[PATCH 0/3] KVM: x86/vPMU: Efficiency optimization by reusing last created perf_event

[Like Xu]

Hi Paolo & Community:

Performance Monitoring Unit is designed to monitor micro architectural
events which helps in analyzing how an application or operating systems
are performing on the processors. In KVM/X86, version 2 Architectural
PMU on Intel and AMD hosts have been enabled. 

This patch series is going to improve vPMU Efficiency for guest perf users
which is mainly measured by guest NMI handler latency for basic perf usage
[1][2][3][4] with hardware PMU. It's not a passthrough solution but based
on the legacy vPMU implementation (since 2011) with backport-friendliness.

The general idea (defined in patch 2/3) is to reuse last created perf_event
for the same vPMC when the new requested config is the exactly same as the
last programed config (used by pmc_reprogram_counter()) AND the new event
period is appropriate and accepted (via perf_event_period() in patch 1/3).
Before the perf_event is resued, it would be disabled until it's could be
reused and reassigned a hw-counter again to serve for vPMC.

If the disabled perf_event is no longer reused, we do a lazy release
mechanism (defined in patch 3/3) which in a short is to release the
disabled perf_events on the first call of vcpu_enter_guest since the
vcpu gets next scheduled in if its MSRs is not accessed in the last
sched time slice. The bitmap pmu->lazy_release_ctrl is added to track.
The kvm_pmu_cleanup() is added to the first time to run vcpu_enter_guest
after the vcpu shced_in and the overhead is very limited.

With this optimization, the average latency of the guest NMI handler is
reduced from 99450 ns to 56195 ns (1.76x speed up on CLX-AP with v5.3).
If host disables the watchdog (echo 0 > /proc/sys/kernel/watchdog), the
minimum latency of guest NMI handler could be speed up at 2994x and in
the average at 685x. The run time of workload with perf attached inside
the guest could be reduced significantly with this optimization. 

Please check each commit for more details and share your comments with us.

Thanks,
Like Xu 

---
[1] multiplexing sampling usage: time perf record  -e \
`perf list | grep Hardware | grep event |\
awk '{print $1}' | head -n 10 |tr '\n' ',' | sed 's/,$//' ` ./ftest
[2] one gp counter sampling usage: perf record -e branch-misses ./ftest
[3] one fixed counter sampling usage: perf record -e instructions ./ftest
[4] event count usage: perf stat -e branch-misses ./ftest

Like Xu (3):
  perf/core: Provide a kernel-internal interface to recalibrate event
    period
  KVM: x86/vPMU: Reuse perf_event to avoid unnecessary
    pmc_reprogram_counter
  KVM: x86/vPMU: Add lazy mechanism to release perf_event per vPMC

 arch/x86/include/asm/kvm_host.h | 10 ++++
 arch/x86/kvm/pmu.c              | 88 ++++++++++++++++++++++++++++++++-
 arch/x86/kvm/pmu.h              | 15 +++++-
 arch/x86/kvm/pmu_amd.c          | 14 ++++++
 arch/x86/kvm/vmx/pmu_intel.c    | 27 ++++++++++
 arch/x86/kvm/x86.c              |  6 +++
 include/linux/perf_event.h      |  5 ++
 kernel/events/core.c            | 28 ++++++++---
 8 files changed, 182 insertions(+), 11 deletions(-)

-- 
2.21.0