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Subject[RFC] [PATCH v2 0/5] Intel Virtual PMU Optimization
As a toolbox treasure to developers, the 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.

Today in KVM, version 2 Architectural PMU on Intel and AMD hosts is
implemented and works. With the joint efforts of the community, it would be
an inspiring journey to enable all available PMU features for guest users
as complete/smooth/accurate as possible.

=== Brief description ===

This proposal for Intel vPMU is still committed to optimize the basic
functionality by reducing the PMU virtualization overhead and not a blind
pass-through of the PMU. The proposal applies to existing models, in short,
is "host perf would hand over control to kvm after counter allocation".

The pmc_reprogram_counter is a heavyweight and high frequency operation
which goes through the host perf software stack to create a perf event for
counter assignment, this could take millions of nanoseconds. The current
vPMU always does reprogram_counter when the guest changes the eventsel,
fixctrl, and global_ctrl msrs. This brings too much overhead to the usage
of perf inside the guest, especially the guest PMI handling and context
switching of guest threads with perf in use.

We optimize the current vPMU to work in this manner:

(1) rely on the existing host perf (perf_event_create_kernel_counter)
to allocate counters for in-use vPMC and always try to reuse events;
(2) vPMU captures guest accesses to the eventsel and fixctrl msr directly
to the hardware msr that the corresponding host event is scheduled on
and avoid pollution from host is also needed in its partial runtime;
(3) save and restore the counter state during vCPU scheduling in hooks;
(4) apply a lazy approach to release the vPMC's perf event. That is, if
the vPMC isn't used in a fixed sched slice, its event will be released.

In the use of vPMC, the vPMU always focus on the assigned resources and
guest perf would significantly benefit from direct access to hardware and
may not care about runtime state of perf_event created by host and always
try not to pay for their maintenance. However to avoid events entering into
any unexpected state, calling pmc_read_counter in appropriate is necessary.

=== vPMU Overhead Comparison ===

For the guest perf usage like "perf stat -e branches,cpu-cycles,\
dTLB-load-misses ./ftest", here are some performance numbers which show the
improvement with this optimization (in nanoseconds) [1]:

(1) Basic operatios latency on legacy Intel vPMU

kvm_pmu_rdpmc 200
pmc_stop_counter: gp 30,000
pmc_stop_counter: fixed 2,000,000
perf_event_create_kernel_counter: gp 30,000,000 <== (mark as 3.1)
perf_event_create_kernel_counter: fixed 25,000

(2) Comparison of max guest behavior latency
legacy v2
enable global_ctrl 57,000,000 17,000,000 <== (3.2)
disable global_ctrl 2,000,000 21,000
r/w fixed_ctrl 21,000 1,100
r/w eventsel 36,000 17,000
rdpmcl 35,000 18,000
x86_pmu.handle_irq 3,500,000 8,800 <== (3.3)

(3) For 3.2, the v2 value is just a maximum value for reprogram and
would be quickly weakened to neglect by reusing perf_events. In general,
we can say this optimization is ~400 times (3.3) faster than the original
for Intel vPMU due to a large number reduction of calls to
perf_event_create_kernel_counter (3.1).

(4) Comparison of guest behavior call time
legacy v2
enable global_ctrl 74,000 3,000 <== (6.1)
rd/wr fixed_ctrl 11,000 1,400
rd/wr eventsel 7,000,000 7,600
rdpmcl 130,000 10,000
x86_pmu.handle_irq 11 14

(5) Comparison of perf-attached thread guest context_switch latency
legacy v2
context_switch, sched_in 350,000,000 4,000,000
context_switch, sched_out 55,000,000 200,000

(6) From 6.1 and table 5, We can see a substantial reduction in the
runtime of a perf attached guest thread and the vPMU is no longer stuck.

=== vPMU Precision Comparison ===

We don't want to lose any precision after optimization and for perf usage
like "perf record -e cpu-cycles --all-user ./ftest"here is the comparison
of the profiling results with and without this optimization [1]:

(1) Test in Guest without optimization:

[ perf record: Woken up 2 times to write data ]
[ perf record: Captured and wrote 0.437 MB (5198 samples) ]

36.95% ftest ftest [.] qux
15.68% ftest ftest [.] foo
15.45% ftest ftest [.] bar
12.32% ftest ftest [.] main
9.56% ftest [.] __random
8.87% ftest [.] __random_r
1.17% ftest ftest [.] random@plt
0.00% ftest [.] _start

(2) Test in Guest with this optimization:

[ perf record: Woken up 4 times to write data ]
[ perf record: Captured and wrote 0.861 MB (22550 samples) ]

36.64% ftest ftest [.] qux
14.35% ftest ftest [.] foo
14.07% ftest ftest [.] bar
12.60% ftest ftest [.] main
11.73% ftest [.] __random
9.18% ftest [.] __random_r
1.42% ftest ftest [.] random@plt
0.00% ftest [.] do_lookup_x
0.00% ftest [.] _dl_new_object
0.00% ftest [.] _dl_sysdep_start
0.00% ftest [.] _start

(3) Test in Host:

[ perf record: Woken up 4 times to write data ]
[ perf record: Captured and wrote 0.789 MB (20652 samples) ]

37.87% ftest ftest [.] qux
15.78% ftest ftest [.] foo
13.18% ftest ftest [.] main
12.14% ftest ftest [.] bar
9.85% ftest [.] __random_r
9.59% ftest [.] __random
1.59% ftest ftest [.] random@plt
0.00% ftest [.] _dl_cache_libcmp
0.00% ftest [.] _dl_start
0.00% ftest [.] _start

=== NEXT ===

This proposal is trying to respected necessary functionality from the host
perf driver and bypasses the host perf subsystem software stack in most
execution paths with no loss of precision compared to the legacy one.
If this proposal is acceptable, here are something we could do for next:

(1) If host perf wants to perceive all the events for scheduling, some
event hooks could be implemented to update host perf_event with the
proper counts/runtimes/state.
(2) Loose the scheduling restrictions on pinned,
but still keeps eyes on special specific requests
(3) This series currently covers the basic perf counter virtualization.
Other features, such as pebs, bts, lbr will come after this series.

May be there is something wrong in the whole series and please help me
reach the other side of the performance improvement with your comments.

[1] Tested on Linux 5.0.0 on Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz,
and added "nowatchdog" to host booting parameter. The values comes from
sched_clock() using tsc as guest clocksource.

=== Changelog ===

v1: Wei Wang (8):
perf/x86: add support to mask counters from host
perf/x86/intel: add pmi callback support
KVM/x86/vPMU: optimize intel vPMU
KVM/x86/vPMU: support msr switch on vmx transitions
KVM/x86/vPMU: intel_pmu_read_pmc
KVM/x86/vPMU: remove some unused functions
KVM/x86/vPMU: save/restore guest perf counters on vCPU switching
KVM/x86/vPMU: return the counters to host if guest is torn down

v2: Like Xu (5):
perf/x86: avoid host changing counter state for kvm_intel events holder
KVM/x86/vPMU: add pmc operations for vmx and count to track release
KVM/x86/vPMU: add Intel vPMC enable/disable and save/restore support
KVM/x86/vPMU: add vCPU scheduling support for hw-assigned vPMC
KVM/x86/vPMU: not do reprogram_counter for Intel hw-assigned vPMC

arch/x86/events/core.c | 37 ++++-
arch/x86/events/intel/core.c | 5 +-
arch/x86/events/perf_event.h | 13 +-
arch/x86/include/asm/kvm_host.h | 2 +
arch/x86/kvm/pmu.c | 34 +++++
arch/x86/kvm/pmu.h | 22 +++
arch/x86/kvm/vmx/pmu_intel.c | 329 +++++++++++++++++++++++++++++++++++++---
arch/x86/kvm/x86.c | 6 +
8 files changed, 421 insertions(+), 27 deletions(-)


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