lat_bench — Kernel Latency Measurement

A minimal library for measuring latency of Linux kernel code paths via /proc/lat_bench/. Includes optional per-probe histograms with fine-grained linear-within-log2 bucketing.

Integration

Copy this directory into your kernel tree and add to the parent Kbuild:

obj-y += lat_bench/

The module is initialized via core_initcall, so it is available early in boot.

Instrumenting code

#include "../lat_bench/lat_bench.h"

DEFINE_LAT_BENCH(lb_foo, "foo");   /* creates /proc/lat_bench/foo */

void my_func(void)
{
    ktime_t start = lat_bench_start();
    /* ... code to measure ... */
    lat_bench_end(&lb_foo, start);
}

static int __init my_init(void)
{
    return lat_bench_register(&lb_foo);
}

Call lat_bench_unregister(&lb_foo) in the exit path for modules.

Measurement points use per-CPU counters, so recording is lock-free on the hot path.

Runtime usage

# Enable measurement (clears all counters first)
echo 1 | sudo tee /proc/lat_bench/enable

# Run your workload, then read results
cat /proc/lat_bench/foo

# Disable and clear all counters
echo 0 | sudo tee /proc/lat_bench/enable

Histogram

An optional latency histogram can be enabled independently:

# Enable histogram collection (clears previous histogram data)
echo 1 | sudo tee /proc/lat_bench/hist_enable

# Disable histogram (data is preserved for reading)
echo 0 | sudo tee /proc/lat_bench/hist_enable

The histogram uses 4 sub-buckets per power-of-2 (linear-within-log2), covering 1 ns to ~16 ms (2^24 ns) in 97 buckets. Bucket layout:

Buckets	Range
0-3	[1, 2) ns
4-7	[2, 4) ns
8-11	[4, 8) ns
...	...
92-95	[2^23, 2^24) ns
96	>= 2^24 ns (overflow)

Output format

count: 142381            # total calls across all CPUs
total_ns: 568743200      # cumulative nanoseconds
avg_ns: 3995             # mean latency per call
hist_shift: 2            # log2(sub-buckets per power-of-2)
hist_buckets: 97         # total number of histogram buckets
hist: 0 0 12 53 ...      # space-separated bucket counts

Files

File	Description
lat_bench.h	Public API: macros, inline start/end functions, histogram bucket mapping
lat_bench.c	procfs setup, registration, enable/hist_enable controls
Makefile	Kbuild makefile

License

GPL-2.0