From mboxrd@z Thu Jan 1 00:00:00 1970 From: Raghavendra K T Subject: Re: Preemptable Ticket Spinlock Date: Tue, 23 Apr 2013 07:12:55 +0530 Message-ID: <5175E71F.8070307@linux.vnet.ibm.com> References: <51745650.9050204@redhat.com> <1366631460.4443.3.camel@laptop> <51753289.70406@redhat.com> <1366660147.6454.6.camel@laptop> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: Rik van Riel , Jiannan Ouyang , LKML , Avi Kivity , Gleb Natapov , Ingo Molnar , Marcelo Tosatti , Srikar , "H. Peter Anvin" , "Nikunj A. Dadhania" , KVM , Thomas Gleixner , Chegu Vinod , "Andrew M. Theurer" , Srivatsa Vaddagiri , Andrew Jones , Karen Noel To: Peter Zijlstra Return-path: In-Reply-To: <1366660147.6454.6.camel@laptop> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 04/23/2013 01:19 AM, Peter Zijlstra wrote: > On Mon, 2013-04-22 at 08:52 -0400, Rik van Riel wrote: >> On 04/22/2013 07:51 AM, Peter Zijlstra wrote: >>> On Sun, 2013-04-21 at 17:12 -0400, Rik van Riel wrote: >>>> >>>> If we always incremented the ticket number by 2 (instead of 1), then >>>> we could use the lower bit of the ticket number as the spinlock. >>> >>> ISTR that paravirt ticket locks already do that and use the lsb to >>> indicate the unlock needs to perform wakeups. >>> >>> Also, since all of this is virt nonsense, shouldn't it live in the >>> paravirt ticket lock code and leave the native code as is? >> >> Sure, but that is still no reason not to have the virt >> implementation be as fast as possible, and share the same >> data type as the non-virt implementation. > > It has to share the same data-type.. > >> Also, is it guaranteed that the native spin_lock code has >> not been called yet before we switch over to the paravirt >> functions? >> >> If the native spin_lock code has been called already at >> that time, the native code would still need to be modified >> to increment the ticket number by 2, so we end up with a >> compatible value in each spin lock's .tickets field, and >> prevent a deadlock after we switch over to the paravirt >> variant. > > I thought the stuff already made it upstream, but apparently not; the > lastest posting I'm aware of is here: > > https://lkml.org/lkml/2012/5/2/105 > > That stuff changes the normal ticket increment as well.. > pv-ticket spinlock went on hold state, after Avi acked because of: though on non-PLE, we get a huge advantage, on PLE machine the benefit was not as impressive (~10% as you stated in email chain) compared to the complexity of the patches. So Avi suggested to try PLE improvements first, so they are going upstream. https://lkml.org/lkml/2012/7/18/247 https://lkml.org/lkml/2013/1/22/104 https://lkml.org/lkml/2013/2/6/345 (on the way in kvm tree) Current status of PV spinlock: I have the rebased patches of pv spinlocks and experimenting with latest kernel.I have Gleb's irq delivery incorporated into the patch series. But I am thinknig whether I can improve some guest side logic in unlock. I will probably setup a githup and post the link soon.