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Message-ID: <alpine.DEB.1.10.0811261151070.26743@gandalf.stny.rr.com>
Date: Wed, 26 Nov 2008 11:54:54 -0500 (EST)
From: Steven Rostedt <rostedt@...dmis.org>
To: "Eric W. Biederman" <ebiederm@...ssion.com>
cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
Andrew Morton <akpm@...ux-foundation.org>,
Frederic Weisbecker <fweisbec@...il.com>,
containers@...ts.osdl.org,
Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
"Serge E. Hallyn" <serue@...ibm.com>,
Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 1/5] ftrace: add function tracing to single thread
On Tue, 25 Nov 2008, Eric W. Biederman wrote:
> Steven Rostedt <rostedt@...dmis.org> writes:
>
> > From: Steven Rostedt <srostedt@...hat.com>
> >
> > Impact: feature to function trace a single thread
> >
> > This patch adds the ability to function trace a single thread.
> > The file:
> >
> > /debugfs/tracing/set_ftrace_pid
> >
> > contains the pid to trace. Valid pids are any positive integer.
>
> Please look below. Using find_get_pid causes this to use the
> same logic as the rest of the kernel on what a valid pid is.
> i.e. 0 is not a valid pid.
0 is not? What happens if I want to trace what the swapper task is doing?
I want the ability to trace kernel threads as well.
>
> I believe I have sketched in what is needed to use the struct
> pid api properly. Certainly enough for you to get the jist
> of the idea.
>
> > Writing any negative number to this file will disable the pid
> > tracing and the function tracer will go back to tracing all of
> > threads.
> >
> > This feature works with both static and dynamic function tracing.
> >
> > Signed-off-by: Steven Rostedt <srostedt@...hat.com>
> > ---
> > Documentation/ftrace.txt | 79 +++++++++++++++++
> > kernel/trace/ftrace.c | 209 ++++++++++++++++++++++++++++++++++++++++------
> > 2 files changed, 262 insertions(+), 26 deletions(-)
> >
> > diff --git a/Documentation/ftrace.txt b/Documentation/ftrace.txt
> > index 35a78bc..de05042 100644
> > --- a/Documentation/ftrace.txt
> > +++ b/Documentation/ftrace.txt
> > @@ -127,6 +127,8 @@ of ftrace. Here is a list of some of the key files:
> > be traced. If a function exists in both set_ftrace_filter
> > and set_ftrace_notrace, the function will _not_ be traced.
> >
> > + set_ftrace_pid: Have the function tracer only trace a single thread.
> > +
> > available_filter_functions: This lists the functions that ftrace
> > has processed and can trace. These are the function
> > names that you can pass to "set_ftrace_filter" or
> > @@ -1073,6 +1075,83 @@ For simple one time traces, the above is sufficent. For
> > anything else,
> > a search through /proc/mounts may be needed to find where the debugfs
> > file-system is mounted.
> >
> > +
> > +Single thread tracing
> > +---------------------
> > +
> > +By writing into /debug/tracing/set_ftrace_pid you can trace a
> > +single thread. For example:
> > +
> > +# cat /debug/tracing/set_ftrace_pid
> > +no pid
> > +# echo 3111 > /debug/tracing/set_ftrace_pid
> > +# cat /debug/tracing/set_ftrace_pid
> > +3111
> > +# echo function > /debug/tracing/current_tracer
> > +# cat /debug/tracing/trace | head
> > + # tracer: function
> > + #
> > + # TASK-PID CPU# TIMESTAMP FUNCTION
> > + # | | | | |
> > + yum-updatesd-3111 [003] 1637.254676: finish_task_switch <-thread_return
> > + yum-updatesd-3111 [003] 1637.254681: hrtimer_cancel <-schedule_hrtimeout_range
> > + yum-updatesd-3111 [003] 1637.254682: hrtimer_try_to_cancel <-hrtimer_cancel
> > + yum-updatesd-3111 [003] 1637.254683: lock_hrtimer_base <-hrtimer_try_to_cancel
> > + yum-updatesd-3111 [003] 1637.254685: fget_light <-do_sys_poll
> > + yum-updatesd-3111 [003] 1637.254686: pipe_poll <-do_sys_poll
> > +# echo -1 > /debug/tracing/set_ftrace_pid
> > +# cat /debug/tracing/trace |head
> > + # tracer: function
> > + #
> > + # TASK-PID CPU# TIMESTAMP FUNCTION
> > + # | | | | |
> > + ##### CPU 3 buffer started ####
> > + yum-updatesd-3111 [003] 1701.957688: free_poll_entry <-poll_freewait
> > + yum-updatesd-3111 [003] 1701.957689: remove_wait_queue <-free_poll_entry
> > + yum-updatesd-3111 [003] 1701.957691: fput <-free_poll_entry
> > + yum-updatesd-3111 [003] 1701.957692: audit_syscall_exit <-sysret_audit
> > + yum-updatesd-3111 [003] 1701.957693: path_put <-audit_syscall_exit
> > +
> > +If you want to trace a function when executing, you could use
> > +something like this simple program:
> > +
> > +#include <stdio.h>
> > +#include <stdlib.h>
> > +#include <sys/types.h>
> > +#include <sys/stat.h>
> > +#include <fcntl.h>
> > +#include <unistd.h>
> > +
> > +int main (int argc, char **argv)
> > +{
> > + if (argc < 1)
> > + exit(-1);
> > +
> > + if (fork() > 0) {
> > + int fd, ffd;
> > + char line[64];
> > + int s;
> > +
> > + ffd = open("/debug/tracing/current_tracer", O_WRONLY);
> > + if (ffd < 0)
> > + exit(-1);
> > + write(ffd, "nop", 3);
> > +
> > + fd = open("/debug/tracing/set_ftrace_pid", O_WRONLY);
> > + s = sprintf(line, "%d\n", getpid());
> > + write(fd, line, s);
> > +
> > + write(ffd, "function", 8);
> > +
> > + close(fd);
> > + close(ffd);
> > +
> > + execvp(argv[1], argv+1);
> > + }
> > +
> > + return 0;
> > +}
> > +
> > dynamic ftrace
> > --------------
> >
> > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> > index 7e2d3b9..00d98c6 100644
> > --- a/kernel/trace/ftrace.c
> > +++ b/kernel/trace/ftrace.c
> > @@ -47,6 +47,9 @@
> > int ftrace_enabled __read_mostly;
> > static int last_ftrace_enabled;
> >
> > +/* ftrace_pid_trace >= 0 will only trace threads with this pid */
> > +static int ftrace_pid_trace = -1;
>
> Why not?
> static struct pid *ftrace_pid_trace = NULL;
> > +
> > /* Quick disabling of function tracer. */
> > int function_trace_stop;
> >
> > @@ -61,6 +64,7 @@ static int ftrace_disabled __read_mostly;
> >
> > static DEFINE_SPINLOCK(ftrace_lock);
> > static DEFINE_MUTEX(ftrace_sysctl_lock);
> > +static DEFINE_MUTEX(ftrace_start_lock);
> >
> > static struct ftrace_ops ftrace_list_end __read_mostly =
> > {
> > @@ -70,6 +74,7 @@ static struct ftrace_ops ftrace_list_end __read_mostly =
> > static struct ftrace_ops *ftrace_list __read_mostly = &ftrace_list_end;
> > ftrace_func_t ftrace_trace_function __read_mostly = ftrace_stub;
> > ftrace_func_t __ftrace_trace_function __read_mostly = ftrace_stub;
> > +ftrace_func_t ftrace_pid_function __read_mostly = ftrace_stub;
> >
> > static void ftrace_list_func(unsigned long ip, unsigned long parent_ip)
> > {
> > @@ -86,6 +91,21 @@ static void ftrace_list_func(unsigned long ip, unsigned long
> > parent_ip)
> > };
> > }
> >
> > +static void ftrace_pid_func(unsigned long ip, unsigned long parent_ip)
> > +{
> > + if (current->pid != ftrace_pid_trace)
> > + return;
>
> And why not?
> if (task_pid(current) != ftrace_pid_trace)
> return;
>
> Racy or not. This is only comparing a different field in struct task_struct.
> So there should be not real difference.
There's no race here. Do kernel threads have their own unique "task_pid"s?
I understand where you are going with this, and I have no issue with it.
As long as I can also pick kernel threads to trace including the idle
thread.
Thanks,
-- Steve
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