[PATCH 17/17] infrun: scheduler-locking reverse

[Metzger, Markus T]

> -----Original Message-----
> From: gdb-patches-owner@sourceware.org [mailto:gdb-patches-
> owner@sourceware.org] On Behalf Of Jan Kratochvil
> Sent: Tuesday, September 15, 2015 7:20 PM

Hello Jan,

> On Tue, 15 Sep 2015 11:28:36 +0200, Metzger, Markus T wrote:
> > > Thanks for bringing it up.  I would have hard time mergin this patch's:
> > > -static const char *scheduler_mode = schedlock_off;
> > > +static const char *scheduler_mode = schedlock_reverse;
> > > with
> > > http://pkgs.fedoraproject.org/cgit/gdb.git/tree/gdb-6.6-
> scheduler_locking-
> > > step-is-default.patch
> > > -static const char *scheduler_mode = schedlock_off;
> > > +static const char *scheduler_mode = schedlock_step;
> >
> > I don't believe that we need to preserve the current behaviour.  If Fedora
> > defaults to schedlock_step, why not also for record targets?
> 
> Yes, I would find logical that during "reverse-step" on Fedora the scheduler
> locking is also turned on.  But I do not see how to achieve it with your
> patch.  If I put there:
> 	static const char *scheduler_mode = schedlock_step;
> then during "reverse-*" commands there will be no scheduler locking.

That's a bug.

Record btrace still has some code that implements an implicit scheduler-
locking on during reverse/replay execution for all-stop targets.  With Pedro's
ASNS patch, scheduler-locking should be honoured.

We could also make record btrace honour scheduler-locking for (soon legacy) all-
stop targets.  The current implementation applies the same stepping command
to all threads matching the argument PTID:

  ALL_NON_EXITED_THREADS (tp)
    if (ptid_match (tp->ptid, ptid))
      record_btrace_resume_thread (tp, flag);

I think what we want instead is to apply the requested stepping command to
INFERIOR_PTID and the corresponding continue command to all other threads
matching PTID.  Is that correct?

We wouldn't even need to check for non-stop mode or distinguish between
non-stop and all-stop targets.  Infrun should take care of all that.

What if INFERIOR_PTID does not match the argument PTID?  Is that an internal
error?  Or would we only resume the argument PTID in this case?  What if it is
a process and not a single thread?

Record full doesn't really support multi-threading so scheduler-locking doesn't
apply.


> > The same holds for mainline, of course.  But the new mode is much closer
> to
> > schedlock_step than it is to schedlock_off.
> 
> Maybe.  But when I make the default "schedlock_reverse" then it will
> regress
> Fedora's forward "step".

Mainline will default to schedlock_reverse to maintain the current behaviour.

Fedora will continue to default to schedlock_step and record btrace will now
behave correctly on (reverse-)continue commands (the current series doesn't
since the underlying target is still all-stop).  It should already do the right thing
for (reverse-)step/next/finish commands.


> > I really only added the new mode because I did not want to change the
> default
> > behaviour completely.  Over time, I think we may want to deprecate it,
> even
> > though schedlock_off isn't very useful with the usually quite short
> execution
> > history.  Maybe mainline will default to schedlock_step one day, as well?
> 
> But how to make the schedlock_step default while still benefiting from your
> patch?

The goal is to have record btrace honour scheduler-locking.  This patch is trying
to do this without changing the current default behaviour by making the current
behaviour a new scheduler-locking mode.


> If I/we make schedlock_step the default then this code of your patch gets
> disabled:
> 
> +  else if ((scheduler_mode == schedlock_reverse)
> +          && ((execution_direction == EXEC_REVERSE)
> +              || target_record_is_replaying (minus_one_ptid)))
> +    {
> +      /* User-settable 'scheduler' mode requires solo thread resume during
> +        reverse/replay stepping.  */
> +      resume_ptid = inferior_ptid;
> +    }

Yes.  But the code above this in infrun.c then applies:

  else if ((scheduler_mode == schedlock_on)
	   || (scheduler_mode == schedlock_step && step))
    {
      /* User-settable 'scheduler' mode requires solo thread
	 resume.  */
      resume_ptid = inferior_ptid;
    }


Regards,
Markus.

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