[PATCH v2 00/19] tracepoint: Avoid double static_branch evaluation at guarded call sites

["Vineeth Pillai (Google)" <vineeth@bitbyteword.org>]

When a caller already guards a tracepoint with an explicit enabled check:

  if (trace_foo_enabled() && cond)
      trace_foo(args);

trace_foo() internally re-evaluates the static_branch_unlikely() key.
Since static branches are patched binary instructions the compiler cannot
fold the two evaluations, so every such site pays the cost twice.

This series introduces trace_call__##name() as a companion to
trace_##name().  It calls __do_trace_##name() directly, bypassing the
redundant static-branch re-check, while preserving all other correctness
properties of the normal path (RCU-watching assertion, might_fault() for
syscall tracepoints).  The internal __do_trace_##name() symbol is not
leaked to call sites; trace_call__##name() is the only new public API.

  if (trace_foo_enabled() && cond)
      trace_call__foo(args);   /* calls __do_trace_foo() directly */

The first patch adds the three-location change to
include/linux/tracepoint.h (__DECLARE_TRACE, __DECLARE_TRACE_SYSCALL,
and the !TRACEPOINTS_ENABLED stub).  The remaining 18 patches
mechanically convert all guarded call sites found in the tree:
kernel/, io_uring/, net/, accel/habanalabs, cpufreq/, devfreq/,
dma-buf/, fsi/, drm/, HID, i2c/, spi/, scsi/ufs/, btrfs/,
net/devlink/, kernel/time/, kernel/trace/, mm/damon/, and arch/x86/.

This series is motivated by Peter Zijlstra's observation in the discussion
around Dmitry Ilvokhin's locking tracepoint instrumentation series, where
he noted that compilers cannot optimize static branches and that guarded
call sites end up evaluating the static branch twice for no reason, and
by Steven Rostedt's suggestion to add a proper API instead of exposing
internal implementation details like __do_trace_##name() directly to
call sites:

  https://lore.kernel.org/linux-trace-kernel/8298e098d3418cb446ef396f119edac58a3414e9.1772642407.git.d@ilvokhin.com

Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>

Changes in v2:
- Renamed trace_invoke_##name() to trace_call__##name() (double
  underscore) per review comments.
- Added 4 new patches covering sites missed in v1, found using
  coccinelle to scan the tree (Keith Busch):
    * net/devlink: guarded tracepoint_enabled() block in trap.c
    * kernel/time: early-return guard in tick-sched.c (tick_stop)
    * kernel/trace: early-return guard in trace_benchmark.c
    * mm/damon: early-return guard in core.c
    * arch/x86: do_trace_*() wrapper functions in lib/msr.c, which
      are called exclusively from tracepoint_enabled()-guarded sites
      in asm/msr.h

v1: https://lore.kernel.org/linux-trace-kernel/abSqrJ1J59RQC47U@kbusch-mbp/

Vineeth Pillai (Google) (19):
  tracepoint: Add trace_call__##name() API
  kernel: Use trace_call__##name() at guarded tracepoint call sites
  io_uring: Use trace_call__##name() at guarded tracepoint call sites
  net: Use trace_call__##name() at guarded tracepoint call sites
  accel/habanalabs: Use trace_call__##name() at guarded tracepoint call
    sites
  cpufreq: Use trace_call__##name() at guarded tracepoint call sites
  devfreq: Use trace_call__##name() at guarded tracepoint call sites
  dma-buf: Use trace_call__##name() at guarded tracepoint call sites
  fsi: Use trace_call__##name() at guarded tracepoint call sites
  drm: Use trace_call__##name() at guarded tracepoint call sites
  HID: Use trace_call__##name() at guarded tracepoint call sites
  i2c: Use trace_call__##name() at guarded tracepoint call sites
  spi: Use trace_call__##name() at guarded tracepoint call sites
  scsi: ufs: Use trace_call__##name() at guarded tracepoint call sites
  btrfs: Use trace_call__##name() at guarded tracepoint call sites
  net: devlink: Use trace_call__##name() at guarded tracepoint call
    sites
  kernel: time, trace: Use trace_call__##name() at guarded tracepoint
    call sites
  mm: damon: Use trace_call__##name() at guarded tracepoint call sites
  x86: msr: Use trace_call__##name() at guarded tracepoint call sites

 arch/x86/lib/msr.c                                |  6 +++---
 drivers/accel/habanalabs/common/device.c          | 12 ++++++------
 drivers/accel/habanalabs/common/mmu/mmu.c         |  3 ++-
 drivers/accel/habanalabs/common/pci/pci.c         |  4 ++--
 drivers/cpufreq/amd-pstate.c                      | 10 +++++-----
 drivers/cpufreq/cpufreq.c                         |  2 +-
 drivers/cpufreq/intel_pstate.c                    |  2 +-
 drivers/devfreq/devfreq.c                         |  2 +-
 drivers/dma-buf/dma-fence.c                       |  4 ++--
 drivers/fsi/fsi-master-aspeed.c                   |  2 +-
 drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c            |  2 +-
 drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c            |  4 ++--
 drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c |  2 +-
 drivers/gpu/drm/scheduler/sched_entity.c          |  4 ++--
 drivers/hid/intel-ish-hid/ipc/pci-ish.c           |  2 +-
 drivers/i2c/i2c-core-slave.c                      |  2 +-
 drivers/spi/spi-axi-spi-engine.c                  |  4 ++--
 drivers/ufs/core/ufshcd.c                         | 12 ++++++------
 fs/btrfs/extent_map.c                             |  4 ++--
 fs/btrfs/raid56.c                                 |  4 ++--
 include/linux/tracepoint.h                        | 11 +++++++++++
 io_uring/io_uring.h                               |  2 +-
 kernel/irq_work.c                                 |  2 +-
 kernel/sched/ext.c                                |  2 +-
 kernel/smp.c                                      |  2 +-
 kernel/time/tick-sched.c                          | 12 ++++++------
 kernel/trace/trace_benchmark.c                    |  2 +-
 mm/damon/core.c                                   |  2 +-
 net/core/dev.c                                    |  2 +-
 net/core/xdp.c                                    |  2 +-
 net/devlink/trap.c                                |  2 +-
 net/openvswitch/actions.c                         |  2 +-
 net/openvswitch/datapath.c                        |  2 +-
 net/sctp/outqueue.c                               |  2 +-
 net/tipc/node.c                                   |  2 +-
 35 files changed, 74 insertions(+), 62 deletions(-)

-- 
2.53.0