Re: [PATCH RFC] sched/fair: Penalty the cfs task which executes mwait/hlt

[Peter Zijlstra]

On Wed, Jan 08, 2020 at 09:50:01AM +0800, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> 
> To deliver all of the resources of a server to instances in cloud, there are no 
> housekeeping cpus reserved. libvirtd, qemu main loop, kthreads, and other agent/tools 
> etc which can't be offloaded to other hardware like smart nic, these stuff will 
> contend with vCPUs even if MWAIT/HLT instructions executed in the guest.
> 
> The is no trap and yield the pCPU after we expose mwait/hlt to the guest [1][2],
> the top command on host still observe 100% cpu utilization since qemu process is 
> running even though guest who has the power management capability executes mwait. 
> Actually we can observe the physical cpu has already enter deeper cstate by 
> powertop on host.
> 
> For virtualization, there is a HLT activity state in CPU VMCS field which indicates 
> the logical processor is inactive because it executed the HLT instruction, but 
> SDM 24.4.2 mentioned that execution of the MWAIT instruction may put a logical 
> processor into an inactive state, however, this VMCS field never reflects this 
> state.

So far I think I can follow, however it does not explain who consumes
this VMCS state if it is set and how that helps. Also, this:

> This patch avoids fine granularity intercept and reschedule vCPU if MWAIT/HLT
> instructions executed, because it can worse the message-passing workloads which 
> will switch between idle and running frequently in the guest. Lets penalty the 
> vCPU which is long idle through tick-based sampling and preemption.

is just complete gibberish. And I have no idea what problem you're
trying to solve how.

Also, I don't think the TSC/MPERF ratio is architected, we can't assume
this is true for everything that has APERFMPERF.

/me tries to reconstruct intent from patch

So what you're doing is, mark the CPU 'idle' when the MPERF/TSC ratio <
1%, and then frob the vruntime such that it will hopefully preempt.
That's pretty disgusting.

Please, write a coherent problem statement and justify the magic
choices. This is unreviewable.