Re: 2.6.21-rc1: known regressions (v2) (part 2)

[Mike Galbraith <efault@gmx.de>]

On Tue, 2007-02-27 at 09:33 +0100, Ingo Molnar wrote:
> * Michal Piotrowski <michal.k.k.piotrowski@gmail.com> wrote:
> 
> > Thomas Gleixner napisał(a):
> > > Adrian,
> > > 
> > > On Mon, 2007-02-26 at 23:05 +0100, Adrian Bunk wrote:
> > >> Subject    : kernel BUG at kernel/time/tick-sched.c:168  (CONFIG_NO_HZ)
> > >> References : http://lkml.org/lkml/2007/2/16/346
> > >> Submitter  : Michal Piotrowski <michal.k.k.piotrowski@gmail.com>
> > >> Handled-By : Thomas Gleixner <tglx@linutronix.de>
> > >> Status     : problem is being debugged
> > > 
> > > The BUG_ON() was replaced by a warning printk(). The BUG_ON() exposed a
> > > problem with the SMT scheduler. See below.
> > > 
> > >> Subject    : BUG: soft lockup detected on CPU#0
> > >>              NOHZ: local_softirq_pending 20  (SMT scheduler)
> > >> References : http://lkml.org/lkml/2007/2/20/257
> > >> Submitter  : Michal Piotrowski <michal.k.k.piotrowski@gmail.com>
> > >> Handled-By : Thomas Gleixner <tglx@linutronix.de>
> > >>              Ingo Molnar <mingo@elte.hu>
> > >> Status     : problem is being debugged
> > > 
> > > Patch available, not confirmed yet.
> > > 
> > 
> > I can confirm that the bug is fixed (over 20 hours of testing should 
> > be enough).
> 
> thanks alot! I think this thing was a long-term performance/latency 
> regression in HT scheduling as well.

Agreed.

I was recently looking at that spot because I found that niced tasks
were taking latency hits, and disabled it, which helped a bunch.  I also
can't understand why it would be OK to interleave a normal task with an
RT task sometimes, but not others.. that's meaningless to the RT task.

IMHO, SMT scheduling should be a buyer beware thing.  Maximizing your
core utilization comes at a price, but so does disabling it, so I think
letting the user decide what he wants is the right thing to do.

	-Mike