Using systemtap to look at futexs

[William Cohen <wcohen@redhat.com>]

Monday I went to lunch with Gavin Romig-Koch. Gavin works in Red Hat 
support. While we were talking he mentioned trying to solve a customer 
problem that appeared to be caused by futex. An attempt was made to use 
GDB to track futex operations from userspace. However, that really 
slowed down the machine and never got the to reported problem. Gavin 
thought that SystemTap might be useful in diagnosing this problem. This 
morning I wrote a quick script to print out futex information while the 
system is running. I don't know whether this would identify the specific 
problem Gavin mentioned.

-Will

# futex.stp
#
# track which things are using futex

probe syscall.futex
{
	printf("pid=%d exec=%s sys_futex(%s)\n", pid(), execname(), argstr)
}

probe syscall.futex.return
{
	printf("pid=%d exec=%s sys_futex()=%s\n", pid(), execname(), retstr)
}