Messages in this thread | | | Date | Thu, 26 Aug 2010 19:36:51 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH 00/11] sched: CFS low-latency features |
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* 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. http://www.efficios.com
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