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

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Date:	Thu, 27 Oct 2011 00:53:30 +0530
From:	Raghavendra K T <raghukt@...ux.vnet.ibm.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
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 12:04 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. 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. ?

>
> The main constraint at present is that you need to update the
> pv_lock_ops structure before pvops patching happens, or you won't see
> any effect from making changes.

Hmm. got it.
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