[PATCH v6 0/10] perf: Implement group-read of events using txn interface

From: Sukadev Bhattiprolu
Date: Thu Sep 03 2015 - 23:12:14 EST

Unlike normal hardware PMCs, the 24x7 counters 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);


- 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/code.

- [Peter Zijlstra] Use the new ->txn_flags to determine if we are in a
transaction and to skip the schedulability tests. This results in
PERF_EVENT_TXN flag being unused and can be dropped. Once PERF_EVENT_TXN
is not used, several architectures no longer need the cpuhw->group_flag
so drop that as well.
- [Peter Zijlstra] Change the type of txn_flags from 'int' to 'unsigned int'.
- [Peter Zijlstra] Add a WARN_ON_ONCE() if the state of txn_flags is not what
we expect.
- Rebase to 4.2.0.

- Invert the sibling-child loop nesting in perf-read-group (re-org
code and drop the patch that defined perf_event_aggregate()).

- Ensure all the transactions operations happen on the same CPU so PMUs
can use per-CPU buffers for the transaction.
- Add lockdep assert and fix a locking issue in perf_read_group().

Changelog [v3]
- Simple changes/reorg of patchset to split/rename functions
- [Peter Zijlstra] Save the transaction flags in ->start_txn() and
drop the flags parameter from ->commit_txn() and ->cancel_txn().
- [Peter Zijlstra] The nop txn interfaces don't need to disable/enable
PMU for PERF_PMU_TXN_READ transactions.

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.

Peter Zijlstra (2):
perf: Add group reads to perf_event_read()
perf: Invert perf_read_group() loops

Peter Zijlstra (Intel) (1):
perf: Rename perf_event_read_{one,group}, perf_read_hw

Sukadev Bhattiprolu (7):
perf/sparc: Remove unnecessary assignment
perf: Add a flags parameter to pmu txn interfaces
perf: Split perf_event_read() and perf_event_count()
perf: Add return value for perf_event_read().
Define PERF_PMU_TXN_READ interface
powerpc/perf/hv-24x7: Use PMU_TXN_READ interface

arch/powerpc/perf/core-book3s.c | 36 +++++--
arch/powerpc/perf/hv-24x7.c | 166 +++++++++++++++++++++++++++++-
arch/s390/kernel/perf_cpum_cf.c | 35 ++++++-
arch/sparc/kernel/perf_event.c | 32 ++++--
arch/x86/kernel/cpu/perf_event.c | 39 +++++--
arch/x86/kernel/cpu/perf_event.h | 2 +-
include/linux/perf_event.h | 15 ++-
kernel/events/core.c | 210 +++++++++++++++++++++++++++++---------
8 files changed, 456 insertions(+), 79 deletions(-)


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