Re: [PATCH] x86: Reduce the default HZ value

[Paul E. McKenney]

On Thu, May 07, 2009 at 01:51:58PM -0400, Christoph Lameter wrote:
> On Thu, 7 May 2009, Paul E. McKenney wrote:
> 
> > On Thu, May 07, 2009 at 01:20:29PM -0400, Christoph Lameter wrote:
> > > On Thu, 7 May 2009, Peter Zijlstra wrote:
> > >
> > > > Another user is RCU, the grace period is tick driven, growing these
> > > > ticks by a factor 50 or so might require some tinkering with forced
> > > > grace periods when we notice our batch queues getting too long.
> > >
> > > One could also schedule RCU via hrtimers with a large fuzz period?
> >
> > You could, but then you would still have a periodic interrupt introducing
> > jitter into your HPC workload.  The approach I suggested allows RCU to be
> > happy with no periodic interrupts on any CPU that has only one runnable
> > task that is a CPU-bound user-level task (in addition to the idle task,
> > of course).
> 
> Sounds good.
> 
> An HPC workload typically has minimal kernel interaction. RCU would
> only need to run once and then the system would be quiet.

Peter Z's post leads me to believe that there might be dragons in
this approach that I am blissfully unaware of.  However, here is what
would have to happen from an RCU perspective, in case it helps:

o	This new mode needs to imply CONFIG_NO_HZ.

o	When a given CPU is transitioning into tickless mode, invoke
	rcu_enter_nohz().  This already happens for dynticks-idle,
	this would be a dynticks-CPU-bound-usermode-task.
	Note that CONFIG_NO_HZ kernels already invokes rcu_enter_nohz()
	from tick_nohz_stop_sched_tick(), and many of the things in
	tick_nohz_stop_sched_tick() would need to be done in this case
	as well.

o	When a given CPU is transitioning out of tickless mode, invoke
	rcu_exit_nohz().  Again, this already happens for dynticks-idle.
	Note that CONFIG_NO_HZ kernels already invoke rcu_exit_nohz() 
	from tick_nohz_restart_sched_tick(), which does other stuff that
	would be required in your case as well.

o	When a given CPU in tickless mode transitions into the kernel
	via a system call or trap, invoke rcu_irq_enter().  Note that
	rcu_irq_enter() is already invoked on irq entry if CONFIG_NO_HZ.
	NMIs are also already handled via rcu_nmi_enter().

o	When a given CPU in tickless mode transitions out of the kernel
	from a system call or trap, invoke rcu_irq_exit().  Note that
	rcu_irq_exit() is already invoked on irq exit if CONFIG_NO_HZ.
	NMIs are also already handled via rcu_nmi_exit().

Then RCU would know that any CPU running a CPU-bound user-mode task
need not be consulted when working out when a grace period ends, since
user-mode code cannot contain kernel-mode RCU read-side critical sections.

							Thanx, Paul
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