Messages in this thread | | | Subject | Re: [PATCH RFC v3 1/6] x86/paravirt: Add pv_idle_ops to paravirt ops | From | Quan Xu <> | Date | Tue, 14 Nov 2017 17:38:57 +0800 |
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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 <quan.xu0@gmail.com> >>>> >>>> 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 <yang.zhang.wz@gmail.com> >>>> Signed-off-by: Quan Xu <quan.xu0@gmail.com> >>>> Cc: Juergen Gross <jgross@suse.com> >>>> Cc: Alok Kataria <akataria@vmware.com> >>>> Cc: Rusty Russell <rusty@rustcorp.com.au> >>>> Cc: Thomas Gleixner <tglx@linutronix.de> >>>> Cc: Ingo Molnar <mingo@redhat.com> >>>> Cc: "H. Peter Anvin" <hpa@zytor.com> >>>> 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
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