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

Srinivas Pandruvada
2019-11-13T16:50:07Z

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@suse.cz>
> ---
> 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;
>