From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeremy Fitzhardinge Subject: Re: [PATCH RFC 2/4] Add yield hypercall for KVM guests Date: Mon, 02 Aug 2010 08:08:13 -0700 Message-ID: <4C56DF5D.807@goop.org> References: <20100726061150.GB21699@linux.vnet.ibm.com> <20100726061445.GB8402@linux.vnet.ibm.com> <4C4DC3AD.7010404@goop.org> <4C5682A3.40409@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: vatsa@linux.vnet.ibm.com, Marcelo Tosatti , Gleb Natapov , linux-kernel@vger.kernel.org, npiggin@suse.de, kvm@vger.kernel.org, bharata@in.ibm.com, Balbir Singh , Jan Beulich To: Avi Kivity Return-path: In-Reply-To: <4C5682A3.40409@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org 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