Unlike normal hardware PMCs, the 24x7 counters[1] in Power8 are stored in
memory and accessed via a hypervisor call (HCALL). A major aspect of the
HCALL is that it allows retireving _SEVERAL_ counters at once (unlike
regular PMCs, which are read one at a time). By reading several counters
at once, we can get a more consistent snapshot of the system.
This patchset extends the transaction interface to accomplish submitting
several events to the PMU and have the PMU read them all at once. User is
expected to submit the set of events they want to read as an "event group".
In the kernel, we submit each event to the PMU using the following logic
(from Peter Zijlstra).
pmu->start_txn(pmu, PMU_TXN_READ);
leader->read();
for_each_sibling()
sibling->read();
pmu->commit_txn();
where:
- the ->read()s queue events to be submitted to the hypervisor, and,
- the ->commit_txn() issues the HCALL, retrieves the result and
updates the event count.
Architectures/PMUs that don't need/implement PMU_TXN_READ type of transactions,
simply ignore the ->start_txn() and ->commit_txn() and continue to read the
counters one at a time in the ->read() call.
Compile/touch tested on x86. Need help testing on s390 and Sparc.
Thanks to Peter Zijlstra for his input.
Changelog [v2]
- Use the transaction interface unconditionally to avoid special-case
code. Architectures/PMUs that don't need the READ transaction types
simply ignore the ->start_txn() and ->commit_txn() calls.
Sukadev Bhattiprolu (5):
perf: Add a flags parameter to pmu txn interfaces
perf: Split perf_event_read() and perf_event_count()
perf: Rename perf_event_read_value to perf_event_compute_values
perf: Define PMU_TXN_READ interface
powerpc/perf/hv-24x7: Use PMU_TXN_READ interface
arch/powerpc/perf/core-book3s.c | 16 +++-
arch/powerpc/perf/hv-24x7.c | 165 ++++++++++++++++++++++++++++++++++++++
arch/s390/kernel/perf_cpum_cf.c | 15 +++-
arch/sparc/kernel/perf_event.c | 15 +++-
arch/x86/kernel/cpu/perf_event.c | 15 +++-
arch/x86/kvm/pmu.c | 6 +-
include/linux/perf_event.h | 17 +++-
kernel/events/core.c | 83 ++++++++++++++-----
8 files changed, 292 insertions(+), 40 deletions(-)
--
1.7.9.5
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