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Re: long I/O delays when strace is running





On 04/21/2017 05:12 PM, Mark Geisert wrote:

Re debugging strace itself, you may not realize that strace is not a Cygwin-native program. It's a Windows-native program. So debugging it with Cygwin gdb is problematic.

I can tell you roughly how strace operates. It launches the target program with CreateProcess(), not with exec(). It acts as the debugger of the target in order to receive Windows DEBUG_EVENTs for things like DLL loads, thread creation, etc. etc. The Cygwin DLL in the target can tell when it's being run under strace and generates trace lines with OutputDebugString() calls that strace receives as DEBUG_EVENTs. strace then just outputs the trace line.

strace does have to explicitly ContinueDebugEvent() on each DEBUG_EVENT it receives, and if there's some issue with that then the target program will remain suspended awaiting that continue. I've never seen an issue with this but it's another possible place to look, I suppose.

Why strace appears to interact badly with /proc accesses, I have no idea.

..mark

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.

Anyway, I can see that the strace process's shared _pinfo object is never fully populated:

_pinfo 0x30000 {
  pid 2800,
  process_state 0x00000001,
  ppid 0,
  exitcode 0
  cygstarted 0,
  dwProcessId 0x00000AF0,
  progname "D:\cygwin64\bin\strace.exe",
  uid 0,
  gid 0,
  pgid 0,
  sid 0,
  ctty 0,
  has_pgid_children 0
  start_time 1492881370,
  nice 0,
  stopsig 0,
  sendsig 0x0,
  exec_sendsig 0x0,
  exec_dwProcessId 0
}

So I would venture to say that is a problem. Also, pinfo::init() should probably issure some error message if it waits 2-ish seconds and the struct still isn't correctly populated.

Is there a way to debug the children of strace? It would make it a lot easier. That's part of why I wrote the _pinfo::debug(), but also when I debug strace with gdb, the _pinfo struct IS properly populated. The best problems are the ones that disappear when you try to debug them.

Thanks,
Daniel



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