Re: [PATCH RFC nohz_full v2 2/7] nohz_full: Add rcu_dyntick data forscalable detection of all-idle state

[Josh Triplett]

On Mon, Jul 01, 2013 at 08:52:20AM -0700, Paul E. McKenney wrote:
> On Mon, Jul 01, 2013 at 08:31:50AM -0700, Josh Triplett wrote:
> > On Fri, Jun 28, 2013 at 01:10:17PM -0700, Paul E. McKenney wrote:
> > > From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> > > 
> > > This commit adds fields to the rcu_dyntick structure that are used to
> > > detect idle CPUs.  These new fields differ from the existing ones in
> > > that the existing ones consider a CPU executing in user mode to be idle,
> > > where the new ones consider CPUs executing in user mode to be busy.
> > 
> > Can you explain, both in the commit messages and in the comments added
> > by the next commit, *why* this code doesn't consider userspace a
> > quiescent state?
> 
> Good point!  Does the following explain it?
> 
> 	Although one of RCU's quiescent states is usermode execution,
> 	it is not a full-system idle state.  This is because the purpose
> 	of the full-system idle state is not RCU, but rather determining
> 	when accurate timekeeping can safely be disabled.  Whenever
> 	accurate timekeeping is required in a CONFIG_NO_HZ_FULL kernel,
> 	at least one CPU must keep the scheduling-clock tick going.
> 	If even one CPU is executing in user mode, accurate timekeeping
> 	is requires, particularly for architectures where gettimeofday()
> 	and friends do not enter the kernel.  Only when all CPUs are
> 	really and truly idle can accurate timekeeping be disabled,
> 	allowing all CPUs to turn off the scheduling clock interrupt,
> 	thus greatly improving energy efficiency.
> 
> 	This naturally raises the question "Why is this code in RCU rather
> 	than in timekeeping?", and the answer is that RCU has the data
> 	and infrastructure to efficiently make this determination.

Good explanation, thanks.

This also naturally raises the question "How can we let userspace get
accurate time without forcing a timer tick?".

- Josh Triplett
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