[tip:sched/core] KVM: Fix steal clock warp during guest CPU hotplug

[tip-bot for Wanpeng Li <tipbot@zytor.com>]

Commit-ID:  2348140d58f4f4245e9635ea8f1a77e940a4d877
Gitweb:     http://git.kernel.org/tip/2348140d58f4f4245e9635ea8f1a77e940a4d877
Author:     Wanpeng Li <wanpeng.li@hotmail.com>
AuthorDate: Mon, 13 Jun 2016 18:32:44 +0800
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 14 Jun 2016 11:13:14 +0200

KVM: Fix steal clock warp during guest CPU hotplug

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 <pbonzini@redhat.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1465813966-3116-2-git-send-email-wanpeng.li@hotmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 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));