linux-kernel - Re: [PATCH 3/5] kvm: Directly account vtime to system on guest switch

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Date:	Thu, 25 Oct 2012 09:56:58 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Tony Luck <tony.luck@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Joerg Roedel <joerg.roedel@....com>,
	Alexander Graf <agraf@...e.de>,
	Xiantao Zhang <xiantao.zhang@...el.com>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: Re: [PATCH 3/5] kvm: Directly account vtime to system on guest switch

2012/10/25 Christian Borntraeger <borntraeger@...ibm.com>:
> On 25/10/12 02:51, Frederic Weisbecker wrote:
>> Switching to or from guest context is done on ioctl context.
>> So by the time we call kvm_guest_enter() or kvm_guest_exit()
>> we know we are not running the idle task.
>>
>> As a result, we can directly account the cputime using
>> vtime_account_system_irqsafe().
>>
>> There are two good reasons to do this:
>>
>> * We avoid some useless checks on guest switch. It optimizes
>> a bit this fast path.
>>
>> * In the case of CONFIG_IRQ_TIME_ACCOUNTING, calling vtime_account()
>> checks for irq time to account. This is pointless since we know
>> we are not in an irq on guest switch. This is wasting cpu cycles
>> for no good reason. vtime_account_system() OTOH is a no-op in
>> this config option.
>>
>> * s390 doesn't disable irqs in its implementation of vtime_account().
>> If vtime_account() in kvm races with an irq, the pending time might
>> be accounted twice. With vtime_account_system_irqsafe() we are protected.
>
> We disable irqs before we call kvm_guest_enter/exit, see kvm-s390.c:
>
>
> [...]
>         if (!kvm_is_ucontrol(vcpu->kvm))
>                 kvm_s390_deliver_pending_interrupts(vcpu);
>         vcpu->arch.sie_block->icptcode = 0;
>         local_irq_disable();
>         kvm_guest_enter();
>         local_irq_enable();
> [...]
>

Ah ok. Hmm I still need to keep it irqsafe for the other archs though,
as it is currently with vtime_account(). So perhaps I can remove your
local_irq_disable there and use vtime_account_system_irqsafe()
instead?

thanks.
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