bcc-wakeuptime - Man Page

wakeuptime [-h] [-u] [-p PID] [-v] [-f] [--stack-storage-size STACK_STORAGE_SIZE] [-m MIN_BLOCK_TIME] [-M MAX_BLOCK_TIME] [duration]

This program shows the kernel stack traces for threads that woke up other  blocked threads, along with the process names of the waker and target, along with a sum of the time that the target was blocked: the "blocked time". It works by tracing when threads block and when they were then woken up, and measuring the time delta. This time measurement will be very similar to off-CPU time, however, off-CPU time may include a little extra time spent waiting on a run queue to be scheduled. The stack traces, process names, and time spent blocked is summarized in the kernel using an eBPF map for efficiency.

The output summary will help you identify reasons why threads were blocking by showing who woke them up, along with the time they were blocked. This spans all types of blocking activity: disk I/O, network I/O, locks, page faults, involuntary context switches, etc.

This can be used in conjunction with offcputime, which shows the stack trace of the blocked thread. wakeuptime shows the stack trace of the waker thread.

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.

CONFIG_BPF and bcc.

-h

Print usage message.

-f

Print output in folded stack format.

-u

Only trace user threads (not kernel threads).

-v

Show raw addresses (for non-folded format).

-p PID

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

--stack-storage-size STACK_STORAGE_SIZE

Change the number of unique stack traces that can be stored and displayed.

duration

Duration to trace, in seconds.

-m MIN_BLOCK_TIME

The amount of time in microseconds over which we store traces (default 1)

-M MAX_BLOCK_TIME

The amount of time in microseconds under which we store traces (default U64_MAX)

Trace all thread blocking events, and summarize (in-kernel) by kernel stack trace and total blocked time:

# wakeuptime

Trace user-mode target threads only:

# wakeuptime -u

Trace for 5 seconds only:

# wakeuptime 5

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

# wakeuptime -f 5

Trace PID 185 only:

# wakeuptime -p 185

This summarizes unique stack traces in-kernel for efficiency, allowing it to trace a higher rate of events than methods 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 min block option. If your aim is to chase down longer blocking events, then this could be increased to filter shorter blocking events, further lowering overhead.

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.

Linux

Unstable - in development.

Brendan Gregg

offcputime(8), stackcount(8)