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[Qemu-devel] [PATCH v11 0/6] trace: [tcg] Optimize per-vCPU tracing stat

From: Lluís Vilanova
Subject: [Qemu-devel] [PATCH v11 0/6] trace: [tcg] Optimize per-vCPU tracing states with separate TB caches
Date: Tue, 4 Jul 2017 10:30:13 +0200
User-agent: StGit/0.17.1-dirty

Optimizes tracing of events with the 'tcg' and 'vcpu' properties (e.g., memory
accesses), making it feasible to statically enable them by default on all QEMU
builds. Last patch shows that overheads are completely eliminated in various
types of benchmarks for linux-user and softmmu (overheads where up to 2x

Right now, events with the 'tcg' property always generate TCG code to trace that
event at guest code execution time, where the event's dynamic state is checked.

This series adds a performance optimization where TCG code for events with the
'tcg' and 'vcpu' properties is not generated if the event is dynamically
disabled. This optimization raises two issues:

* An event can be dynamically disabled/enabled after the corresponding TCG code
  has been generated (i.e., a new TB with the corresponding code should be

* Each vCPU can have a different dynamic state for the same event (i.e., tracing
  the memory accesses of only one process pinned to a vCPU).

To handle both issues, this series integrates the dynamic tracing event state
into the TB hashing function, so that vCPUs tracing different events will use
separate TBs. Note that only events with the 'vcpu' property are used for
hashing (as stored in the bitmap of #CPUState::trace_dstate).

This makes dynamic event state changes on vCPUs very efficient, since they can
use TBs produced by other vCPUs while on the same event state combination (or
produced by the same vCPU, earlier).

Discarded alternatives:

* Emitting TCG code to check if an event needs tracing, where we should still
  move the tracing call code to either a cold path (making tracing performance
  worse), or leave it inlined (making non-tracing performance worse).

* Eliding TCG code only when *zero* vCPUs are tracing an event, since enabling
  it on a single vCPU will impact the performance of all other vCPUs that are
  not tracing that event.

Signed-off-by: Lluís Vilanova <address@hidden>

Changes in v11

* Rebase on 931892e8a6.
* Drop patch for flushing per-CPU virtual TB cache.
* Fix typo in comments.
* Change documentation format in TranslationBlock.

Changes in v10

* Replace lingering trace_get_vcpu_event_count() with
* Add performance results for dbt-bench [Emilio G. Cota].

Changes in v9

* Rebase on 931892e8a6.
* Undo renaming of tb->trace_vcpu_dstate to the shorter tb->trace_ds.
* Add measurements to commit enabling all guest events.

Changes in v8

[Emilio G. Cota]

* Ported to current dev tree.

* Allocate cpu->trace_dstate statically. This
  * allows us to drop the event_count inline patch.
  * simplifies and improves the performance of accessing cpu->trace_dstate:
    we just need to dereference, instead of going through bitmap_copy and
    an intermediate unsigned long.

* If we try to register more CPU events than the max we support (there's a
  constant for it), drop the event and tell the user with error_report. But
  really this is a bug, since we control what CPU events are traceable. Should
  we abort() as well?

* Added rth's R-b tag

* Addressed my own comments:
  * rename tb->trace_vcpu_dstate to the shorter tb->trace_ds
  * use uint32_t for tb->trace_ds instead of a typedef
  * add BUILD_BUG_ON check to make sure tb->trace_ds is big enough
  * fix xxhash

* Do not add trace_dstate to tb_htable_lookup, since we can grab it from

This patchset applies cleanly on top of rth's tcg-next (a01792e1e).

Changes in v7

* Fix delayed dstate changes (now uses async_run_on_cpu() as suggested by Paolo

* Note to Richard: patch 4 has been adapted to the new patch 3 async callback,
  but is essentially the same.

Changes in v6

* Check hashing size error with QEMU_BUILD_BUG_ON [Richard Henderson].

Changes in v5

* Move define into "qemu-common.h" to allow compilation of tests.

Changes in v4

* Incorporate trace_dstate into the TB hashing function instead of using
  multiple physical TB caches [suggested by Richard Henderson].

Changes in v3

* Rebase on 0737f32daf.
* Do not use reserved symbol prefixes ("__") [Stefan Hajnoczi].
* Refactor trace_get_vcpu_event_count() to be inlinable.
* Optimize cpu_tb_cache_set_requested() (hottest path).

Changes in v2

* Fix bitmap copy in cpu_tb_cache_set_apply().
* Split generated code re-alignment into a separate patch [Daniel P. Berrange].

Lluís Vilanova (6):
      trace: Allocate cpu->trace_dstate in place
      trace: [tcg] Delay changes to dynamic state when translating
      exec: [tcg] Use different TBs according to the vCPU's dynamic tracing 
      trace: [tcg] Do not generate TCG code to trace dynamically-disabled events
      trace: [tcg,trivial] Re-align generated code
      trace: [trivial] Statically enable all guest events

 accel/tcg/cpu-exec.c                     |    8 ++++++--
 accel/tcg/translate-all.c                |   11 +++++++++--
 include/exec/exec-all.h                  |    3 +++
 include/exec/tb-hash-xx.h                |    7 +++++--
 include/exec/tb-hash.h                   |    5 +++--
 include/qom/cpu.h                        |   12 ++++++------
 qom/cpu.c                                |    8 --------
 scripts/tracetool/__init__.py            |    3 ++-
 scripts/tracetool/backend/dtrace.py      |    4 ++--
 scripts/tracetool/backend/ftrace.py      |   20 ++++++++++----------
 scripts/tracetool/backend/log.py         |   19 ++++++++++---------
 scripts/tracetool/backend/simple.py      |    4 ++--
 scripts/tracetool/backend/syslog.py      |    6 +++---
 scripts/tracetool/backend/ust.py         |    4 ++--
 scripts/tracetool/format/h.py            |   26 +++++++++++++++++++-------
 scripts/tracetool/format/tcg_h.py        |   21 +++++++++++++++++----
 scripts/tracetool/format/tcg_helper_c.py |    5 +++--
 tcg/tcg-runtime.c                        |    3 ++-
 tests/qht-bench.c                        |    2 +-
 trace-events                             |    6 +++---
 trace/control-target.c                   |   21 ++++++++++++++++++---
 trace/control.c                          |    9 ++++++++-
 trace/control.h                          |    3 +++
 23 files changed, 137 insertions(+), 73 deletions(-)

To: address@hidden
Cc: Stefan Hajnoczi <address@hidden>
Cc: Eduardo Habkost <address@hidden>
Cc: Eric Blake <address@hidden>
Cc: Emilio G. Cota <address@hidden>

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