Date: Mon, 4 Oct 2004 14:03:06 -0400 From: John Baldwin <jhb@FreeBSD.org> To: freebsd-arch@FreeBSD.org Cc: Stephan Uphoff <ups@tree.com> Subject: Re: scheduler (sched_4bsd) questions Message-ID: <200410041403.06187.jhb@FreeBSD.org> In-Reply-To: <1096911278.44307.17.camel@palm.tree.com> References: <1095468747.31297.241.camel@palm.tree.com> <200410041131.35387.jhb@FreeBSD.org> <1096911278.44307.17.camel@palm.tree.com>
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On Monday 04 October 2004 01:34 pm, Stephan Uphoff wrote:
> On Mon, 2004-10-04 at 11:31, John Baldwin wrote:
> > On Friday 01 October 2004 12:13 am, Stephan Uphoff wrote:
> > > On Wed, 2004-09-29 at 18:14, Stephan Uphoff wrote:
> > > > I was looking at the MUTEX_WAKE_ALL undefined case when I used the
> > > > critical section for turnstile_claim().
> > > > However there are bigger problems with MUTEX_WAKE_ALL undefined
> > > > so you are right - the critical section for turnstile_claim is pretty
> > > > useless.
> > >
> > > Arghhh !!!
> > >
> > > MUTEX_WAKE_ALL is NOT an option in GENERIC.
> > > I recall verifying that it is defined twice. Guess I must have looked
> > > at the wrong source tree :-(
> > > This means yes - we have bigger problems!
> > >
> > > Example:
> > >
> > > Thread A holds a mutex x contested by Thread B and C and has priority
> > > pri(A).
> > >
> > > Thread C holds a mutex y and pri(B) < pri(C)
> > >
> > > Thread A releases the lock wakes thread B but lets C on the turnstile
> > > wait queue.
> > >
> > > An interrupt thread I tries to lock mutex y owned by C.
> > >
> > > However priority inheritance does not work since B needs to run first
> > > to take ownership of the lock.
> > >
> > > I is blocked :-(
> >
> > Ermm, if the interrupt happens after x is released then I's priority
> > should propagate from I to C to B.
>
> There is a hole after the mutex x is released by A - but before B can
> claim the mutex. The turnstile for mutex x is unowned and interrupt
> thread I when trying to donate its priority will run into:
>
> if (td == NULL) {
> /*
> * This really isn't quite right. Really
> * ought to bump priority of thread that
> * next acquires the lock.
> */
> return;
> }
>
> So B needs to run and acquire the mutex before priority inheritance
> works again and does not get a priority boost to do so.
>
> This is easy to fix and MUTEX_WAKE_ALL can be removed again at that time
> - but my time budget is limited and Peter has an interesting bug left
> that has priority.
Isn't this handled by the mtx_lock == MTX_CONTESTED case that calls into
turnstile_claim() which bumps the priority of the new owner to the highest
priority waiting thread? I guess this won't happen until B gets to run again
which is the problem. You don't know which thread is going to get the lock,
so what do you do? You don't even have a way to get to the threads that you
might have just woken up.
BTW, Solaris uses MUTEX_WAKE_ALL by default, but for performance reasons. It
is a kernel option because the idea was to benchmark it both ways and then
choose the default based on those numbers. It's off by default as the
wake-one was the original behavior. I'm pretty sure BSD/OS has this same
issue.
--
John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve" = http://www.FreeBSD.org
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