On Thu, 2008-04-24 at 12:39 +0200, stephane eranian wrote: > Hello everyone, > > I am running into a new problem with perfmon on Itanium and 2.6.25. > > The pfmon tool is able to monitor across fork(). For that it relies on > ptrace() to receive notifications on fork. This works fine on X86 and 2.6.25 > however it is currently broken on IA-64. > > Normally, on fork(), the ptracing parent (here pfmon) receives 2 notifications: > > 1. SIGTRAP with event PTRACE_EVENT_FORK to indicate a new process > is being created. New pid is extracted via PTRACE_GETEVENTMSG > > 2. SIGSTOP with for new pid indicating that child is ready to > execute its first > instruction > > > The first message allow the tool to create the data structure to for > new process, > the second marks the point where a perfmon context can actually be attached. > > With 2.6.25 on Itanium, the notifications are received out of order, > i.e., the SIGTOP > first and the FORK notification next. Of course, the tool is confused > because until > it sees the FORK event, it does not know the new process. > > This situation never happens on X86 with the same kernel. > > To demonstrate the problem, I have attached a simple test program. You need > to pass the name of a command that creates child processes. Look at the order > between the FORK and SIGSTOP notifications. There is a forktest program in > pfmon/tests. > > I don't have time to track this down. However, I am highly suspicious of this > new TIF_RESTORE_RSE and the arch_ptrace_stop_needed() code. The do_fork() > routine does indeed set SIGSTOP, before it call ptrace_notify(). But this does > not impact X86, which, by the way, does not define arch_ptrace_stop_needed(). > I don't have an older kernel handy to run the test. Hopefully someone > on this list > will try this on 2.6.24 or older. I tried it on SLES10, which is basically a 2.6.16 with a simplified version of the patch (one which only uses arch_ptrace_stop, but not TIF_RESTORE_RSE) and it works as expected: glass:~/ptrace-wrong-notify # ./task_ptrace_attach ./forktest 10 10 creating 10 additional process(es) 10 iterations pid=6198 errno=0 exited=0 stopped=1 signaled=0 stopsig=5 FORK new_pid [6199] pid=6199 errno=0 exited=0 stopped=1 signaled=0 stopsig=19 SIGSTOP from [6199] pid=6198 errno=0 exited=0 stopped=1 signaled=0 stopsig=5 FORK new_pid [6200] pid=6200 errno=0 exited=0 stopped=1 signaled=0 stopsig=19 SIGSTOP from [6200] pid=6198 errno=0 exited=0 stopped=1 signaled=0 stopsig=5 FORK new_pid [6201] pid=6199 errno=0 exited=1 stopped=0 signaled=0 stopsig=0 EXITED [6199] pid=6200 errno=0 exited=1 stopped=0 signaled=0 stopsig=0 EXITED [6200] pid=6201 errno=0 exited=0 stopped=1 signaled=0 stopsig=19 SIGSTOP from [6201] pid=6198 errno=0 exited=0 stopped=1 signaled=0 stopsig=17 pid=6198 errno=0 exited=0 stopped=1 signaled=0 stopsig=17 pid=6201 errno=0 exited=1 stopped=0 signaled=0 stopsig=0 EXITED [6201] pid=6198 errno=0 exited=0 stopped=1 signaled=0 stopsig=17 pid=6198 errno=0 exited=0 stopped=1 signaled=0 stopsig=5 FORK new_pid [6202] pid=6202 errno=0 exited=0 stopped=1 signaled=0 stopsig=19 SIGSTOP from [6202] pid=6198 errno=0 exited=0 stopped=1 signaled=0 stopsig=5 FORK new_pid [6203] pid=6202 errno=0 exited=1 stopped=0 signaled=0 stopsig=0 EXITED [6202] pid=6203 errno=0 exited=0 stopped=1 signaled=0 stopsig=19 SIGSTOP from [6203] pid=6198 errno=0 exited=0 stopped=1 signaled=0 stopsig=17 pid=6203 errno=0 exited=1 stopped=0 signaled=0 stopsig=0 EXITED [6203] pid=6198 errno=0 exited=0 stopped=1 signaled=0 stopsig=17 pid=6198 errno=0 exited=0 stopped=1 signaled=0 stopsig=5 FORK new_pid [6204] pid=6204 errno=0 exited=0 stopped=1 signaled=0 stopsig=19 SIGSTOP from [6204] pid=6198 errno=0 exited=0 stopped=1 signaled=0 stopsig=5 FORK new_pid [6205] pid=6204 errno=0 exited=1 stopped=0 signaled=0 stopsig=0 EXITED [6204] pid=6205 errno=0 exited=0 stopped=1 signaled=0 stopsig=19 SIGSTOP from [6205] pid=6198 errno=0 exited=0 stopped=1 signaled=0 stopsig=17 pid=6198 errno=0 exited=0 stopped=1 signaled=0 stopsig=5 FORK new_pid [6206] pid=6205 errno=0 exited=1 stopped=0 signaled=0 stopsig=0 EXITED [6205] pid=6206 errno=0 exited=0 stopped=1 signaled=0 stopsig=19 SIGSTOP from [6206] pid=6198 errno=0 exited=0 stopped=1 signaled=0 stopsig=17 pid=6198 errno=0 exited=0 stopped=1 signaled=0 stopsig=5 FORK new_pid [6207] pid=6206 errno=0 exited=1 stopped=0 signaled=0 stopsig=0 EXITED [6206] pid=6207 errno=0 exited=0 stopped=1 signaled=0 stopsig=19 SIGSTOP from [6207] pid=6198 errno=0 exited=0 stopped=1 signaled=0 stopsig=17 pid=6198 errno=0 exited=0 stopped=1 signaled=0 stopsig=5 FORK new_pid [6208] pid=6207 errno=0 exited=1 stopped=0 signaled=0 stopsig=0 EXITED [6207] pid=6208 errno=0 exited=0 stopped=1 signaled=0 stopsig=19 SIGSTOP from [6208] pid=6198 errno=0 exited=0 stopped=1 signaled=0 stopsig=17 pid=6208 errno=0 exited=1 stopped=0 signaled=0 stopsig=0 EXITED [6208] pid=6198 errno=0 exited=0 stopped=1 signaled=0 stopsig=17 pid=6198 errno=0 exited=1 stopped=0 signaled=0 stopsig=0 EXITED [6198] So, if something is broken, it must be the TIF_RESTORE_RSE part of the patch, or an unexpected side effect of switching to the generic sys_ptrace. I plan to have a look at mainline later today... Kind regards, Petr Tesarik -- To unsubscribe from this list: send the line "unsubscribe linux-ia64" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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