From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ot0-f196.google.com ([74.125.82.196]:53664 "EHLO mail-ot0-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754243AbdKNJjF (ORCPT ); Tue, 14 Nov 2017 04:39:05 -0500 Subject: Re: [PATCH RFC v3 1/6] x86/paravirt: Add pv_idle_ops to paravirt ops To: Juergen Gross , Quan Xu , kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, x86@kernel.org, xen-devel@lists.xenproject.org Cc: Yang Zhang , Alok Kataria , Rusty Russell , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" References: <1510567565-5118-1-git-send-email-quan.xu0@gmail.com> <1510567565-5118-2-git-send-email-quan.xu0@gmail.com> <07fac696-e3d4-8f35-8f3d-764d7ab41204@suse.com> <902da704-1e4f-583b-91c3-1a62ccd6e73d@gmail.com> From: Quan Xu Message-ID: <79dad15c-2d26-bcf3-7283-293e42a161ea@gmail.com> Date: Tue, 14 Nov 2017 17:38:57 +0800 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On 2017/11/14 15:30, Juergen Gross wrote: > On 14/11/17 08:02, Quan Xu wrote: >> >> On 2017/11/13 18:53, Juergen Gross wrote: >>> On 13/11/17 11:06, Quan Xu wrote: >>>> From: Quan Xu >>>> >>>> So far, pv_idle_ops.poll is the only ops for pv_idle. .poll is called >>>> in idle path which will poll for a while before we enter the real idle >>>> state. >>>> >>>> In virtualization, idle path includes several heavy operations >>>> includes timer access(LAPIC timer or TSC deadline timer) which will >>>> hurt performance especially for latency intensive workload like message >>>> passing task. The cost is mainly from the vmexit which is a hardware >>>> context switch between virtual machine and hypervisor. Our solution is >>>> to poll for a while and do not enter real idle path if we can get the >>>> schedule event during polling. >>>> >>>> Poll may cause the CPU waste so we adopt a smart polling mechanism to >>>> reduce the useless poll. >>>> >>>> Signed-off-by: Yang Zhang >>>> Signed-off-by: Quan Xu >>>> Cc: Juergen Gross >>>> Cc: Alok Kataria >>>> Cc: Rusty Russell >>>> Cc: Thomas Gleixner >>>> Cc: Ingo Molnar >>>> Cc: "H. Peter Anvin" >>>> Cc: x86@kernel.org >>>> Cc: virtualization@lists.linux-foundation.org >>>> Cc: linux-kernel@vger.kernel.org >>>> Cc: xen-devel@lists.xenproject.org >>> Hmm, is the idle entry path really so critical to performance that a new >>> pvops function is necessary? >> Juergen, Here is the data we get when running benchmark netperf: >>  1. w/o patch and disable kvm dynamic poll (halt_poll_ns=0): >>     29031.6 bit/s -- 76.1 %CPU >> >>  2. w/ patch and disable kvm dynamic poll (halt_poll_ns=0): >>     35787.7 bit/s -- 129.4 %CPU >> >>  3. w/ kvm dynamic poll: >>     35735.6 bit/s -- 200.0 %CPU >> >>  4. w/patch and w/ kvm dynamic poll: >>     42225.3 bit/s -- 198.7 %CPU >> >>  5. idle=poll >>     37081.7 bit/s -- 998.1 %CPU >> >> >> >>  w/ this patch, we will improve performance by 23%.. even we could improve >>  performance by 45.4%, if we use w/patch and w/ kvm dynamic poll. also the >>  cost of CPU is much lower than 'idle=poll' case.. > I don't question the general idea. I just think pvops isn't the best way > to implement it. > >>> Wouldn't a function pointer, maybe guarded >>> by a static key, be enough? A further advantage would be that this would >>> work on other architectures, too. >> I assume this feature will be ported to other archs.. a new pvops makes       sorry, a typo.. /other archs/other hypervisors/       it refers hypervisor like Xen, HyperV and VMware).. >> code >> clean and easy to maintain. also I tried to add it into existed pvops, >> but it >> doesn't match. > You are aware that pvops is x86 only? yes, I'm aware.. > I really don't see the big difference in maintainability compared to the > static key / function pointer variant: > > void (*guest_idle_poll_func)(void); > struct static_key guest_idle_poll_key __read_mostly; > > static inline void guest_idle_poll(void) > { > if (static_key_false(&guest_idle_poll_key)) > guest_idle_poll_func(); > } thank you for your sample code :) I agree there is no big difference.. I think we are discussion for two things:  1) x86 VM on different hypervisors  2) different archs VM on kvm hypervisor What I want to do is x86 VM on different hypervisors, such as kvm / xen / hyperv .. > And KVM would just need to set guest_idle_poll_func and enable the > static key. Works on non-x86 architectures, too. > .. referred to 'pv_mmu_ops', HyperV and Xen can implement their own functions for 'pv_mmu_ops'. I think it is the same to pv_idle_ops. with above explaination, do you still think I need to define the static key/function pointer variant? btw, any interest to port it to Xen HVM guest? :) Quan Alibaba Cloud