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Subject: Re: [PATCH v3 1/3] x86: Use HLT in default_idle when idle=nomwait
 cmdline arg is passed
From:   Zhang Rui <rui.zhang@intel.com>
To:     Wyes Karny <wyes.karny@amd.com>, linux-kernel@vger.kernel.org
Cc:     Lewis.Carroll@amd.com, Mario.Limonciello@amd.com,
        gautham.shenoy@amd.com, Ananth.Narayan@amd.com, bharata@amd.com,
        len.brown@intel.com, x86@kernel.org, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
        hpa@zytor.com, peterz@infradead.org, chang.seok.bae@intel.com,
        keescook@chromium.org, metze@samba.org, zhengqi.arch@bytedance.com,
        mark.rutland@arm.com, puwen@hygon.cn, rafael.j.wysocki@intel.com,
        andrew.cooper3@citrix.com, jing2.liu@intel.com,
        jmattson@google.com, pawan.kumar.gupta@linux.intel.com
Date:   Fri, 20 May 2022 23:38:17 +0800
In-Reply-To: <1b24cadcf1d86a91352baa13905218015b9d4fed.1652176835.git-series.wyes.karny@amd.com>
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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. 

        } 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@amd.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;
>