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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* 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.

Thanks,

Mathieu

> 
> Thanks,
> 
> 	tglx

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