Re: [PATCH tip/core/rcu 11/17] rcu: Bind grace-period kthreads to non-NO_HZ_FULL CPUs

[Frederic Weisbecker]

On Fri, Jul 11, 2014 at 12:08:16PM -0700, Paul E. McKenney wrote:
> On Fri, Jul 11, 2014 at 08:57:33PM +0200, Frederic Weisbecker wrote:
> > On Fri, Jul 11, 2014 at 11:45:28AM -0700, Paul E. McKenney wrote:
> > > On Fri, Jul 11, 2014 at 08:25:43PM +0200, Frederic Weisbecker wrote:
> > > > On Fri, Jul 11, 2014 at 01:10:41PM -0500, Christoph Lameter wrote:
> > > > > On Tue, 8 Jul 2014, Frederic Weisbecker wrote:
> > > > > 
> > > > > > > I was figuring that a fair number of the kthreads might eventually
> > > > > > > be using this, not just for the grace-period kthreads.
> > > > > >
> > > > > > Ok makes sense. But can we just rename the cpumask to housekeeping_mask?
> > > > > 
> > > > > That would imply that all no-nohz processors are housekeeping? So all
> > > > > processors with a tick are housekeeping?
> > > > 
> > > > Well, now that I think about it again, I would really like to keep housekeeping
> > > > to CPU 0 when nohz_full= is passed.
> > > 
> > > When CONFIG_NO_HZ_FULL_SYSIDLE=y, then housekeeping kthreads are bound to
> > > CPU 0.  However, doing this causes significant slowdowns according to
> > > Fengguang's testing, so when CONFIG_NO_HZ_FULL_SYSIDLE=n, I bind the
> > > housekeeping kthreads to the set of non-nohz_full CPUs.
> > 
> > But did he see these slowdowns with nohz_full= parameter passed? I doubt he
> > tested that. And I'm not sure that people who need full dynticks will run
> > the usecases that trigger slowdowns with grace period kthreads.
> > 
> > I also doubt that people will often omit other CPUs than CPU 0 nohz_full=
> > range.
> 
> Agreed, this is only a problem when people run workloads for which
> NO_HZ_FULL is not well-suited.  Which is why I settled on designating
> the non-nohz_full= CPUs as the housekeeping CPUs -- people wanting to
> run general workloads not suited to NO_HZ_FULL probably won't specify
> nohz_full=.  If they don't, then any CPU can be a housekeeping CPU.

Right. So affining GP kthread to all non-nohz-full CPU works in all case. It's convenient
but it requires some plumbing:

* add a housekeeping cpumask and implement housekeeping_affine on top
* add kthread_bind_cpumask()

So what I propose is to skip these complications and just do:

        if (tick_nohz_full_enabled()) // means that somebody passed nohz_full= kernel parameter
            kthread_bind_cpu(GP kthread, 0)

Moreover Thomas didn't like the idea of extending housekeeping duty further CPU 0, arguing that
it's too early for that. He meant that for timekeeping but the idea is expandable.

> 
> > > > > Could we make that set configurable? Ideally I'd like to have the ability
> > > > > restrict the housekeeping to one processor.
> > > > 
> > > > Ah, I'm curious about your usecase. But I think we can do that. And we should.
> > > > 
> > > > In fact I think that Paul could keep affining grace period kthread to CPU 0
> > > > for the sole case when we have nohz_full= parameter passed.
> > > > 
> > > > I think the performance issues reported to him refer to CONFIG_NO_HZ_FULL=y
> > > > config without nohz_full= parameter passed. That's the most important to address.
> > > > 
> > > > Optimizing the "nohz_full= passed" case is probably not very useful and worse
> > > > it complicate things a lot.
> > > > 
> > > > What do you think Paul? Can we simplify things that way? I'm pretty sure that
> > > > nobody cares about optimizing the nohz_full= case. That would really simplify
> > > > things to stick to CPU 0.
> > > 
> > > When we have CONFIG_NO_HZ_FULL_SYSIDLE=y, agreed.  In that case, having
> > > housekeeping CPUs on CPUs other than CPU 0 means that you never reach
> > > full-system-idle state.
> > 
> > That said I expect CONFIG_NO_HZ_FULL_SYSIDLE=y to be always enable for those
> > who run NO_HZ_FULL in the long run.
> 
> Hmmm...  That probably means that we need boot-time parameters to
> make sysidle detection really happen.  Otherwise, many users will
> get a nasty surprise once CONFIG_NO_HZ_FULL_SYSIDLE=y is enabled on
> systems that really aren't running HPC or RT workloads.
> 
> I suppose that I could confine SYSIDLE's attention to the nohz_full=
> CPUs -- that might actually make things work nicely in all cases with
> no configuration of any sort required.  I will need to give this some
> thought.

Exactly, nohz_full= gives all the information we need for sysidle.

> 
> > > But in other cases, we appear to need more than one housekeeping CPU.
> > > This is especially the case when people run general workloads on systems
> > > that have NO_HZ_FULL=y, which appears to be a significant fraction of
> > > the systems these days.
> > 
> > Yeah NO_HZ_FULL=y is likely to be enabled in many distros. But you know the
> > amount of nohz_full= users.
> 
> Indeed!  ;-)
> 
> 								Thanx, Paul
> 
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