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

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

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

Changes in v1:
- rework how the resource is allocated and released
- rework when the resource is allocated that happens only when the resource is
  requested 
- rework the structure shared between the tool and Xen to make it extensible to
  new counters and declare it in a public header

There are still the following questions:
   - consumer may fetch inconsistency data
   - resource shall be released when there are no more readers otherwise we keep
     updating it during a hot path
   - one frame can host up to 512 vcpus. Should I check to this limit when
     updating? Should it be possible to allocate more than one frame for vcpu
     counters?  

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

 tools/misc/Makefile          |  6 +++
 tools/misc/xen-vcpus-stats.c | 76 +++++++++++++++++++++++++++++
 xen/arch/x86/hvm/hvm.c       |  2 +
 xen/common/memory.c          | 94 ++++++++++++++++++++++++++++++++++++
 xen/common/sched/core.c      |  7 +++
 xen/include/public/memory.h  |  3 ++
 xen/include/public/vcpu.h    | 10 ++++
 xen/include/xen/mm.h         |  2 +
 xen/include/xen/sched.h      |  5 ++
 9 files changed, 205 insertions(+)
 create mode 100644 tools/misc/xen-vcpus-stats.c

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2.25.1