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From: Ingo Molnar on 16 Mar 2010 09:20

* Nick Piggin <npiggin(a)suse.de> wrote:

> On Tue, Mar 16, 2010 at 08:36:35AM +0100, Ingo Molnar wrote:
> >
> > * Mathieu Desnoyers <mathieu.desnoyers(a)efficios.com> wrote:
> >
> > > Unless this question is answered, Ingo's SA_RUNNING signal proposal, as
> > > appealing as it may look at a first glance, falls into the
> > > "fundamentally broken" category. [...]
> >
> > How is it different from your syscall? I.e. which lines of code make the
> > difference? We could certainly apply the (trivial) barrier change to
> > context_switch().
>
> I think it is just easy for userspace to misuse or think it does something
> that it doesn't (because of races).

That wasnt my question though. The question i asked Mathieu was to show how
SA_RUNNING is "fundamentally broken" for librcu use while sys_membarrier() is
not?

This is really what he claims above. (i preserved the quote)

It must be a misunderstanding either on my side or on his side. (Once that is
cleared we can discuss further usecases for SA_RUNNING.)

Thanks,

Ingo
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From: Ingo Molnar on 16 Mar 2010 10:00

* Mathieu Desnoyers <mathieu.desnoyers(a)efficios.com> wrote:

> * Ingo Molnar (mingo(a)elte.hu) wrote:
> >
> > * Nick Piggin <npiggin(a)suse.de> wrote:
> >
> > > On Tue, Mar 16, 2010 at 08:36:35AM +0100, Ingo Molnar wrote:
> > > >
> > > > * Mathieu Desnoyers <mathieu.desnoyers(a)efficios.com> wrote:
> > > >
> > > > > Unless this question is answered, Ingo's SA_RUNNING signal proposal, as
> > > > > appealing as it may look at a first glance, falls into the
> > > > > "fundamentally broken" category. [...]
> > > >
> > > > How is it different from your syscall? I.e. which lines of code make the
> > > > difference? We could certainly apply the (trivial) barrier change to
> > > > context_switch().
> > >
> > > I think it is just easy for userspace to misuse or think it does something
> > > that it doesn't (because of races).
> >
> > That wasnt my question though. The question i asked Mathieu was to show how
> > SA_RUNNING is "fundamentally broken" for librcu use while sys_membarrier() is
> > not?
> >
> > This is really what he claims above. (i preserved the quote)
> >
> > It must be a misunderstanding either on my side or on his side. (Once that is
> > cleared we can discuss further usecases for SA_RUNNING.)
>
> Well, it's not broken for sys_membarrier() specifically if we add the proper
> memory barriers to the scheduler, but it's broken when we try to use it for
> anything else. [...]

That's quite an important distinction to an unqualified "fundamentally
broken", right?

> [...] What makes it broken is that it requires that the scheduler switch
> guarantee to have the same side-effect on a running thread than execution on
> the per-running-thread signal handler.
>
> What's different with the sys_membarrier system call is that it does not try
> to make generic something that should probably stay case-specific due to its
> close coupling with the scheduler.

Yeah, that's a fair point.

Without another realistic usecase SA_RUNNING would just essentially be a
SA_BARRIER special-case. (IMO even in that case signal handling speedups
driven via this usecase would still be tempting though.)

But note that some other usecase is possible as well:

In theory we could inject signals at context-switch time (if that signal is
not pending yet) - signals are fairly atomic [with a preallocated pool] and
the 'wakeup' property of signals is not needed as the to-be-running task is
obviously up to execution. (so there's no deadlock. It doesnt have to run with
the rq lock taken in any case - it can run from sched_tail() i suspect.)

So all this could be done via the ret-to-user framework that KVM uses at
essentially no extra scheduler overhead. I think :-) It would be a bit like
SIGALRM for timers.

Plus another performance optimization would be useful as well: signals could
be turned on/off without having to enter the kernel. This could be done via a
in-user-memory enable/disable-signals flag/mask associated with each task. (it
would pin a page of memory.)

The question is, do we want to enable user-space to trigger a signal upon
context-switches?

It probably cannot be a queued one, as preemption from the signal handler
itself would be rather yucky. As long as concurrency control is involved,
user-space only wants a callback for the _first_ reschedule - subsequent
reschedules dont need to trigger a signal, until the signal handler has
finished.

Ingo
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