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

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Date:	Fri, 27 Aug 2010 10:43:21 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Tony Lindgren <tony@...mide.com>,
	Mike Galbraith <efault@....de>
Subject: Re: [RFC PATCH 00/11] sched: CFS low-latency features

On Thu, 26 Aug 2010, Mathieu Desnoyers wrote:
> * Mathieu Desnoyers (mathieu.desnoyers@...icios.com) wrote:
> > * Thomas Gleixner (tglx@...utronix.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.

That's easy.
 
> 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 ?

Why should a single timer fork many threads? Just because a previous
thread did not complete before the timer fires again? That's
braindamage as all threads call the same function which then needs to
be serialized anyway. We really do not need a function which creates
tons of threads which get all stuck on the same resource.

Thanks,

	tglx
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