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

From: Jeremy Fitzhardinge
Date: Mon Aug 02 2010 - 11:08:19 EST

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.


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