From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx> I observed that sometimes st is 100% instantaneous, then idle is 100% even if there is a cpu hog on the guest cpu after the cpu hotplug comes back(N.B. this can not always be readily reproduced). I add trace to capture it as below: 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 steal clock warp and then steal_jiffies underflow. Rik also pointed out to me: | I have seen stuff like that with live migration too, in the past The root cause of steal clock warp during hotplug is kvm_steal_time reset to 0 after cpu hotplug comes back which should be preexiting guest value. This patch fix it by don't reset kvm_steal_time during guest cpu hotplug. Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx> Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Cc: Rik van Riel <riel@xxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx> --- 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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html