Re: [PATCH v9 3/3] cpus-common: implement dirty page limit on vCPU

[Hyman]

在 2021/12/7 10:57, Peter Xu 写道:
> On Mon, Dec 06, 2021 at 11:19:21PM +0800, Hyman wrote:
> > > > +    if (has_cpu_index) {
> > > > +        info = dirtylimit_query_vcpu(cpu_index);
> > > > +        QAPI_LIST_APPEND(tail, info);
> > > > +    } else {
> > > > +        CPUState *cpu;
> > > > +        CPU_FOREACH(cpu) {
> > > > +            if (!cpu->unplug) {
> > > > +                info = dirtylimit_query_vcpu(cpu->cpu_index);
> > > > +                QAPI_LIST_APPEND(tail, info);
> > > > +            }
> > >
> > > There're special handling for unplug in a few places.  Could you explain why?
> > > E.g. if the vcpu is unplugged then dirty rate is zero, then it seems fine to
> > > even keep it there?
> > > The dirty limit logic only allow plugged vcpu to be enabled throttle, so
> > that the "dirtylimit-{cpu-index}" thread don't need to be forked and we can
> > save the overhead. So in query logic we just filter the unplugged vcpu.
>
> I've commented similarly in the other thread - please consider not using NVCPU
> threads only for vcpu throttling, irrelevant of vcpu hot plug/unplug.
>
> Per-vcpu throttle is totally not a cpu intensive workload, 1 thread should be
> enough globally, imho.
>
> A guest with hundreds of vcpus are becoming more common, we shouldn't waste OS
> thread resources just for this.
Ok, i'll try this out next version
> > Another reason is that i thought it could make user confused when we return
> > the unplugged vcpu dirtylimit info. Uh, in most time of vm lifecycle,
> > hotplugging vcpu may never happen.
>
> I just think if plug/unplug does not affect the throttle logic then we should
> treat them the same, it avoids unnecessary special care on those vcpus too.
Indeed, i'm struggling too :), i'll remove the plug/unplug logic the 
next version.