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

[Jan Beulich <jbeulich@xxxxxxxx>]

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?

Jan