Re: [PATCH v4 5/6] x86,sched: Add support for frequency invariance on ATOM

[Srinivas Pandruvada]

On Wed, 2019-11-13 at 13:46 +0100, Giovanni Gherdovich wrote:
> The scheduler needs the ratio freq_curr/freq_max for frequency-
> invariant
> accounting. On all ATOM CPUs prior to Goldmont, set freq_max to the
> 1-core
> turbo ratio.
> 
> We intended to perform tests validating that this patch doesn't
> regress in
> terms of energy efficiency, given that this is the primary concern on
> Atom
> processors. Alas, we found out that turbostat doesn't support reading
> RAPL
> interfaces on our test machine (Airmont), and we don't have external
> equipment
> to measure power consumption; all we have is the performance results
> of the
> benchmarks we ran.
> 
I can run some benchmarks on this.

Thanks,
Srinivas

> Test machine:
> 
> Platform    : Dell Wyse 3040 Thin Client[1]
> CPU Model   : Intel Atom x5-Z8350 (aka Cherry Trail, aka Airmont)
> Fam/Mod/Ste : 6:76:4
> Topology    : 1 socket, 4 cores / 4 threads
> Memory      : 2G
> Storage     : onboard flash, XFS filesystem
> 
> [1] 
> https://www.dell.com/en-us/work/shop/wyse-endpoints-and-software/wyse-3040-thin-client/spd/wyse-3040-thin-client
> 
> Base frequency and available turbo levels (MHz):
> 
>     Min Operating Freq   266 |***
>     Low Freq Mode        800 |********
>     Base Freq           2400 |************************
>     4 Cores             2800 |****************************
>     3 Cores             2800 |****************************
>     2 Cores             3200 |********************************
>     1 Core              3200 |********************************
> 
> Tested kernels:
> 
> Baseline      : v5.4-rc1,              intel_pstate
> passive,  schedutil
> Comparison #1 : v5.4-rc1,              intel_pstate active
> ,  powersave
> Comparison #2 : v5.4-rc1, this patch,  intel_pstate
> passive,  schedutil
> 
> tbench, hackbench and kernbench performed the same under all three
> kernels;
> dbench ran faster with intel_pstate/powersave and the git unit tests
> were a
> lot faster with intel_pstate/powersave and invariant schedutil wrt
> the
> baseline. Not that any of this is terrbily interesting anyway, one
> doesn't buy
> an Atom system to go fast. Power consumption regressions aren't
> expected but
> we lack the equipment to make that measurement. Turbostat seems to
> think that
> reading RAPL on this machine isn't a good idea and we're trusting
> that
> decision.
> 
> comparison ratio of performance with baseline; 1.00 means neutral,
> lower is better:
> 
>                       I_PSTATE      FREQ-INV
>     ----------------------------------------
>     dbench                0.90             ~
>     kernbench             0.98          0.97
>     gitsource             0.63          0.43
> 
> Signed-off-by: Giovanni Gherdovich <ggherdovich@xxxxxxx>
> ---
>  arch/x86/kernel/smpboot.c | 29 +++++++++++++++++++++--------
>  1 file changed, 21 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 4d192abf337d..8988177064be 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1821,6 +1821,24 @@ static bool turbo_disabled(void)
>  	return (misc_en & MSR_IA32_MISC_ENABLE_TURBO_DISABLE);
>  }
>  
> +static bool slv_set_cpu_max_freq(u64 *ratio, u64 *turbo_ratio)
> +{
> +	int err;
> +
> +	err = rdmsrl_safe(MSR_ATOM_CORE_RATIOS, ratio);
> +	if (err)
> +		return false;
> +
> +	err = rdmsrl_safe(MSR_ATOM_CORE_TURBO_RATIOS, turbo_ratio);
> +	if (err)
> +		return false;
> +
> +	*ratio = (*ratio >> 16) & 0x3F;      /* max P state ratio */
> +	*turbo_ratio = *turbo_ratio & 0x3F;  /* 1C turbo ratio */
> +
> +	return true;
> +}
> +
>  #include <asm/cpu_device_id.h>
>  #include <asm/intel-family.h>
>  
> @@ -1984,19 +2002,14 @@ static bool core_set_cpu_max_freq(u64 *ratio,
> u64 *turbo_ratio)
>  
>  static void intel_set_cpu_max_freq(void)
>  {
> -	/*
> -	 * TODO: add support for:
> -	 *
> -	 * - Atom Silvermont
> -	 *
> -	 * which all now get by default arch_max_freq =
> SCHED_CAPACITY_SCALE
> -	 */
> -
>  	u64 ratio = 1, turbo_ratio = 1;
>  
>  	if (turbo_disabled())
>  		return;
>  
> +	if (slv_set_cpu_max_freq(&ratio, &turbo_ratio))
> +		goto set_value;
> +
>  	if (glm_set_cpu_max_freq(&ratio, &turbo_ratio))
>  		goto set_value;
>