Re: [PATCH 2/2] x86/idle: use dynamic halt poll

[Alexander Graf <agraf@suse.de>]

On 13.07.17 13:49, Yang Zhang wrote:
> On 2017/7/4 22:13, Radim Krčmář wrote:
>> 2017-07-03 17:28+0800, Yang Zhang:
>>> The background is that we(Alibaba Cloud) do get more and more complaints
>>> from our customers in both KVM and Xen compare to bare-mental.After
>>> investigations, the root cause is known to us: big cost in message 
>>> passing
>>> workload(David show it in KVM forum 2015)
>>>
>>> A typical message workload like below:
>>> vcpu 0                             vcpu 1
>>> 1. send ipi                     2.  doing hlt
>>> 3. go into idle                 4.  receive ipi and wake up from hlt
>>> 5. write APIC time twice        6.  write APIC time twice to
>>>    to stop sched timer              reprogram sched timer
>>
>> One write is enough to disable/re-enable the APIC timer -- why does
>> Linux use two?
> 
> One is to remove the timer and another one is to reprogram the timer. 
> Normally, only one write to remove the timer.But in some cases, it will 
> reprogram it.
> 
>>
>>> 7. doing hlt                    8.  handle task and send ipi to
>>>                                     vcpu 0
>>> 9. same to 4.                   10. same to 3
>>>
>>> One transaction will introduce about 12 vmexits(2 hlt and 10 msr 
>>> write). The
>>> cost of such vmexits will degrades performance severely.
>>
>> Yeah, sounds like too much ... I understood that there are
>>
>>   IPI from 1 to 2
>>   4 * APIC timer
>>   IPI from 2 to 1
>>
>> which adds to 6 MSR writes -- what are the other 4?
> 
> In the worst case, each timer will touch APIC timer twice.So it will add 
> additional 4 msr writse. But this is  not always true.
> 
>>
>>>                                                          Linux kernel
>>> already provide idle=poll to mitigate the trend. But it only 
>>> eliminates the
>>> IPI and hlt vmexit. It has nothing to do with start/stop sched timer. A
>>> compromise would be to turn off NOHZ kernel, but it is not the default
>>> config for new distributions. Same for halt-poll in KVM, it only 
>>> solve the
>>> cost from schedule in/out in host and can not help such workload much.
>>>
>>> The purpose of this patch we want to improve current idle=poll 
>>> mechanism to
>>
>> Please aim to allow MWAIT instead of idle=poll -- MWAIT doesn't slow
>> down the sibling hyperthread.  MWAIT solves the IPI problem, but doesn't
>> get rid of the timer one.
> 
> Yes, i can try it. But MWAIT will not yield CPU, it only helps the 
> sibling hyperthread as you mentioned.

If you implement proper MWAIT emulation that conditionally gets en- or 
disabled depending on the same halt poll dynamics that we already have 
for in-host HLT handling, it will also yield the CPU.

As for the timer - are you sure the problem is really the overhead of 
the timer configuration, not the latency that it takes to actually fire 
the guest timer?

One major problem I see is that we configure the host hrtimer to fire at 
the point in time when the guest wants to see a timer event. But in a 
virtual environment, the point in time when we have to start switching 
to the VM really should be a bit *before* the guest wants to be woken 
up, as it takes quite some time to switch back into the VM context.


Alex