linux-kernel - Re: [PATCH] sched/cputime: add steal clock warps handling during cpu hotplug

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Message-ID: <CANRm+Cy1iHnUUOgfoLmK=qqq3gmcTDi7mApLSrWF+pvVGw+qgQ@mail.gmail.com>
Date:	Tue, 7 Jun 2016 15:35:27 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>, Wanpeng Li <wanpeng.li@...mail.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Radim <rkrcmar@...hat.com>
Subject: Re: [PATCH] sched/cputime: add steal clock warps handling during cpu hotplug

2016-06-07 15:31 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
>
>
> On 07/06/2016 03:24, Rik van Riel wrote:
>> On Mon, 2016-06-06 at 15:40 +0200, Paolo Bonzini wrote:
>>>
>>> On 02/06/2016 15:59, Rik van Riel wrote:
>>>>
>>>> If a guest is saved to disk and later restored (eg. after
>>>> a host reboot), or live migrated to another host, I would
>>>> expect to get totally disjoint steal time statistics from
>>>> the "new run" of the guest (which is the same run of the
>>>> guest OS).
>>> Why?  The preexisting guest steal time is always added to by
>>> KVM, so the time won't restart from zero.
>>>
>>> Continuing the previous count on CPU hot-unplug followed by hot-plug
>>> is less obvious, but I think it's overall the right thing to do.
>>>
>>> In fact, I was going to test a patch this week as simple as this:
>>>
>>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>>> index eea2a6f72b31..1ef5e48b3a36 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));
>>
>> By removing the memset from initial bootup allocation,
>> can't that cause the steal time to "increase by a ludicrous
>> amount" the very first time it is compared with the arch
>> independent value in the scheduler code?
>>
>> In other words, would removal of the memset result in still
>> requiring Wanpeng's patch?
>
> The percpu area is initialized to zero, isn't it?

Your proposal can fix the steal clock warp during guest cpu hotplug, I
will send out a new version later.

Regards,
Wanpeng Li