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

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

On Mon, Jul 01, 2013 at 11:16:01AM -0700, Josh Triplett wrote:
> 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@linux.vnet.ibm.com>
> > > > 
> > > > 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?".

We don't.  ;-)

Without CONFIG_NO_HZ_FULL, if a CPU is running in user mode, that CPU
takes scheduling-clock interrupts.  User-mode code will therefore always
see accurate time.  For some definition of "accurate", anyway.

With CONFIG_NO_HZ_FULL and without CONFIG_NO_HZ_FULL_SYSIDLE, a single
designated CPU will always be taking scheduling-clock interrupts, which
again ensures that user-mode code will always see accurate time.

With both CONFIG_NO_HZ_FULL and CONFIG_NO_HZ_FULL_SYSIDLE, if
any CPU other than the timekeeping CPU is nonidle (where "nonidle"
includes usermode execution), then the timekeeping CPU will be taking
scheduling-clock interrupts, yet again ensuring that user-mode code will
always see accurate time.  If all CPUs are idle (in other words, we are
in RCU_SYSIDLE_FULL_NOTED state and the timekeeping CPU is also idle),
scheduling-clock interrupts will be globally disabled.  Or will be,
once I fix the bug noted by Frederic.

I am guessing that you would like this added to the explanation?  ;-)

							Thanx, Paul