Re: [RFC PATCH v1 0/2] Add a new acquire resource to query vcpu statistics

[Matias Ezequiel Vara Larsen <matiasevara@gmail.com>]

On Thu, Sep 29, 2022 at 05:55:50PM +0200, Jan Beulich wrote:
> On 24.08.2022 15:27, Matias Ezequiel Vara Larsen wrote:
> > The purpose of this RFC is to get feedback about a new acquire resource that
> > exposes vcpu statistics for a given domain. The current mechanism to get those
> > statistics is by querying the hypervisor. This mechanism relies on a hypercall
> > and holds the domctl spinlock during its execution. When a pv tool like xcp-rrdd
> > periodically samples these counters, it ends up affecting other paths that share
> > that spinlock. By using acquire resources, the pv tool only requires a few
> > hypercalls to set the shared memory region and samples are got without issuing
> > any other hypercall. The original idea has been suggested by Andrew Cooper to
> > which I have been discussing about how to implement the current PoC. You can
> > find the RFC patch series at [1]. The series is rebased on top of stable-4.15.
> > 
> > I am currently a bit blocked on 1) what to expose and 2) how to expose it. For
> > 1), I decided to expose what xcp-rrdd is querying, e.g., XEN_DOMCTL_getvcpuinfo.
> > More precisely, xcp-rrd gets runstate.time[RUNSTATE_running]. This is a uint64_t
> > counter. However, the time spent in other states may be interesting too.
> > Regarding 2), I am not sure if simply using an array of uint64_t is enough or if
> > a different interface should be exposed. The remaining question is when to get
> > new values. For the moment, I am updating this counter during
> > vcpu_runstate_change().
> > 
> > The current series includes a simple pv tool that shows how this new interface is
> > used. This tool maps the counter and periodically samples it.
> > 
> > Any feedback/help would be appreciated.
> 
> Before looking more closely - was there perhaps kind-of-review feedback
> during the summit, which would make it more reasonable to look through
> this once a v2 has appeared?
> 

Yes, there was. I will submit v2 from feedback during summit. Thanks for point
it.

Matias