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

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Message-ID: <20100826233651.GB28177@Krystal>
Date:	Thu, 26 Aug 2010 19:36:51 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Thomas Gleixner <tglx@...utronix.de>
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

* 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.

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.
http://www.efficios.com
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