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

Paul E. McKenney
2010-08-27T00:09:17Z

On Thu, Aug 26, 2010 at 07:53:23PM -0400, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > On Thu, Aug 26, 2010 at 07:28:58PM -0400, Mathieu Desnoyers wrote:
> > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > > > On Fri, Aug 27, 2010 at 12:22:46AM +0200, Thomas Gleixner 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 ?
> > > >
> > > > C'mon, Thomas!!! That is entirely too sensible!!! ;-)
> > > >
> > > > But if you are going to create the thread at timer_create() time,
> > > > why not just have the new thread block for the desired duration?
> > >
> > > The timer infrastructure allows things like periodic timer which restarts when
> > > it fires, detection of missed timer events, etc. If you try doing this in a
> > > userland thread context with a simple sleep, then your period becomes however
> > > long you sleep for _and_ the thread execution time. This is all in all quite
> > > different from the timer semantic.
> >
> > Hmmm... Why couldn't the thread in question set the next sleep time based
> > on the timer period? Yes, if the function ran for longer than the period,
> > there would be a delay, but the POSIX semantics allow such a delay, right?
>
> I'm afraid you'll have a large error accumulation over time, and getting the
> precise picture of how much time between now and where the the period end is
> expected to be is kind of hard to do precisely from user-space. In a few words,
> this solution would be terrible for jitter. This is why we usually rely on
> timers rather than delays in these periodic workloads.

Why couldn't the timer_create() call record the start time, and then
compute the sleeps from that time? So if timer_create() executed at
time t=100 and the period is 5, upon awakening and completing the first
invocation of the function in question, the thread does a sleep calculated
to wake at t=110.

Thanx, Paul