Re: 2.6.14-rt22 (and mainline) excessive latency

["Paul E. McKenney" <paulmck@us.ibm.com>]

On Wed, Dec 21, 2005 at 02:54:18PM -0500, Lee Revell wrote:
> On Wed, 2005-12-21 at 05:36 -0800, Paul E. McKenney wrote:
> > On Tue, Dec 20, 2005 at 10:32:48PM -0500, Lee Revell wrote:
> > > On Tue, 2005-12-20 at 17:47 -0800, Paul E. McKenney wrote:
> > > > On Tue, Dec 20, 2005 at 05:24:42AM +0100, Ingo Molnar wrote:
> > > > > 
> > > > > * Lee Revell <rlrevell@joe-job.com> wrote:
> > > > > 
> > > > > > I captured this 3+ ms latency trace when killing a process with a few 
> > > > > > thousand threads.  Can a cond_resched be added to this code path?
> > > > > 
> > > > > >     bash-17992 0.n.1   29us : eligible_child (do_wait)
> > > > > > 
> > > > > >     [ 3000+ of these deleted ]
> > > > > > 
> > > > > >     bash-17992 0.n.1 3296us : eligible_child (do_wait)
> > > > > 
> > > > > Atomicity of signal delivery is pretty much a must, so i'm not sure this 
> > > > > particular latency can be fixed, short of running PREEMPT_RT. Paul E.  
> > > > > McKenney is doing some excellent stuff by RCU-ifying the task lookup and 
> > > > > signal code, but i'm not sure whether it could cover do_wait().
> > > > 
> > > > Took a quick break from repeatedly shooting myself in the foot with
> > > > RCU read-side priority boosting (still have a few toes left) to take
> > > > a quick look at this.  The TASK_TRACED and TASK_STOPPED cases seem
> > > > non-trivial, and I am concerned about races with exit.
> > > > 
> > > > Any thoughts on whether the latency is due to contention on the
> > > > tasklist lock vs. the "goto repeat" in do_wait()?
> > > 
> > > It's a UP system so I'd be surprised if there were any contention.
> > 
> > Couldn't there be contention due to preemption of someone holding
> > the tasklist lock?
> 
> But I'm running with PREEMPT_DESKTOP (specifically I configured a system
> to have the exact same preemption model as mainline - PREEMPT_DESKTOP
> with no soft/hardirq preemption) so holding a spinlock will disable
> preemption.

My head just exploded.  I will see about getting you a
CONFIG_FREAKING_INSANE patch, if you are willing to test it.

						Thanx, Paul
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