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Subject[PATCH 0/5] [RFC] binary reading of ftrace ring buffers

RFC only, not for pulling, unless everyone is fine with these :-)

After telling the folks from Google that I had splice code that I needed
to get working, I finally got around to doing it.

Not only did I get the splice code working, but I also made a perl
script (might want to cover you eyes if you look at that code) that
can automagically create the output from the binary files reading
the format arguments in the /debugfs/tracing/events/<subsys>/<event>/format

From previous patches, we have in include/trace/sched_event_types.h:

#define TRACE_SYSTEM sched

TPPROTO(struct rq *rq, struct task_struct *prev,
struct task_struct *next),
TPARGS(rq, prev, next),
TPFMT("task %s:%d ==> %s:%d",
prev->comm, prev->pid, next->comm, next->pid),
TRACE_FIELD(pid_t, prev_pid, prev->pid)
TRACE_FIELD(int, prev_prio, prev->prio)
TRACE_FIELD(pid_t, next_pid, next->pid)
TRACE_FIELD(int, next_prio, next->prio)
TPRAWFMT("prev %d:%d ==> next %s:%d:%d")

By making that, we automagically get this file:

# cat /debug/tracing/events/sched/sched_switch/format
name: sched_switch
ID: 29
field:unsigned char type; offset:0; size:1;
field:unsigned char flags; offset:1; size:1;
field:unsigned char preempt_count; offset:2; size:1;
field:int pid; offset:4; size:4;
field:int tgid; offset:8; size:4;

field:pid_t prev_pid; offset:12; size:4;
field:int prev_prio; offset:16; size:4;
field special:char next_comm[TASK_COMM_LEN]; offset:20; size:16;
field:pid_t next_pid; offset:36; size:4;
field:int next_prio; offset:40; size:4;

print fmt: "prev %d:%d ==> next %s:%d:%d"

Now with this patch set, we create a way to read the ftrace ring buffers
directly, as a binary page. Splice has been used such that the user
could splice the ring buffers without need to copy the pages.
The pages are taken from the ring buffers and can be placed directly
into files, without extra copies.

# ls /debug/tracing/binary_buffers/
0 1 2 3 4 5 6 7

One can either just use the read/write to grab live data from these
buffers, or they could use splice. I have a simple file that reads
this buffers using splice. Note, it only runs on one file, you
can make multiple copies to run more. The ring buffers in ftrace
are per cpu and they are not dependent on each other.

Also, if there is no data in the buffer, it returns -EAGAIN.

# find /debug/tracing/events -name 'type' | while read f; do
> echo raw > $f; done

# find /debug/tracing/events -name 'enable' | while read f; do
> echo 1 > $f; done

# ./splice /debug/tracing/binary_buffers/0 /tmp/buf-0

Yes you can run multiple instances of this on different buffers.

# ./ /tmp/buf-0


0 [000] 7071.936459: (irq_handler_entry) irq 48
0 [000] 7071.936462: (irq_handler_exit) irq 48 ret 1
0 [000] 7071.988801: (sched_signal_send) sig: 14 task 0
0 [000] 7071.988813: (sched_wakeup) task 0 success=1
0 [000] 7071.988823: (sched_switch) prev 0:140 ==> next ntpd:6582272:0
3303 [000] 7071.988916: (sched_switch) prev 0:120 ==> next swap:7497072:0
0 [000] 7072.020370: (sched_wakeup) task 0 success=1
0 [000] 7072.020407: (sched_switch) prev 0:140 ==> next even:808416116:0
16 [000] 7071.936157: (sched_switch) prev 0:115 ==> next swap:7497072:0

Notice the "(sched_switch)" lines.

The splice C code:

The perl script to parse:

And yes I know, they are both ugly :-p

The following patches are in:


branch: rfc/splice/tip/tracing/ftrace

Steven Rostedt (5):
ring-buffer: reset write field for ring_buffer_read_page
ring-buffer: fix ring_buffer_read_page
ring-buffer: replace sizeof of event header with offsetof
ring-buffer: make ring_buffer_read_page read from start on partial page
tracing: add binary buffer files for use with splice

include/linux/ring_buffer.h | 7 +-
kernel/trace/ring_buffer.c | 118 +++++++++++++------
kernel/trace/trace.c | 274 +++++++++++++++++++++++++++++++++++++++++-
kernel/trace/trace.h | 1 +
4 files changed, 357 insertions(+), 43 deletions(-)

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