Re: [PATCH v4 1/3] x86: Handle idle=nomwait cmdline properly for x86_idle

[Wyes Karny <wyes.karny@amd.com>]

Hi Rui,

On 5/25/2022 1:36 PM, Zhang Rui wrote:
> On Mon, 2022-05-23 at 22:25 +0530, Wyes Karny wrote:
>> 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.
>>
>> This patch fixes it by checking for the idle=nomwait boot option in
>> prefer_mwait_c1_over_halt().
>>
>> Also update the documentation around idle=nomwait appropriately.
>>
>> Signed-off-by: Wyes Karny <wyes.karny@amd.com>
>> ---
>> Changes in v4:
>> - Update documentation around idle=nomwait
>> - Rename patch subject
>>
>>  Documentation/admin-guide/pm/cpuidle.rst | 15 +++++++++------
>>  arch/x86/kernel/process.c                |  6 +++++-
>>  2 files changed, 14 insertions(+), 7 deletions(-)
>>
>> diff --git a/Documentation/admin-guide/pm/cpuidle.rst
>> b/Documentation/admin-guide/pm/cpuidle.rst
>> index aec2cd2aaea7..19754beb5a4e 100644
>> --- a/Documentation/admin-guide/pm/cpuidle.rst
>> +++ b/Documentation/admin-guide/pm/cpuidle.rst
>> @@ -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``
>> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
>> index b370767f5b19..4e0178b066c5 100644
>> --- a/arch/x86/kernel/process.c
>> +++ b/arch/x86/kernel/process.c
>> @@ -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;
>>  
>> @@ -932,7 +936,7 @@ 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
>> +		 * it means that mwait will be disabled for CPU
>> C1/C2/C3
>>  		 * states. In such case it won't touch the variable
>>  		 * of boot_option_idle_override.
> 
> the code didn't change boot_option_idle_override when it was
> introduced, but this has changed since commit d18960494f65 ("ACPI,
> intel_idle: Cleanup idle= internal variables")

Could you please clarify bit more why the commit you mentioned is
related to this patch?

> 
> thanks,
> rui
>