Re: [PATCH 2/2] x86, sched: Bail out of frequency invariance if turbo frequency is unknown

[Peter Zijlstra]

On Tue, Apr 28, 2020 at 03:24:50PM +0200, Giovanni Gherdovich wrote:
> There may be CPUs that support turbo boost but don't declare any turbo
> ratio, i.e. their MSR_TURBO_RATIO_LIMIT is all zeroes. In that condition
> scale-invariant calculations can't be performed.
> 
> Signed-off-by: Giovanni Gherdovich <ggherdovich@xxxxxxx>
> Suggested-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> Fixes: 1567c3e3467c ("x86, sched: Add support for frequency invariance")
> ---
>  arch/x86/kernel/smpboot.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 4718f29a3065..ab2a0df7d1fb 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -1991,9 +1991,11 @@ static bool intel_set_max_freq_ratio(void)
>  	/*
>  	 * Some hypervisors advertise X86_FEATURE_APERFMPERF
>  	 * but then fill all MSR's with zeroes.
> +	 * Some CPUs have turbo boost but don't declare any turbo ratio
> +	 * in MSR_TURBO_RATIO_LIMIT.
>  	 */
> -	if (!base_freq) {
> -		pr_debug("Couldn't determine cpu base frequency, necessary for scale-invariant accounting.\n");
> +	if (!base_freq || !turbo_freq) {
> +		pr_debug("Couldn't determine cpu base or turbo frequency, necessary for scale-invariant accounting.\n");
>  		return false;
>  	}

I've added the below, imagine base_freq > turbo_freq *
SCHED_CAPACITY_SCALE.

--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1975,6 +1975,7 @@ static bool core_set_max_freq_ratio(u64
 static bool intel_set_max_freq_ratio(void)
 {
 	u64 base_freq, turbo_freq;
+	u64 turbo_ratio;
 
 	if (slv_set_max_freq_ratio(&base_freq, &turbo_freq))
 		goto out;
@@ -2008,9 +2009,15 @@ static bool intel_set_max_freq_ratio(voi
 		return false;
 	}
 
-	arch_turbo_freq_ratio = div_u64(turbo_freq * SCHED_CAPACITY_SCALE,
-					base_freq);
+	turbo_ratio = div_u64(turbo_freq * SCHED_CAPACITY_SCALE, base_freq);
+	if (!turbo_ratio) {
+		pr_debug("Non-zero turbo and base frequencies led to a 0 ratio.\n");
+		return false;
+	}
+
+	arch_turbo_freq_ratio = turbo_ratio;
 	arch_set_max_freq_ratio(turbo_disabled());
+
 	return true;
 }