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

[Peter Zijlstra]

On Mon, Jul 21, 2014 at 08:57:41AM -0700, Paul E. McKenney wrote:
> 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.

You mean that is not currently the case? Yes that seems like a fairly
sane thing to do.

> > > 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).

ack

> 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.

Yeah, I don't see a problem with that, but then I'm not entirely sure
why we use RCU to track system idle state.

> 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.  ;-)

But you still haven't talked about the battery people... I don't think
nohz_full is something they should care about / use.
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