From: Wanpeng Li <wanpeng...@hotmail.com>
Sometimes, after CPU hotplug you can observe a spike in stolen time
(100%) followed by the CPU being marked as 100% idle when it's actually
busy with a CPU hog task. The trace looks like the following:
cpuhp/1-12 [001] d.h1 167.461657: account_process_tick: steal =
1291385514, prev_steal_time = 0
cpuhp/1-12 [001] d.h1 167.461659: account_process_tick: steal_jiffies =
1291
<idle>-0 [001] d.h1 167.462663: account_process_tick: steal = 18732255,
prev_steal_time = 1291000000
<idle>-0 [001] d.h1 167.462664: account_process_tick: steal_jiffies =
18446744072437
The sudden decrease of "steal" causes steal_jiffies to underflow.
The root cause is kvm_steal_time being reset to 0 after hot-plugging
back in a CPU. Instead, the preexisting value can be used, which is
what the core scheduler code expects.
John Stultz also reported a similar issue after guest S3.
Suggested-by: Paolo Bonzini <pbonz...@redhat.com>
Cc: Paolo Bonzini <pbonz...@redhat.com>
Cc: Radim Krčmář <rkrc...@redhat.com>
Cc: Ingo Molnar <mi...@kernel.org>
Cc: Peter Zijlstra (Intel) <pet...@infradead.org>
Cc: Rik van Riel <r...@redhat.com>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Frederic Weisbecker <fweis...@gmail.com>
Cc: John Stultz <john.stu...@linaro.org>
Signed-off-by: Wanpeng Li <wanpeng...@hotmail.com>
---
v4 -> v5:
* improve commit message
v2 -> v3:
* fix the root cause
v1 -> v2:
* update patch subject, description and comments
* deal with the case where steal time suddenly increases by a ludicrous amount
arch/x86/kernel/kvm.c | 2 --
1 file changed, 2 deletions(-)
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index eea2a6f..1ef5e48 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -301,8 +301,6 @@ static void kvm_register_steal_time(void)
if (!has_steal_clock)
return;
- memset(st, 0, sizeof(*st));
-
wrmsrl(MSR_KVM_STEAL_TIME, (slow_virt_to_phys(st) | KVM_MSR_ENABLED));
pr_info("kvm-stealtime: cpu %d, msr %llx\n",
cpu, (unsigned long long) slow_virt_to_phys(st));
--
1.9.1
