linux-kernel - Re: [PATCH v3 1/3] x86: Use HLT in default_idle when idle=nomwait cmdline arg is passed

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Date:   Mon, 23 May 2022 10:32:48 +0530
From:   Wyes Karny <wyes.karny@....com>
To:     Zhang Rui <rui.zhang@...el.com>, linux-kernel@...r.kernel.org
Cc:     Lewis.Carroll@....com, Mario.Limonciello@....com,
        gautham.shenoy@....com, Ananth.Narayan@....com, bharata@....com,
        len.brown@...el.com, x86@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, peterz@...radead.org, chang.seok.bae@...el.com,
        keescook@...omium.org, metze@...ba.org, zhengqi.arch@...edance.com,
        mark.rutland@....com, puwen@...on.cn, rafael.j.wysocki@...el.com,
        andrew.cooper3@...rix.com, jing2.liu@...el.com,
        jmattson@...gle.com, pawan.kumar.gupta@...ux.intel.com
Subject: Re: [PATCH v3 1/3] x86: Use HLT in default_idle when idle=nomwait
 cmdline arg is passed

Hello Rui,

On 5/20/2022 9:08 PM, Zhang Rui wrote:
> Hi, Wyes,
> 
> On Tue, 2022-05-10 at 15:48 +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.
> 
> I think we also need to update the comment in idle_setup() as well. 

Agreed. Will update. Thanks!

> 
>         } 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.
>                  */
>                 boot_op
> tion_idle_override = IDLE_NOMWAIT;
>         } else
> 
> thanks,
> rui
>>
>> Signed-off-by: Wyes Karny <wyes.karny@....com>
>> ---
>> Changes in v3:
>> - Update documentation around idle=mwait
>>
>>  Documentation/admin-guide/pm/cpuidle.rst | 15 +++++++++------
>>  arch/x86/kernel/process.c                |  4 ++++
>>  2 files changed, 13 insertions(+), 6 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..49b915d1b7b4 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;
>>  
> 

Thanks,
Wyes