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Re: long I/O delays when strace is running




Well, waiting for GNU/Linux tests to run, so I had a little more time to play with this.

On 04/23/2017 02:42 AM, Mark Geisert wrote:
Daniel Santos wrote:
Well thank you, I wish I had read this earlier. I've been trying to debug (with gdb) strace (following children) and now I know why the debugger is blowing through breakpoints, because I may be debugging strace, but strace debugging it's own child. I added a _pinfo::dump () member function that I'm calling from pinfo::init () after the test "if (!created && !(flag & PID_NEW))" and it's crashing because for some processes, the entire _pinfo struct isn't mapped in, just the first page. Of course, I'm a newbie in this sphere, so maybe I've missed something and there's sometimes a reason for this? (The first part of
the struct looks normal.) Otherwise, that would be another odd flaw.

I don't know enough about pinfo.h and pinfo.cc to say anything definite on this.

Well here's one mystery solved. I modified my dump() function to first dump the first page of the struct and then the second, so that when it crashes I can see the contents of the first page at least:

_pinfo 0x30000 {
  pid 2044,
  process_state 0x00004001,
  ppid 2044,
  exitcode 0
  cygstarted 0,
  dwProcessId 0x00000000,
  progname "",

The process_state's bit 0x00004000 happens to be PID_EXECED, so this is intentionally shorter because there's no more information.

As for the strange _pinfo object that causes the delay, this appears to be an error in the process database because the windows PID is a duplicate of another process entry. (I modified ps.exe to output the _pinfo::process_state field.)

$ strace --trace-children --output=/tmp/ps.log --mask=all ps -el
PID PPID PGID WINPID TTY UID STIME STATE COMMAND 948 1788 948 1772 pty0 197608 00:04:22 000000c1 /usr/bin/bash 2720 1 2720 2720 ? 18 00:02:42 00000041 /usr/bin/cygrunsrv 860 0 0 860 pty0 0 00:04:30 00000001 /usr/bin/cygrunsrv 984 1 2168 984 pty0 197608 00:04:30 00000041 /usr/bin/ps 2376 1628 2376 2612 ? 197610 00:02:45 000000c1 /usr/sbin/sshd 2524 2720 2524 1404 ? 18 00:02:42 000000c1 /usr/sbin/cygserver 2168 948 2168 860 pty0 197608 00:04:30 00000061 /usr/bin/strace 1788 2376 1788 2788 ? 197610 00:04:22 000000c1 /usr/sbin/sshd 1628 1 1628 1628 ? 197610 00:02:45 00000041 /usr/bin/cygrunsrv

The entry that causes the delay is the one with a zero PPID and PGID and the progname field is wrong -- It isn't "cygrunsrv". It always has the same progname name of the previous process. Notice that the WINPID field has the same value as the PID of the strace process that's real (2168).

So my guess is that this object shouldn't even be there.

Daniel

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