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Subject[PATCH 0/8] arm64/ftrace: Add support for DYNAMIC_FTRACE_WITH_CALL_OPS
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This series adds a new DYNAMIC_FTRACE_WITH_CALL_OPS mechanism, and
enables support for this on arm64. This significantly reduces the
overhead of tracing when a callsite/tracee has a single associated
tracer, avoids a number of issues that make it undesireably and
infeasible to use dynamically-allocated trampolines (e.g. branch range
limitations), and makes it possible to implement support for
DYNAMIC_FTRACE_WITH_DIRECT_CALLS in future.

The main idea is to give each ftrace callsite an associated pointer to
an ftrace_ops. The architecture's ftrace_caller trampoline can recover
the ops pointer and invoke ops->func from this without needing to use
ftrace_ops_list_func, which has to iterate through all registered ops.

To do this, we use -fpatchable-function-entry=M,N, there N NOPs are
placed before the function entry point. On arm64 NOPs are always 4
bytes, so by allocating 2 per-function NOPs, we have enaough space to
place a 64-bit value. So that we can manipulate the pointer atomically,
we need to align instrumented functions to at least 8 bytes.

The first three patches enable this function alignment, requiring
changes to the ACPICA Makefile, and working around cases where GCC drops
alignment.

The final four patches implement support for arm64. As noted in the
final patch, this results in a significant reduction in overhead:

Before this patch:

Number of tracers || Total time | Per-call average time (ns)
Relevant | Irrelevant || (ns) | Total | Overhead
=========+============++=============+==============+============
0 | 0 || 94,583 | 0.95 | -
0 | 1 || 93,709 | 0.94 | -
0 | 2 || 93,666 | 0.94 | -
0 | 10 || 93,709 | 0.94 | -
0 | 100 || 93,792 | 0.94 | -
---------+------------++-------------+--------------+------------
1 | 1 || 6,467,833 | 64.68 | 63.73
1 | 2 || 7,509,708 | 75.10 | 74.15
1 | 10 || 23,786,792 | 237.87 | 236.92
1 | 100 || 106,432,500 | 1,064.43 | 1063.38
---------+------------++-------------+--------------+------------
1 | 0 || 1,431,875 | 14.32 | 13.37
2 | 0 || 6,456,334 | 64.56 | 63.62
10 | 0 || 22,717,000 | 227.17 | 226.22
100 | 0 || 103,293,667 | 1032.94 | 1031.99
---------+------------++-------------+--------------+--------------

Note: per-call overhead is estiamated relative to the baseline case
with 0 relevant tracers and 0 irrelevant tracers.

After this patch

Number of tracers || Total time | Per-call average time (ns)
Relevant | Irrelevant || (ns) | Total | Overhead
=========+============++=============+==============+============
0 | 0 || 94,541 | 0.95 | -
0 | 1 || 93,666 | 0.94 | -
0 | 2 || 93,709 | 0.94 | -
0 | 10 || 93,667 | 0.94 | -
0 | 100 || 93,792 | 0.94 | -
---------+------------++-------------+--------------+------------
1 | 1 || 281,000 | 2.81 | 1.86
1 | 2 || 281,042 | 2.81 | 1.87
1 | 10 || 280,958 | 2.81 | 1.86
1 | 100 || 281,250 | 2.81 | 1.87
---------+------------++-------------+--------------+------------
1 | 0 || 280,959 | 2.81 | 1.86
2 | 0 || 6,502,708 | 65.03 | 64.08
10 | 0 || 18,681,209 | 186.81 | 185.87
100 | 0 || 103,550,458 | 1,035.50 | 1034.56
---------+------------++-------------+--------------+------------

Note: per-call overhead is estiamated relative to the baseline case
with 0 relevant tracers and 0 irrelevant tracers.

Thanks,
Mark.

Mark Rutland (8):
Compiler attributes: GCC function alignment workarounds
ACPI: Don't build ACPICA with '-Os'
arm64: Extend support for CONFIG_FUNCTION_ALIGNMENT
ftrace: Add DYNAMIC_FTRACE_WITH_CALL_OPS
arm64: insn: Add helpers for BTI
arm64: patching: Add aarch64_insn_write_literal_u64()
arm64: ftrace: Update stale comment
arm64: Implement HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS

arch/arm64/Kconfig | 3 +
arch/arm64/Makefile | 5 +-
arch/arm64/include/asm/ftrace.h | 15 +--
arch/arm64/include/asm/insn.h | 1 +
arch/arm64/include/asm/linkage.h | 10 +-
arch/arm64/include/asm/patching.h | 2 +
arch/arm64/kernel/asm-offsets.c | 4 +
arch/arm64/kernel/entry-ftrace.S | 32 +++++-
arch/arm64/kernel/ftrace.c | 158 +++++++++++++++++++++++++++-
arch/arm64/kernel/patching.c | 17 +++
drivers/acpi/acpica/Makefile | 2 +-
include/linux/compiler_attributes.h | 23 +++-
include/linux/ftrace.h | 15 ++-
kernel/trace/Kconfig | 7 ++
kernel/trace/ftrace.c | 109 ++++++++++++++++++-
15 files changed, 371 insertions(+), 32 deletions(-)

--
2.30.2

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