[tip: sched/urgent] x86, sched: Bail out of frequency invariance if base frequency is unknown

[tip-bot2 for Giovanni Gherdovich]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     9a6c2c3c7a73ce315c57c1b002caad6fcc858d0f
Gitweb:        https://git.kernel.org/tip/9a6c2c3c7a73ce315c57c1b002caad6fcc858d0f
Author:        Giovanni Gherdovich <ggherdovich@xxxxxxx>
AuthorDate:    Thu, 16 Apr 2020 07:47:42 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Wed, 22 Apr 2020 23:10:13 +02:00

x86, sched: Bail out of frequency invariance if base frequency is unknown

Some hypervisors such as VMWare ESXi 5.5 advertise support for
X86_FEATURE_APERFMPERF but then fill all MSR's with zeroes. In particular,
MSR_PLATFORM_INFO set to zero tricks the code that wants to know the base
clock frequency of the CPU (highest non-turbo frequency), producing a
division by zero when computing the ratio turbo_freq/base_freq necessary
for frequency invariant accounting.

It is to be noted that even if MSR_PLATFORM_INFO contained the appropriate
data, APERF and MPERF are constantly zero on ESXi 5.5, thus freq-invariance
couldn't be done in principle (not that it would make a lot of sense in a
VM anyway). The real problem is advertising X86_FEATURE_APERFMPERF. This
appears to be fixed in more recent versions: ESXi 6.7 doesn't advertise
that feature.

Fixes: 1567c3e3467c ("x86, sched: Add support for frequency invariance")
Signed-off-by: Giovanni Gherdovich <ggherdovich@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
Link: https://lkml.kernel.org/r/20200416054745.740-2-ggherdovich@xxxxxxx
---
 arch/x86/kernel/smpboot.c |  9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index fe3ab96..3a318ec 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1985,6 +1985,15 @@ static bool intel_set_max_freq_ratio(void)
 	return false;
 
 out:
+	/*
+	 * Some hypervisors advertise X86_FEATURE_APERFMPERF
+	 * but then fill all MSR's with zeroes.
+	 */
+	if (!base_freq) {
+		pr_debug("Couldn't determine cpu base frequency, necessary for scale-invariant accounting.\n");
+		return false;
+	}
+
 	arch_turbo_freq_ratio = div_u64(turbo_freq * SCHED_CAPACITY_SCALE,
 					base_freq);
 	arch_set_max_freq_ratio(turbo_disabled());