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

[Juliet Kim]

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.