Re: [ANNOUNCE] 3.8-rc6-nohz4

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

On Thu, Feb 07, 2013 at 11:12:00AM -0500, Steven Rostedt wrote:
> On Thu, 2013-02-07 at 12:10 +0100, Ingo Molnar wrote:
> > * Steven Rostedt <rostedt@goodmis.org> wrote:
> > 
> > > I'll reply to this as I come up with comments.
> > > 
> > > First thing is, don't call it NO_HZ_FULL. A better name would 
> > > be NO_HZ_CPU. I would like to reserve NO_HZ_FULL when we 
> > > totally remove jiffies :-)
> > 
> > I don't think we want yet another config option named in a 
> > weird way.
> > 
> > What we want instead is to just split NO_HZ up into its 
> > conceptual parts:
> > 
> >    CONFIG_NO_HZ_IDLE
> >    CONFIG_NO_HZ_USER_SPACE
> >    CONFIG_NO_HZ_KERNEL_SPACE
> > 
> > Where the current status quo is NO_HZ_IDLE=y, and Frederic is 
> > about to introduce NO_HZ_USER_SPACE=y. When jiffies get removed 
> > we get NO_HZ_KERNEL_SPACE=y.
> 
> Saying NO_HZ_USER_SPACE is a bit of a misnomer. As we don't just stop
> the tick for user space, but it may remained stopped when entering the
> kernel. The rule is that when there's just a single task on a CPU, the
> tick can stop (no scheduling work needed). But if the task triggers
> something that may require a tick (like printk) then the tick will start
> again. But just going into the kernel does not designate a tick restart.
> 
> Maybe a better name would be NO_HZ_SINGLE_TASK ?
> 
> > 
> > The 'CONFIG_NO_HZ' meta-option, which we should leave for easy 
> > configurability and for compatibility, should get us the 
> > currently recommended default, which for the time being might 
> > be:
> > 
> >    CONFIG_NO_HZ_IDLE=y
> >    # CONFIG_NO_HZ_USER_SPACE is disabled
> > 
> > Btw., you could add CONFIG_NO_HZ_KERNEL_SPACE right away, just 
> > keep it false all the time. That would document our future plans 
> > pretty well.
> 
> Maybe the removal of jiffies would be NO_HZ_COMPLETE?

I suspect that removal of jiffies from the kernel will take a few stages,
with RCU being one of the laggards for awhile.  Making RCU's state
machine depend wholly on process-based execution will take some care
and experimentation, especially for extreme and corner-case workloads.
For example, having RCU OOM the system just because a specific CPU was
unable to run some RCU kthread for an extended time is something to
be avoided.  ;-)

							Thanx, Paul