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

Thanx, Paul



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