From: Frederic Weisbecker <fweisbec@gmail.com>
To: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
Tony Luck <tony.luck@intel.com>,
Fenghua Yu <fenghua.yu@intel.com>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
Heiko Carstens <heiko.carstens@de.ibm.com>,
Martin Schwidefsky <schwidefsky@de.ibm.com>,
Avi Kivity <avi@redhat.com>,
Marcelo Tosatti <mtosatti@redhat.com>,
Joerg Roedel <joerg.roedel@amd.com>,
Alexander Graf <agraf@suse.de>,
Xiantao Zhang <xiantao.zhang@intel.com>,
Cornelia Huck <cornelia.huck@de.ibm.com>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Steven Rostedt <rostedt@goodmis.org>,
Paul Gortmaker <paul.gortmaker@windriver.com>
Subject: Re: [PATCH 3/5] kvm: Directly account vtime to system on guest switch
Date: Thu, 25 Oct 2012 09:56:58 +0200 [thread overview]
Message-ID: <CAFTL4hyFiE-G4VvmO+J2MMrJBFtH07ZcYp0GEnqoQ846_eivEQ@mail.gmail.com> (raw)
In-Reply-To: <5088EF85.4080002@de.ibm.com>
2012/10/25 Christian Borntraeger <borntraeger@de.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.
next prev parent reply other threads:[~2012-10-25 7:57 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-10-25 0:51 [PATCH 0/5] cputime: Moar cleanups / enhancements v2 Frederic Weisbecker
2012-10-25 0:51 ` [PATCH 1/5] vtime: Gather vtime declarations to their own header file Frederic Weisbecker
2012-10-25 0:51 ` [PATCH 2/5] vtime: Provide an irq safe version of vtime_account_system() Frederic Weisbecker
2012-10-25 7:48 ` Ingo Molnar
2012-10-25 0:51 ` [PATCH 3/5] kvm: Directly account vtime to system on guest switch Frederic Weisbecker
2012-10-25 7:51 ` Christian Borntraeger
2012-10-25 7:56 ` Frederic Weisbecker [this message]
2012-10-25 7:59 ` Christian Borntraeger
2012-10-25 0:51 ` [PATCH 4/5] cputime: Specialize irq vtime hooks Frederic Weisbecker
2012-10-25 0:51 ` [PATCH 5/5] cputime: Separate irqtime accounting from generic vtime Frederic Weisbecker
2012-10-25 17:47 [PATCH 0/5] cputime: Moar cleanups / enhancements v3 Frederic Weisbecker
2012-10-25 17:48 ` [PATCH 3/5] kvm: Directly account vtime to system on guest switch Frederic Weisbecker
2012-10-26 13:55 ` Frederic Weisbecker
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