linux-kernel - Re: [PATCH RFC V2 5/5] kvm guest : pv-ticketlocks support for linux guests running on KVM hypervisor

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Message-ID: <4EA86387.5040108@goop.org>
Date:	Wed, 26 Oct 2011 12:46:15 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Raghavendra K T <raghukt@...ux.vnet.ibm.com>
CC:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	"H. Peter Anvin" <hpa@...or.com>, Gleb Natapov <gleb@...hat.com>,
	Virtualization <virtualization@...ts.linux-foundation.org>,
	"x86@...nel.org" <x86@...nel.org>, KVM <kvm@...r.kernel.org>,
	Dave Jiang <dave.jiang@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	Yinghai Lu <yinghai@...nel.org>,
	Sedat Dilek <sedat.dilek@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Xen <xen-devel@...ts.xensource.com>, Avi Kivity <avi@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Suzuki Poulose <suzuki@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH RFC V2 5/5] kvm guest : pv-ticketlocks support for linux
 guests running on KVM hypervisor

On 10/26/2011 12:23 PM, Raghavendra K T wrote:
> On 10/26/2011 12:04 AM, Jeremy Fitzhardinge wrote:
>> On 10/23/2011 12:07 PM, Raghavendra K T wrote:
>>> This patch extends Linux guests running on KVM hypervisor to support
>>> +/*
>>> + * Setup pv_lock_ops to exploit KVM_FEATURE_WAIT_FOR_KICK if present.
>>> + * This needs to be setup really early in boot, before the first
>>> call to
>>> + * spinlock is issued!
>>
>> Actually, it doesn't matter that much.  The in-memory format is the same
>> for regular and PV spinlocks, and the PV paths only come into play if
>> the "slowpath" flag is set in the lock, which it never will be by the
>> non-PV code.
>>
>> In principle, you could defer initializing PV ticketlocks until some
>> arbitrarily late point if you notice that the system is oversubscribed
>> enough to require it.
>
> ok.. so this means it will not affect even if it is initialized in
> middle somewhere, but better to do it before we start seeing lock
> contention.

Right.  Or more specifically, lock contention while you have VCPU
overcommit.

> our current aim was to have before any printk happens.
> So I  'll trim the comment to somethings like :
>
> Setup pv_lock_ops to exploit KVM_FEATURE_WAIT_FOR_KICK if present.
> This needs to be setup early in boot. ?

You can hook the smp_ops.smp_prepare_cpus call and initialize it there. 
There's no need to add new hook code.

    J
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