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

From: Thomas Gleixner
Date: Mon Jul 03 2017 - 06:09:17 EST

On Mon, 3 Jul 2017, Yang Zhang wrote:
> 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
> 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. 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.

You still can turn if off on the kernel command line via nohz=off