Manual: offcputime-bpfcc

offcputime(8) System Manager's Manual offcputime(8)

NAME
offcputime - Summarize off-CPU time by kernel stack trace. Uses Linux eBPF/bcc.

SYNOPSIS
offcputime [-h] [-p PID | -t TID | -u | -k] [-U | -K] [-d] [-f] [--stack-storage-size STACK_STORAGE_SIZE] [-m
MIN_BLOCK_TIME] [-M MAX_BLOCK_TIME] [--state STATE] [duration]

DESCRIPTION
This program shows stack traces and task names that were blocked and "off-CPU", and the total duration they were not
running: their "off-CPU time". It works by tracing when threads block and when they return to CPU, measuring both
the time they were off-CPU and the blocked stack trace and the task name. This data is summarized in the kernel us‐
ing an eBPF map, and by summing the off-CPU time by unique stack trace and task name.

The output summary will help you identify reasons why threads were blocking, and quantify the time they were off-CPU.
This spans all types of blocking activity: disk I/O, network I/O, locks, page faults, involuntary context switches,
etc.

This is complementary to CPU profiling (e.g., CPU flame graphs) which shows the time spent on-CPU. This shows the
time spent off-CPU, and the output, especially the -f format, can be used to generate an "off-CPU time flame graph".

See http://www.brendangregg.com/FlameGraphs/offcpuflamegraphs.html

This tool only works on Linux 4.6+. It uses the new `BPF_STACK_TRACE` table APIs to generate the in-kernel stack
traces. For kernels older than 4.6, see the version under tools/old.

Note: this tool only traces off-CPU times that began and ended while tracing.

REQUIREMENTS
CONFIG_BPF and bcc.

OPTIONS
-h Print usage message.

-p PID Trace this process ID only (filtered in-kernel).

-t TID Trace this thread ID only (filtered in-kernel).

-u Only trace user threads (no kernel threads).

-k Only trace kernel threads (no user threads).

-U Show stacks from user space only (no kernel space stacks).

-K Show stacks from kernel space only (no user space stacks).

-d Insert delimiter between kernel/user stacks.

-f Print output in folded stack format.

--stack-storage-size STACK_STORAGE_SIZE
Change the number of unique stack traces that can be stored and displayed.

-m MIN_BLOCK_TIME
The minimum time in microseconds over which we store traces (default 1)

-M MAX_BLOCK_TIME
The maximum time in microseconds under which we store traces (default U64_MAX)

--state
Filter on this thread state bitmask (eg, 2 == TASK_UNINTERRUPTIBLE). See include/linux/sched.h for states.

duration
Duration to trace, in seconds.

EXAMPLES
Trace all thread blocking events, and summarize (in-kernel) by kernel stack trace and total off-CPU time:
# offcputime

Trace for 5 seconds only:
# offcputime 5

Trace for 5 seconds, and emit output in folded stack format (suitable for flame graphs):
# offcputime -f 5

Trace PID 185 only:
# offcputime -p 185

OVERHEAD
This summarizes unique stack traces in-kernel for efficiency, allowing it to trace a higher rate of events than meth‐
ods that post-process in user space. The stack trace and time data is only copied to user space once, when the output
is printed. While these techniques greatly lower overhead, scheduler events are still a high frequency event, as they
can exceed 1 million events per second, and so caution should still be used. Test before production use.

If the overhead is still a problem, take a look at the MINBLOCK_US tunable in the code. If your aim is to chase down
longer blocking events, then this could be increased to filter shorter blocking events, further lowering overhead.

SOURCE
This is from bcc.

https://github.com/iovisor/bcc

Also look in the bcc distribution for a companion _examples.txt file containing example usage, output, and commentary
for this tool.

OS
Linux

STABILITY
Unstable - in development.

AUTHOR
Brendan Gregg

Manual: offcputime-bpfcc

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