The preempt and IRQ tracepoints currently impose measurable overhead
even when they are compiled in but not actively enabled. This overhead
stems from external function calls and unconditional tracepoint checks
in highly active code paths.
The v2 series optimized within the existing CONFIG_TRACE_IRQFLAGS
path, which still required the heavy lockdep and irqsoff
infrastructure to be enabled. This v3 takes a different approach by
providing independent, user-selectable configurations
(CONFIG_TRACE_PREEMPT_TOGGLE and CONFIG_TRACE_IRQFLAGS_TOGGLE) that
expose the tracepoints without pulling in the heavier infrastructure.
The preempt optimization uses inline static key checks, while the IRQ
optimization employs lightweight wrapper functions that check raw
hardware state. Making both configurations explicitly user-selectable
addresses upstream feedback regarding the impact on code generation,
ensuring that this optimization remains strictly opt-in.
---
Performance Measurements
Measurements were taken using the tracer-benchmark kernel module [1].
The module creates one kthread per online CPU. Each thread performs
a configurable number of iterations of
local_irq_disable()/local_irq_enable() and
preempt_disable()/preempt_enable() pairs, timing each pair with
ktime_get_ns(). All threads start simultaneously via a completion
to maximize contention. Per-CPU results (average, median) are aggregated across
CPUs. The 99th percentile is measured separately
on a single pinned CPU. The kernel used was version 7.0.0. All
values are in nanoseconds. Each run collected 10^7 samples.
Configurations compared:
- 7.0.0: stock kernel
- irqsoff: stock kernel with CONFIG_IRQSOFF_TRACER=y and
CONFIG_PREEMPT_TRACER=y
- preemptirq: patched kernel with CONFIG_TRACE_PREEMPT_TOGGLE=y
and CONFIG_TRACE_IRQFLAGS_TOGGLE=y
The '+' suffix indicates the test ran with tracepoints enabled.
IRQ Metrics
Metric 7.0.0 irqsoff irqsoff+ preemptirq preemptirq+
average 19 27 175 19 166
median 19 27 172 19 164
99 percentile 21 29 234 21 221
Preempt Metrics
Metric 7.0.0 irqsoff irqsoff+ preemptirq preemptirq+
average 16 21 169 16 160
median 16 21 165 17 159
99 percentile 18 23 236 18 217
The preemptirq configuration matches the stock kernel performance
when tracepoints are disabled, while the irqsoff configuration adds
~40% overhead even when inactive. When tracepoints are enabled,
preemptirq is also slightly faster than irqsoff.
Binary size impact (stripped vmlinux, defconfig):
7.0.0: 43404576 bytes
preemptirq: 43429152 bytes (+24576 bytes, +0.057%)
Suggested-by: Steven Rostedt <[email protected]>
---
References:
[1] https://github.com/walac/tracer-benchmark
Changes in v3:
- Reworked series from 2 to 4 patches
- IRQ tracing rearchitected: instead of optimizing within
CONFIG_TRACE_IRQFLAGS, introduced independent
CONFIG_TRACE_IRQFLAGS_TOGGLE that provides irq_disable/irq_enable
tracepoints without pulling in lockdep or irqsoff infrastructure
- Made TRACE_PREEMPT_TOGGLE user-selectable in Kconfig, addressing
upstream feedback about code generation impact
- Preempt optimization now handles CONFIG_PREEMPT_TRACER alongside
CONFIG_DEBUG_PREEMPT in the three-way #if split
- Fixed __preempt_trace_enabled() macro to accept val as parameter
- Resolved circular header dependency on m68k by placing
tracepoint-defs.h include inside conditional blocks instead of
at the top of preempt.h and irqflags.h
- Moved atomic.h include from tracepoint-defs.h to tracepoint.h
to break circular dependency chain on ARM32
- Used EXPORT_TRACEPOINT_SYMBOL() instead of raw
EXPORT_SYMBOL(__tracepoint_*) for proper tracepoint registration
- Narrowed core.c compilation guard to CONFIG_DEBUG_PREEMPT ||
CONFIG_PREEMPT_TRACER since TRACE_PREEMPT_TOGGLE is now handled
inline
- Updated performance benchmarks on 7.0.0, including
tracepoint-enabled measurements and binary size impact
Changes in v2:
- Fixed build failure on arm32 (circular dependency:
atomic.h -> cmpxchg.h -> irqflags.h -> tracepoint.h -> atomic.h)
Wander Lairson Costa (4):
tracing/preemptirq: Optimize preempt_disable/enable() tracepoint
overhead
trace/preemptirq: make TRACE_PREEMPT_TOGGLE user-selectable
trace/preemptirq: add TRACE_IRQFLAGS_TOGGLE
trace/preemptirq: Implement trace_irqflags hooks
include/linux/irqflags.h | 62 +++++++++++++++++++++++++++-
include/linux/preempt.h | 49 ++++++++++++++++++++--
include/linux/tracepoint-defs.h | 1 -
include/linux/tracepoint.h | 1 +
include/trace/events/preemptirq.h | 2 +-
kernel/sched/core.c | 2 +-
kernel/trace/Kconfig | 19 +++++++--
kernel/trace/trace_preemptirq.c | 68 +++++++++++++++++++++++++++++++
8 files changed, 193 insertions(+), 11 deletions(-)
--
2.53.0
