeps
>> putting CPUs back online, that would never finish, and would keepgenerating
>> new offline requests
>>
>>> Second, Linux checks the H_VASI_STATE result on every retry. If the
>>> platform wants to terminate the migration (say, if it imposes a
>>> timeout), Linux will abandon it when H_VASI_STATE fails to return
>>> H_VASI_SUSPENDING. And it seems incorrect to bail out before that
>>> happens, absent hard errors on the Linux side such as allocation
>>> failures.
>> I confirmed with the PHYP and HMC folks that they wouldn't time out the LPM
>> request including H_VASI_STATE, so if the LPM retries were unlucky enough to
>> encounter repeated CPU offline attempts (maybe some customer code retrying
>> that), then the retries could continue indefinitely, or until some manual
>> intervention.  And in the mean time, the LPM delay here would cause PHYP to
>> block other operations.
> That sounds like a PHYP bug to me.
>
> cheers


PHYP doesn’t time out because they have no idea how long it will take for OS to
get to the point that it suspends. Other OS allows application to prepare for LPM.
They cannot predict the length of time that is appropriate in all cases and in any
case, it’s unlikely they’d make a change to that would come in time to help with
the current issue.