linux-kernel - Re: [PATCH v6 1/3] KVM: fix steal clock warp during guest cpu hotplug

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Date:	Mon, 13 Jun 2016 12:44:46 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc:	Wanpeng Li <wanpeng.li@...mail.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>,
	John Stultz <john.stultz@...aro.org>
Subject: Re: [PATCH v6 1/3] KVM: fix steal clock warp during guest cpu hotplug



On 13/06/2016 12:32, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@...mail.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 <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>
> Cc: John Stultz <john.stultz@...aro.org>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
>  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));
> 

Because there's no cover letter, I guess I have to ack each patch
independently.

Acked-by: Paolo Bonzini <pbonzini@...hat.com>

Also, there's really no relation between patches 1-2 and 3...

Paolo