Re: [Xen-devel] [PATCH RFC v3 1/6] x86/paravirt: Add pv_idle_ops to paravirt ops

[Quan Xu <quan.xu0@xxxxxxxxx>]

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@xxxxxxxxx>
> > > >
> > > > 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@xxxxxxxxx>
> > > > Signed-off-by: Quan Xu <quan.xu0@xxxxxxxxx>
> > > > Cc: Juergen Gross <jgross@xxxxxxxx>
> > > > Cc: Alok Kataria <akataria@xxxxxxxxxx>
> > > > Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
> > > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > > > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > > > Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> > > > Cc: x86@xxxxxxxxxx
> > > > Cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
> > > > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > > > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
> > > 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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