Re: systemtap application to find applications doing polling

[Jim Keniston <jkenisto at us dot ibm dot com>]

On Wed, 2009-01-28 at 11:52 -0500, William Cohen wrote:
> Hi All,
> 
> Uli Drepper mentions in a blog entry need "avoid unnecessary wakeups" and that a
> systemtap script to monitor this would be useful:
...
> 
> I plan to check this into systemtap.examples directory in next day or so. Just
> looking to see if people have additional suggestions.
> 
> -Will
> # Copyright (C) 2009 Red Hat, Inc.
> # Written by Ulrich Drepper <drepper@redhat.com>
> # Modified by William Cohen <wcohen@redhat.com>
> 
> global process, timeout_count, to
> global poll_timeout, epoll_timeout, select_timeout, itimer_timeout
> global nanosleep_timeout, futex_timeout, signal_timeout
> 
> probe syscall.poll, syscall.epoll_wait {
>   if (timeout) to[pid()]=timeout
> }
> 
> probe syscall.poll.return {
>   p = pid()
>   if ($return == 0 && to[p] > 0 ) {
>     poll_timeout[p]++
>     timeout_count[p]++
>     process[p] = execname()
>     delete to[p]
>   }
> }

I'm wondering if this script could be shorter (though perhaps less
efficient) if you used a function like this:

global timeouts
function tally_timeout(name:string) {
	p = pid()
	timeouts[name, p]++
	timeout_count[p]++
	process[p] = execname()
}
> 
> probe syscall.epoll_wait.return {
>   if ($return == 0 && to[p] > 0 ) {

Here p is used before it is set.

>     p = pid()
>     epoll_timeout[p]++
>     timeout_count[p]++
>     process[p] = execname()
>     delete to[p]
>   }
> }

The above could turn into

	p = pid()
	if ($return == 0 && to[p] > 0) {
		tally_timeout(name)
		delete to[p]
	}
> 
> probe syscall.select.return {
>   if ($return == 0) {
>     p = pid()
>     select_timeout[p]++
>     timeout_count[p]++
>     process[p] = execname()
>   }
> }
> 
> probe syscall.futex.return {
>   if (errno_str($return) == "ETIMEDOUT") {
>     p = pid()
>     futex_timeout[p]++
>     timeout_count[p]++
>     process[p] = execname()
>   }
> }
> 
> probe syscall.nanosleep.return {
>   if ($return == 0) {
>     p = pid()
>     nanosleep_timeout[p]++
>     timeout_count[p]++
>     process[p] = execname()
>   }
> }
> 
> probe kernel.function("it_real_fn") {
>   p = pid()
>   itimer_timeout[p]++
>   timeout_count[p]++
>   process[p] = execname()
> }
> 
> probe syscall.rt_sigtimedwait.return {
>   if (errno_str($return) == "EAGAIN") {
>     p = pid()
>     signal_timeout[p]++
>     timeout_count[p]++
>     process[p] = execname()
>   }
> }
> 
> probe syscall.exit {
>   p = pid()
>   if (p in process) {
>     delete process[p]
>     delete timeout_count[p]
>     delete poll_timeout[p]
>     delete epoll_timeout[p]
>     delete select_timeout[p]
>     delete itimer_timeout[p]
>     delete futex_timeout[p]
>     delete nanosleep_timeout[p]
>     delete signal_timeout[p]
>   }
> }
> 
> probe timer.s(1) {
>   printf("\033[2J\033[1;1H") /* clear screen */
>   printf ("  uid |   poll  select   epoll  itimer   futex nanosle  signal| process\n")
>   foreach (p in timeout_count- limit 20) {
>      printf ("%5d |%7d %7d %7d %7d %7d %7d %7d| %-.38s\n", p,
>               poll_timeout[p], select_timeout[p],
>               epoll_timeout[p], itimer_timeout[p],
>               futex_timeout[p], nanosleep_timeout[p],
>               signal_timeout[p], process[p])
>   }
> }

Jim