Re: [PATCH tip/core/rcu] Do not keep timekeeping CPU tick running for non-nohz_full= CPUs

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

On Sun, Jul 20, 2014 at 10:34:17PM +0200, Peter Zijlstra wrote:
> On Sun, Jul 20, 2014 at 04:47:59AM -0700, Paul E. McKenney wrote:
> > So we really have to have -all- the CPUs be idle to turn off the timekeeper.
> 
> That seems to be pretty unavoidable any which way around.

Hmmm...  The exception would be the likely common case where none of
the CPUs are flagged as nohz_full= CPUs.  If we handled that case as
if CONFIG_NO_HZ_FULL=n, we would have handled almost all of
the problem.

> > This won't make the battery-powered embedded guys happy...
> > 
> > Other thoughts on this?  We really should not be setting
> > CONFIG_NO_HZ_FULL_SYSIDLE by default until this is solved.
> 
> What are those same guys doing with nohz_full to begin with?

If CONFIG_NO_HZ_FULL_SYSIDLE=y is the default, my main concern is for
people who didn't really want it, and who thus did not set the nohz_full=
boot parameter.  Hence my suggestion above that we treat that case as
if CONFIG_NO_HZ_FULL=n (and thus also as if CONFIG_NO_HZ_FULL_SYSIDLE=n).

There have been some people saying that they want only a subset of
their CPUs in nohz_full= state, and these guys seem to want to run a
mixed workload.  For example, they have HPC (or RT) workloads on the
nohz_full= CPUs, and also want normal high-throughput processing on the
remaining CPUs.  If software was trivial (and making other unlikely
assumptions about the perfection of the world and the invalidity of
Murphy's lawy), we would want the timekeeping CPU to be able to move
among the non-nohz_full= CPUs.

However, this should be a small fraction of the users, and many of
these guys would probably be open to making a few changes.  Thus, a
less-proactive approach should allow us to solve their actual problems, as
opposed to the problems that we speculate that they might encounter.  ;-)

							Thanx, Paul