linux-kernel - [PATCH v4 1/3] KVM: fix steal clock warp during guest cpu hotplug

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Message-Id: <1465288389-3542-1-git-send-email-wanpeng.li@hotmail.com>
Date:	Tue,  7 Jun 2016 16:33:07 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:	Wanpeng Li <wanpeng.li@...mail.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Radim Krčmář <rkrcmar@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Peter Zijlstra (Intel)" <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: [PATCH v4 1/3] KVM: fix steal clock warp during guest cpu hotplug

From: Wanpeng Li <wanpeng.li@...mail.com>

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@...hat.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>
Cc: Radim Krčmář <rkrcmar@...hat.com>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Rik van Riel <riel@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
---
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