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x86: Handle idle=nomwait cmdline properly for x86_idle 

When kernel is booted with idle=nomwait do not use MWAIT as the
default idle state.

If the user boots the kernel with idle=nomwait, it is a clear
direction to not use mwait as the default idle state.
However, the current code does not take this into consideration
while selecting the default idle state on x86.

Fix it by checking for the idle=nomwait boot option in
prefer_mwait_c1_over_halt().

Also update the documentation around idle=nomwait appropriately.

[ dhansen: tweak commit message ]

Signed-off-by: Wyes Karny <wyes.karny@amd.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Tested-by: Zhang Rui <rui.zhang@intel.com>
Link: https://lkml.kernel.org/r/fdc2dc2d0a1bc21c2f53d989ea2d2ee3ccbc0dbe.1654538381.git-series.wyes.karny@amd.com
This commit is contained in:
Wyes Karny 2022-06-06 23:33:34 +05:30 committed by Dave Hansen
parent 2147c438fd
commit 8bcedb4ce0
2 changed files with 15 additions and 9 deletions
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15
Documentation/admin-guide/pm/cpuidle.rst
View File

@ -612,8 +612,8 @@ the ``menu`` governor to be used on the systems that use the ``ladder`` governor
by default this way, for example.
The other kernel command line parameters controlling CPU idle time management
described below are only relevant for the *x86* architecture and some of
them affect Intel processors only.
described below are only relevant for the *x86* architecture and references
to ``intel_idle`` affect Intel processors only.
The *x86* architecture support code recognizes three kernel command line
options related to CPU idle time management: ``idle=poll``, ``idle=halt``,
@ -635,10 +635,13 @@ idle, so it very well may hurt single-thread computations performance as well as
energy-efficiency. Thus using it for performance reasons may not be a good idea
at all.]
The ``idle=nomwait`` option disables the ``intel_idle`` driver and causes
``acpi_idle`` to be used (as long as all of the information needed by it is
there in the system's ACPI tables), but it is not allowed to use the
``MWAIT`` instruction of the CPUs to ask the hardware to enter idle states.
The ``idle=nomwait`` option prevents the use of ``MWAIT`` instruction of
the CPU to enter idle states. When this option is used, the ``acpi_idle``
driver will use the ``HLT`` instruction instead of ``MWAIT``. On systems
running Intel processors, this option disables the ``intel_idle`` driver
and forces the use of the ``acpi_idle`` driver instead. Note that in either
case, ``acpi_idle`` driver will function only if all the information needed
by it is in the system's ACPI tables.
In addition to the architecture-level kernel command line options affecting CPU
idle time management, there are parameters affecting individual ``CPUIdle``

9
arch/x86/kernel/process.c
View File

@ -824,6 +824,10 @@ static void amd_e400_idle(void)
*/
static int prefer_mwait_c1_over_halt(const struct cpuinfo_x86 *c)
{
/* User has disallowed the use of MWAIT. Fallback to HALT */
if (boot_option_idle_override == IDLE_NOMWAIT)
return 0;
if (c->x86_vendor != X86_VENDOR_INTEL)
return 0;
@ -932,9 +936,8 @@ static int __init idle_setup(char *str)
} else if (!strcmp(str, "nomwait")) {
/*
* If the boot option of "idle=nomwait" is added,
* it means that mwait will be disabled for CPU C2/C3
* states. In such case it won't touch the variable
* of boot_option_idle_override.
* it means that mwait will be disabled for CPU C1/C2/C3
* states.
*/
boot_option_idle_override = IDLE_NOMWAIT;
} else