powerpc/pseries: remove cede offline state for CPUs

This effectively reverts commit 3aa565f53c39 ("powerpc/pseries: Add
hooks to put the CPU into an appropriate offline state"), which added
an offline mode for CPUs which uses the H_CEDE hcall instead of the
architected stop-self RTAS function in order to facilitate "folding"
of dedicated mode processors on PowerVM platforms to achieve energy
savings. This has been the default offline mode since its
introduction.

There's nothing about stop-self that would prevent the hypervisor from
achieving the energy savings available via H_CEDE, so the original
premise of this change appears to be flawed.

I also have encountered the claim that the transition to and from
ceded state is much faster than stop-self/start-cpu. Certainly we
would not want to use stop-self as an *idle* mode. That is what H_CEDE
is for. However, this difference is insignificant in the context of
Linux CPU hotplug, where the latency of an offline or online operation
on current systems is on the order of 100ms, mainly attributable to
all the various subsystems' cpuhp callbacks.

The cede offline mode also prevents accurate accounting, as discussed
before:
https://lore.kernel.org/linuxppc-dev/1571740391-3251-1-git-send-email-ego@linux.vnet.ibm.com/

Unconditionally use stop-self to offline processor threads. This is
the architected method for offlining CPUs on PAPR systems.

The "cede_offline" boot parameter is rendered obsolete.

Removing this code enables the removal of the partition suspend code
which temporarily onlines all present CPUs.

Fixes: 3aa565f53c39 ("powerpc/pseries: Add hooks to put the CPU into an appropriate offline state")
Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200612051238.1007764-2-nathanl@linux.ibm.com

1 files changed, 0 insertions, 7 deletions

diff --git a/Documentation/core-api/cpu_hotplug.rst b/Documentation/core-api/cpu_hotplug.rst
index 4a50ab7817f7..b1ae1ac159cf 100644
--- a/Documentation/core-api/cpu_hotplug.rst
+++ b/Documentation/core-api/cpu_hotplug.rst
@@ -50,13 +50,6 @@ Command Line Switches
 
   This option is limited to the X86 and S390 architecture.
 
-``cede_offline={"off","on"}``
-  Use this option to disable/enable putting offlined processors to an extended
-  ``H_CEDE`` state on supported pseries platforms. If nothing is specified,
-  ``cede_offline`` is set to "on".
-
-  This option is limited to the PowerPC architecture.
-
 ``cpu0_hotplug``
   Allow to shutdown CPU0.