Re: [PATCH] x86 idle: repair large-server 50-watt idle-power regression

[Ingo Molnar <mingo@kernel.org>]

* Len Brown <lenb@kernel.org> wrote:

> From: Len Brown <len.brown@intel.com>
> 
> Linux 3.10 changed the timing of how thread_info->flags is touched:
> 
> 	x86: Use generic idle loop
> 	(7d1a941731fabf27e5fb6edbebb79fe856edb4e5)
> 
> This caused Intel NHM-EX and WSM-EX servers to experience a large number
> of immediate MONITOR/MWAIT break wakeups, which caused cpuidle to demote
> from deep C-states to shallow C-states, which caused these platforms
> to experience a significant increase in idle power.
> 
> Note that this issue was already present before the commit above,
> however, it wasn't seen often enough to be noticed in power measurements.
> 
> Here we extend an errata workaround from the Core2 EX "Dunnington"
> to extend to NHM-EX and WSM-EX, to prevent these immediate
> returns from MWAIT, reducing idle power on these platforms.
> 
> While only acpi_idle ran on Dunnington, intel_idle
> may also run on these two newer systems.
> As of today, there are no other models that are known
> to need this tweak.
> 
> ref: https://lkml.org/lkml/2013/12/7/22
> Signed-off-by: Len Brown <len.brown@intel.com>
> Cc: <stable@vger.kernel.org> # 3.12.x, 3.11.x, 3.10.x
> ---
>  arch/x86/kernel/cpu/intel.c | 3 ++-
>  drivers/idle/intel_idle.c   | 3 +++
>  2 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> index dc1ec0d..ea04b34 100644
> --- a/arch/x86/kernel/cpu/intel.c
> +++ b/arch/x86/kernel/cpu/intel.c
> @@ -387,7 +387,8 @@ static void init_intel(struct cpuinfo_x86 *c)
>  			set_cpu_cap(c, X86_FEATURE_PEBS);
>  	}
>  
> -	if (c->x86 == 6 && c->x86_model == 29 && cpu_has_clflush)
> +	if (c->x86 == 6 && cpu_has_clflush &&
> +	    (c->x86_model == 29 || c->x86_model == 46 || c->x86_model == 47))
>  		set_cpu_cap(c, X86_FEATURE_CLFLUSH_MONITOR);
>  
>  #ifdef CONFIG_X86_64
> diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
> index 92d1206..f80b700 100644
> --- a/drivers/idle/intel_idle.c
> +++ b/drivers/idle/intel_idle.c
> @@ -377,6 +377,9 @@ static int intel_idle(struct cpuidle_device *dev,
>  
>  	if (!current_set_polling_and_test()) {
>  
> +		if (this_cpu_has(X86_FEATURE_CLFLUSH_MONITOR))
> +			clflush((void *)&current_thread_info()->flags);
> +
>  		__monitor((void *)&current_thread_info()->flags, 0, 0);

I don't think either of these casts to '(void *)' is needed, both the 
clflush() and __monitor() will take pointers.

Looks good to me otherwise - except that maybe the best way to 
represent this quirk would be for the CLFLUSH+MONITOR sequence to be a 
single 'instruction' which is patched in dynamically during bootup, 
using our usual alternatives framework.

On non-affected CPUs a NOP would remain in place of the CLFLUSH, 
eliminating the branch above.

So the whole thing could be thought of as a slightly more complex 
'monitor' instruction - not exposing the quirk details to actual usage 
sites.

Thanks,

	Ingo