KVM: x86/pmu: Introduce pmc->is_paused to reduce the call time of perf interfaces

Based on our observations, after any vm-exit associated with vPMU, there
are at least two or more perf interfaces to be called for guest counter
emulation, such as perf_event_{pause, read_value, period}(), and each one
will {lock, unlock} the same perf_event_ctx. The frequency of calls becomes
more severe when guest use counters in a multiplexed manner.

Holding a lock once and completing the KVM request operations in the perf
context would introduce a set of impractical new interfaces. So we can
further optimize the vPMU implementation by avoiding repeated calls to
these interfaces in the KVM context for at least one pattern:

After we call perf_event_pause() once, the event will be disabled and its
internal count will be reset to 0. So there is no need to pause it again
or read its value. Once the event is paused, event period will not be
updated until the next time it's resumed or reprogrammed. And there is
also no need to call perf_event_period twice for a non-running counter,
considering the perf_event for a running counter is never paused.

Based on this implementation, for the following common usage of
sampling 4 events using perf on a 4u8g guest:

  echo 0 > /proc/sys/kernel/watchdog
  echo 25 > /proc/sys/kernel/perf_cpu_time_max_percent
  echo 10000 > /proc/sys/kernel/perf_event_max_sample_rate
  echo 0 > /proc/sys/kernel/perf_cpu_time_max_percent
  for i in `seq 1 1 10`
  do
  taskset -c 0 perf record \
  -e cpu-cycles -e instructions -e branch-instructions -e cache-misses \
  /root/br_instr a
  done

the average latency of the guest NMI handler is reduced from
37646.7 ns to 32929.3 ns (~1.14x speed up) on the Intel ICX server.
Also, in addition to collecting more samples, no loss of sampling
accuracy was observed compared to before the optimization.

Signed-off-by: Like Xu <likexu@tencent.com>
Message-Id: <20210728120705.6855-1-likexu@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
index 67e753edfa22..0e4f2b1fa9fb 100644
--- a/arch/x86/kvm/pmu.h
+++ b/arch/x86/kvm/pmu.h
@@ -55,7 +55,7 @@ static inline u64 pmc_read_counter(struct kvm_pmc *pmc)
 	u64 counter, enabled, running;
 
 	counter = pmc->counter;
-	if (pmc->perf_event)
+	if (pmc->perf_event && !pmc->is_paused)
 		counter += perf_event_read_value(pmc->perf_event,
 						 &enabled, &running);
 	/* FIXME: Scaling needed? */