Linux BPF Superpowers

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Linux
4.x
Performance

Using
BPF
Superpowers

Brendan Gregg
Senior Performance Architect
Feb

2016

Ten
years
ago,

I
gave
a
talk
here

about
DTrace
tools…

Superpowers
are
coming
to
Linux

Solve performance issues that were previously impossible
For example, ...

Ideal
Thread
States

A starting point for
deeper analysis

Linux
Thread
States

Based on:
TASK_RUNNING
TASK_INTERRUPTIBLE
TASK_UNINTERRUPTIBLE
Still a useful
starting point

Linux
On-‐CPU
Analysis

CPU
Flame
Graph

•  I'll start with on-CPU analysis:
•  Split into user/kernel ...

Broken
Stacks

Missing Java
stacks

Missing
Symbols

"[unknown]"

Java
Mixed-‐Mode
CPU
Flame
Graph

Java
JVM
Kernel
GC
•  Fixed!
–  Java –XX:+PreserveFramePointer
–  Java p...

Stack depth
Samples
(alphabetical sort)

Also,
CPI
Flame
Graph

Cycles Per Instruction
-  red == instruction
heavy
-  blue == cycle heavy
(likely mem s...

Linux
Oﬀ-‐CPU
Analysis

On Linux, the state
isn't helpful, but the
code path is
Off-CPU analysis by
measuring bl...

Oﬀ-‐CPU
Time
Flame
Graph

From
hRp://www.brendangregg.com/blog/2016-‐02-‐01/linux-‐wakeup-‐oﬀwake-‐pr...

Oﬀ-‐CPU
Time
(zoomed):
tar(1)

file read
from disk
directory read
from disk
Currently kernel stacks only; use...

Oﬀ-‐CPU
Time:
more
states

lock
contention sleep
run queue
latency
Flame graph quantifies total time spent in...

CPU
+
Oﬀ-‐CPU
==
See
Everything?

Oﬀ-‐CPU
Time
(zoomed):
gzip(1)

Off-CPU doesn't always make sense:
what is gzip blocked on?

Wakeup
Time
Flame
Graph

Wakeup
Time
(zoomed):
gzip(1)

gzip(1) is blocked on tar(1)!
tar cf - * | gzip > out.tar.gz
Can't we associate...

Oﬀ-‐Wake
Time
Flame
Graph

Wakeup stacks
are associated
and merged
in-kernel
using
BPF
We couldn't do
this before

•  One wakeup stack is often
not enough…
•  Who woke the waker?
Haven't
Solved
Everything
Yet…

Chain
Graphs

Merging multiple
wakeup stacks
in kernel using
BPF
With enough
stacks, all paths
lead to metal

Solve
Everything

CPU + off-CPU analysis can solve most issues
Flame graph (profiling) types:
1.  CPU
2.  CPI
3.  Of...

2.
BPF

"One
of
the
more
interesbng
features
in
this

cycle
is
the
ability
to
...

2.
BPF

"crazy
stuﬀ"

–
Alexei
Starovoitov
(eBPF
lead)

Source:
hRp://www.slideshare.net/Ale...

BPF

•  eBPF == enhanced Berkeley Packet Filter; now just BPF
•  Integrated into Linux (in stages: 3.15, 3.19, 4.1, 4.5...

BPF
for
Tracing

•  Can do per-event output and in-kernel summary
statistics (histograms, etc).
BPF
bytecode
...

Old
way:
TCP
Retransmits

•  tcpdump of all send & receive, dump to FS, post-process
•  Overheads adds up on 1...

New
way:
BPF
TCP
Retransmits

•  Just trace the retransmit functions
•  Negligible overhead
send

receiv...

BPF:
TCP
Retransmits

# ./tcpretrans
TIME PID IP LADDR:LPORT T> RADDR:RPORT STATE
01:55:05 0 4 10.153.223.157:22 ...

Old:
Oﬀ-‐CPU
Time
Stack
Proﬁling

•  perf_events tracing of sched events, post-process
•  Despite bufferin...

New:
BPF
Oﬀ-‐CPU
Time
Stacks

•  Measure off-CPU time, add to map with key = stack,
value = total time. As...

Stack
Trace
Hack

•  For my offcputime tool, I wrote a BPF stack walker:

"Crazy
Stuﬀ"

•  … using unrolled loops & goto:

BPF
Stack
Traces

•  Proper BPF stack support just landed in net-next:
•  Allows more than just chain graphs
Date...

memleak

•  Real-time memory growth and leak analysis:
•  Uses my stack hack, but will switch to BPF stacks soon
•  By ...

3.
bcc

•  BPF Compiler Collection
–  https://github.com/iovisor/bcc
•  Python front-end, C instrumentation
•  Curre...

execsnoop

•  Trace new processes:
# ./execsnoop
PCOMM PID RET ARGS
bash 15887 0 /usr/bin/man ls
preconv 15894 0 /usr/b...

biolatency

•  Block device (disk) I/O latency distribution:
# ./biolatency -mT 1 5
Tracing block device I/O... Hit Ctr...

ext4slower

•  ext4 file system I/O, slower than a threshold:
# ./ext4slower 1
Tracing ext4 operations slower than 1 ms...

bashreadline

•  Trace bash interactive commands system-wide:
# ./bashreadline
TIME PID COMMAND
05:28:25 21176 ls -l
05...

gethostlatency

•  Show latency for getaddrinfo/gethostbyname[2] calls:
# ./gethostlatency
TIME PID COMM LATms HOST
06:...

trace

•  Trace custom events. Ad hoc analysis multitool:
# trace 'sys_read (arg3 > 20000) "read %d bytes", arg3'
TIME ...

Linux
bcc/BPF
Tracing
Tools

4.
Future
Work

•  All event sources
•  Language improvements
•  More tools: eg, TCP
•  GUI support

Linux
Event
Sources

done

XXX:
todo
XXX:
todo

XXX:
todo

done

BPF/bcc
Language
Improvements

More
Tools

•  eg, netstat(8)…
$ netstat -s
Ip:
7962754 total packets received
8 with invalid addresses
0 forwarded
...

BeRer
TCP
Tools

•  TCP retransmit by type and time
•  Congestion algorithm metrics
•  etc.

GUI
Support

•  eg, Netflix Vector: open source instance analyzer:

Summary

•  BPF in Linux 4.x makes many new things possible
–  Stack-based thread state analysis (solve all issues!)
– ...

Links

•  iovisor bcc:
•  https://github.com/iovisor/bcc
•  http://www.brendangregg.com/blog/2015-09-22/bcc-linux-4.3-t...

Feb

2016

•  Questions?
•  http://slideshare.net/brendangregg
•  http://www.brendangregg.com
•  bgregg@netflix.com
...

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