Re: SIGPROF + SIGCHLD and igc

[Pip Cet]

"Helmut Eller" <eller.helmut@gmail.com> writes:

> On Tue, Dec 24 2024, Gerd Möllmann wrote:
>
>>> I've looked at SIGPROF. From an admittedly brief look at this, I'd
>>> summarize my results as:
>>>
>>> - The important part is get_backtrace. The rest could be done elsewhere
>>>   by posting that to a message board, or whatever the mechanism is at
>>>   the end.
>
> I have an idea how to make a safer profiler.  First, remember that MPS

"Safer" how?

> will stop mutator threads to protect its own consistency.  What happens
> if we make the profiler its own thread?  MPS will stop the profiler like
> normal mutator threads.  This is useful and is as it should be.

If we tell MPS about it, it'll be stopped, yes.

> The problem now is how the profiler thread can do what get_backtrace
> does.  We can use a protocol between the profiler thread and the main
> thread that goes like this:
>
> 1. The profiler periodically sends a signal, say SIGPROF, to the main
>    thread.
>
> 2. In the signal handler for SIGPROF, the main thread synchronizes
>    itself with the profiler by communicating over a pipe (like the Unix
>    fathers did it).  It sends some token to the profiler and waits.

...forever, if the profiler thread has been suspended at this point.

> 3. The profiler receives the token and can now access the virtual
>    machine state because the main thread waits.  The profiler now does
>    what get_backtrace does.  After logging the stack snapshot, the
>    profiler sends the token back to the main thread
>
> 4. The main thread receives the token, returns from the signal handler,
>    and continues execution.
>
> Note that the signal handler only communicates over the pipe and doesn't
> read or write any memory that could mess up MPS.  The profiler thread
> doesn't run any signal handlers (other than those that MPS may use
> behind the scenes).

Can you explain how this would produce different behavior than what we
have currently, apart from the deadlock?  I'm sorry, but it's not
obvious to me.

If the main thread was performing GC when SIGPROF arrived, the SIGPROF
handler cannot do get_backtrace, and the profiler thread can't, either.

> Another observation: MPS sends SIGXFSZ and SIGXCPU to stop and resume
> threads.  We could intercept those signals and block SIGPROF while the
> thread is stopped.  Obviously a hack, but could still be useful.

MPS blocks all signals (except for the wakeup signal) while threads are
suspended.  This is suboptimal behavior and not something we should
expect and rely on.

Pip