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

[Juergen Gross <jgross@xxxxxxxx>]

On 14/11/17 12:43, Quan Xu wrote:
> 
> 
> On 2017/11/14 18:27, Juergen Gross wrote:
>> On 14/11/17 10:38, Quan Xu wrote:
>>>
>>> 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 ..
>> Why limit the solution to x86 if the more general solution isn't
>> harder?
>>
>> As you didn't give any reason why the pvops approach is better other
>> than you don't care for non-x86 platforms you won't get an "Ack" from
>> me for this patch.
> 
> 
> It just looks a little odder to me. I understand you care about no-x86
> arch.
> 
> Are you aware 'pv_time_ops' for arm64/arm/x86 archs, defined in
>    - arch/arm64/include/asm/paravirt.h
>    - arch/x86/include/asm/paravirt_types.h
>    - arch/arm/include/asm/paravirt.h

Yes, I know. This is just a hack to make it compile. Other than the
same names this has nothing to do with pvops, but is just a function
vector.

> I am unfamilar with arm code. IIUC, if you'd implement pv_idle_ops
> for arm/arm64 arch, you'd define a same structure in
>    - arch/arm64/include/asm/paravirt.h     or
>    - arch/arm/include/asm/paravirt.h
> 
> .. instead of static key / fuction.
> 
> then implement a real function in
>    - arch/arm/kernel/paravirt.c.

So just to use pvops you want to implement it in each arch instead
of using a mechanism available everywhere?

> Also I wonder HOW/WHERE to define a static key/function, then to benifit
> x86/no-x86 archs?

What? There are plenty of examples in the kernel.

Please stop wasting my time. Either write a patch which is acceptable
or let it be. I won't take your pvops approach without a really good
reason to do so. And so far you haven't given any reason other than
you are too lazy to write a proper patch, sorry.


Juergen

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