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authorNathan Lynch <nathanl@linux.ibm.com>2020-06-12 08:12:21 +0300
committerMichael Ellerman <mpe@ellerman.id.au>2020-07-16 06:12:34 +0300
commit48f6e7f6d948b56489da027bc3284c709b939d28 (patch)
tree7850ae1d01e58ab858f5cb637d313642ee652598 /Documentation/core-api/cpu_hotplug.rst
parent4d24e21cc694e7253a532fe5a9bde12b284f1317 (diff)
downloadlinux-48f6e7f6d948b56489da027bc3284c709b939d28.tar.xz
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
Diffstat (limited to 'Documentation/core-api/cpu_hotplug.rst')
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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.