Re: [RFC PATCH 00/11] sched: CFS low-latency features

Mathieu Desnoyers
2010-08-26T23:36:56Z

* Mathieu Desnoyers (mathieu.desnoyers@efficios.com) wrote:
> * Thomas Gleixner (tglx@linutronix.de) wrote:
> > On Thu, 26 Aug 2010, Thomas Gleixner wrote:
> > > On Thu, 26 Aug 2010, Peter Zijlstra wrote:
> > > >
> > > > Fudging fork seems dubious at best, it seems generated by the use of
> > > > timer_create(.evp->sigev_notify = SIGEV_THREAD), which is a really
> > > > broken thing to do, it has very ill defined semantics and is utterly
> > > > unable to properly cope with error cases. Furthermore its trivial to
> > > > actually correctly implement the desired behaviour, so I'm really
> > > > skeptical on this front; friends don't let friends use SIGEV_THREAD.
> > >
> > > SIGEV_THREAD is the best proof that the whole posix timer interface
> > > was comitte[e]d under the influence of not to be revealed
> > > mind-altering substances.
> > >
> > > I completely object to add timer specific wakeup magic and support for
> > > braindead fork orgies to the kernel proper. All that mess can be fixed
> > > in user space by using sensible functionality.
> > >
> > > Providing support for misdesigned crap just for POSIX compliance
> > > reasons and to make some of the blind abusers of that very same crap
> > > happy would be a completely stupid decision.
> > >
> > > In fact that would make a brilliant precedence case for forcing the
> > > kernel to solve user space madness at the expense of kernel
> > > complexity. If we follow down that road we get requests for extra
> > > functionality for AIO, networking and whatever in a split second with
> > > no real good reason to reject them anymore.
> >
> > I really risked eye cancer and digged into the glibc code.
> >
> > /* There is not much we can do if the allocation fails. */
> > (void) pthread_create (&th, &tk->attr, timer_sigev_thread, td);
> >
> > So if the helper thread which gets the signal fails to create the
> > thread then everything is toast.
> >
> > What about fixing the f*cked up glibc implementation in the first place
> > instead of fiddling in the kernel to support this utter madness?
> >
> > WTF can't the damned delivery thread not be created when timer_create
> > is called and the signal be delivered to that very thread directly via
> > SIGEV_THREAD_ID ?
>
> Yeah, that sounds exactly like what I proposed about an hour ago on IRC ;) I'm
> pretty sure that would work.
>
> The only thing we might have to be careful about is what happens if the timer
> re-fires before the thread completes its execution. We might want to let the
> signal handler detect these overruns somehow.

Hrm, thinking about it a little more, one of the "plus" sides of these
SIGEV_THREAD timers is that a single timer can fork threads that will run on
many cores on a multi-core system. If we go for preallocation of a single
thread, we lose that. Maybe we could think of a way to preallocate a thread pool
instead ?

Thanks,

Mathieu

>
> Thanks,
>
> Mathieu
>
> >
> > Thanks,
> >
> > tglx
>
> --
> Mathieu Desnoyers
> Operating System Efficiency R&D Consultant
> EfficiOS Inc.
> http://www.efficios.com

--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.