linux-kernel - Re: [PATCH RFC 2/4] Add yield hypercall for KVM guests

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Date:	Mon, 02 Aug 2010 08:08:13 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Avi Kivity <avi@...hat.com>
CC:	vatsa@...ux.vnet.ibm.com, Marcelo Tosatti <mtosatti@...hat.com>,
	Gleb Natapov <gleb@...hat.com>, linux-kernel@...r.kernel.org,
	npiggin@...e.de, kvm@...r.kernel.org, bharata@...ibm.com,
	Balbir Singh <balbir@...ibm.com>,
	Jan Beulich <JBeulich@...ell.com>
Subject: Re: [PATCH RFC 2/4] Add yield hypercall for KVM guests

  On 08/02/2010 01:32 AM, Avi Kivity wrote:
>  On 07/26/2010 08:19 PM, Jeremy Fitzhardinge wrote:
>>  On 07/25/2010 11:14 PM, Srivatsa Vaddagiri wrote:
>>> Add KVM hypercall for yielding vcpu timeslice.
>>
>> Can you do a directed yield?
>>
>
> A problem with directed yield is figuring out who to yield to.  One 
> idea is to look for a random vcpu that is not running and donate some 
> runtime to it.  In the best case, it's the lock holder and we cause it 
> to start running.  Middle case it's not the lock holder, but we lose 
> enough runtime to stop running, so at least we don't waste cpu.  Worst 
> case we continue running not having woken the lock holder.  Spin 
> again, yield again hoping to find the right vcpu.

That can help with lockholder preemption, but on unlock you need to wake 
up exactly the right vcpu - the next in the ticket queue - in order to 
avoid burning masses of cpu.  If each cpu records what lock it is 
spinning on and what its ticket is in a percpu variable, then the 
unlocker can search for the next person to kick.

     J

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