[PATCH 0/3] Add walltime support for ring-buffer

Zhaolei
2009-07-24T10:37:50Z

Hello, Steven

It is a prototype code to make ftrace display walltime of events.

It need to applys on my patch of:
[PATCH v3 1/2] Add function to convert between calendar time and broken-down time for universal use

This code can run but need to be fixed, it only for demostrate what is result
looks like.

I think display walltime is valuable for ftrace, it can provide more
recognizable information to system manager that he can known "when"
event happened.
And it is also helpful for flight-recorder we are doing now, we can get
walltime of events after panic for analyse.

We can get following result on this patch:
# cd /mnt/debugfs/tracing/
# echo sched_switch > current_tracer
# cat trace
# tracer: sched_switch
#
# TASK-PID CPU# TIMESTAMP FUNCTION
# | | | | |
bash-2457 [000] 214.895446: 2457:120:S + [000] 2457:120:S bash
bash-2457 [000] 214.898298: 2457:120:R + [000] 5:115:S events/0
bash-2457 [000] 214.898684: 2457:120:R ==> [000] 5:115:R events/0
events/0-5 [000] 214.899458: 5:115:R + [000] 2455:120:S sshd
events/0-5 [000] 214.899495: 5:115:S ==> [000] 2455:120:R sshd
...
# echo 1 > options/walltime
# cat trace
# tracer: sched_switch
#
# TASK-PID CPU# TIMESTAMP FUNCTION
# | | | | |
<idle>-0 [000] 2009-07-25 18:01:00.848468: 0:140:R ==> [000] 5:115:R events/0
events/0-5 [000] 2009-07-25 18:01:00.848523: 5:115:S ==> [000] 0:140:R <idle>
<idle>-0 [000] 2009-07-25 18:01:01.613479: 0:140:R + [000] 2455:120:S sshd
<idle>-0 [000] 2009-07-25 18:01:01.613678: 0:140:R ==> [000] 2455:120:R sshd
sshd-2455 [000] 2009-07-25 18:01:01.614015: 2455:120:S + [000] 2455:120:S sshd
...

This patch is prototype because we have following things to do:
1: function to get walltime is not near rb_time_stamp()
So walltime is not absolutely synchronize with trace time.
But if we place code to get walltime near rb_time_stamp(), it will be called
on every reserve-event and will make system slow.
2: We can't call getnstimeofday() freely in ring-buffer's code, because
it is possibility already hold xtime_lock. Prototype code used a ugly way
to get gid of this problem, maybe we need to ftrace_disable_cpu() before
every write_seqlock() instead.
3: People maybe change walltime when tracing, but current ring-buffer page's
walltime can not reflect this change. We need to add tracepoints to watch
this change, and add a walltime-change event(already reserved in
ring-buffer.c) to record this change.

I'll continue to finish remain steps, what is your opinion on it?

Thanks
Zhaolei