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

[Matias Ezequiel Vara Larsen <matiasevara@xxxxxxxxx>]

Hello all,

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.

Thanks, Matias.

[1] https://github.com/MatiasVara/xen/tree/feature_stats

Matias Ezequiel Vara Larsen (2):
  xen/memory : Add stats_table resource type
  tools/misc: Add xen-stats tool

 tools/misc/Makefile         |  5 +++
 tools/misc/xen-stats.c      | 83 +++++++++++++++++++++++++++++++++++++
 xen/common/domain.c         | 42 +++++++++++++++++++
 xen/common/memory.c         | 29 +++++++++++++
 xen/common/sched/core.c     |  5 +++
 xen/include/public/memory.h |  1 +
 xen/include/xen/sched.h     |  5 +++
 7 files changed, 170 insertions(+)
 create mode 100644 tools/misc/xen-stats.c

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2.25.1