From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH 0/9] qspinlock stuff -v15 Date: Thu, 26 Mar 2015 21:21:53 +0100 Message-ID: <20150326202153.GD27490__23711.5776319571$1427401452$gmane$org@worktop.programming.kicks-ass.net> References: <20150316131613.720617163@infradead.org> <20150325194739.GK25884@l.oracle.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Received: from mail6.bemta14.messagelabs.com ([193.109.254.103]) by lists.xen.org with esmtp (Exim 4.72) (envelope-from ) id 1YbEI4-0003qq-1t for xen-devel@lists.xenproject.org; Thu, 26 Mar 2015 20:22:08 +0000 Content-Disposition: inline In-Reply-To: <20150325194739.GK25884@l.oracle.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Konrad Rzeszutek Wilk Cc: Waiman.Long@hp.com, linux-arch@vger.kernel.org, riel@redhat.com, x86@kernel.org, kvm@vger.kernel.org, scott.norton@hp.com, raghavendra.kt@linux.vnet.ibm.com, paolo.bonzini@gmail.com, oleg@redhat.com, linux-kernel@vger.kernel.org, mingo@redhat.com, david.vrabel@citrix.com, hpa@zytor.com, luto@amacapital.net, xen-devel@lists.xenproject.org, tglx@linutronix.de, paulmck@linux.vnet.ibm.com, torvalds@linux-foundation.org, boris.ostrovsky@oracle.com, virtualization@lists.linux-foundation.org, doug.hatch@hp.com List-Id: xen-devel@lists.xenproject.org On Wed, Mar 25, 2015 at 03:47:39PM -0400, Konrad Rzeszutek Wilk wrote: > Ah nice. That could be spun out as a seperate patch to optimize the existing > ticket locks I presume. Yes I suppose we can do something similar for the ticket and patch in the right increment. We'd need to restructure the code a bit, but its not fundamentally impossible. We could equally apply the head hashing to the current ticket implementation and avoid the current bitmap iteration. > Now with the old pv ticketlock code an vCPU would only go to sleep once and > be woken up when it was its turn. With this new code it is woken up twice > (and twice it goes to sleep). With an overcommit scenario this would imply > that we will have at least twice as many VMEXIT as with the previous code. An astute observation, I had not considered that. > I presume when you did benchmarking this did not even register? Thought > I wonder if it would if you ran the benchmark for a week or so. You presume I benchmarked :-) I managed to boot something virt and run hackbench in it. I wouldn't know a representative virt setup if I ran into it. The thing is, we want this qspinlock for real hardware because its faster and I really want to avoid having to carry two spinlock implementations -- although I suppose that if we really really have to we could.