x86: Handle idle=nomwait cmdline properly for x86_idle

[ Upstream commit 45c2933ed65fb55142548be0f145dc88764cbab4 ]

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
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/Documentation/admin-guide/pm/cpuidle.rst b/Documentation/admin-guide/pm/cpuidle.rst
index e70b365dbc6030e758c20f6aa6165498393dd42d..80cf2ef2a5062104b64d923232e9f3cb46bf4665 100644 (file)
--- a/Documentation/admin-guide/pm/cpuidle.rst
+++ b/Documentation/admin-guide/pm/cpuidle.rst
@@ -676,8 +676,8 @@ 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``,
@@ -699,10 +699,13 @@ 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``

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 571e38c9ee1d34ec5dbe37b1980878b1ea33e73f..068715a52ac104a4ccf7ba0ac46819134849f83c 100644 (file)
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -659,6 +659,10 @@ 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;
 
@@ -769,9 +773,8 @@ 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