Re: [RFC PATCH 7/9] housekeeping: Use own boot option, independant from nohz

[Luiz Capitulino <lcapitulino@redhat.com>]

On Mon, 14 Aug 2017 19:01:09 +0200
Frederic Weisbecker <fweisbec@gmail.com> wrote:

> > > Perhaps I should remove the nohz_full= parameter altogether and let nohz_full controlled
> > > by housekeeping= only. How much can kernel parameters be considered as kernel ABIs?  
> > 
> > That's a very good question, I don't have an answer for that.  
> 
> That said, "nohz_full=" never implied too much isolation features so far, and those have
> often changed over time, as in RCU. I think unbound timer affinity is the most important
> one.
> 
> Perhaps we can keep "nohz_full=1-15" as an alias for a future "cpu_isolation=nohz,1-15"
> and at least imply unbound timer affinity with it.

That would work for me.

> > > Also I'm wondering if "housekeeping=" is a clear name for users. "isolation=" or
> > > "cpu_isolation=" would be better and more obvious. Housekeeping based naming would only be
> > > internal implementation detail. And deactivating the tick through "cpu_isolation=" would
> > > be clearer than if we did through "housekeeping=".  
> > 
> > That's exactly my thinking while I was reviewing the series!
> >   
> > > Of course the problem is that we already have "isolcpus=". But re-implementing isolcpus
> > > on top of housekeeping might be a good idea. I believe that the current implementation on
> > > top of NULL domains isn't much beloved. A less controversial implementation might even
> > > allow us to control it though cpusets.  
> > 
> > You're completely right. Some people don't use isolcpus= because it
> > disables load balancing and that may be a problem for setups where
> > tasks are pinned to a set of CPUs where the number of tasks is greater
> > than the number of CPUs. However, for the cases where you have a
> > single task pinned to a CPU, having load balancing taking place adds
> > an extra latency (I won't remember how much, but I guess it was more
> > than 10us).  
> 
> What is the source of the load balancing inducing such latency when a single
> task is affine to a CPU? If this is idle load balancing, it is now affine to
> housekeepers. If this is task wakeup then it's suprising because select_task_rq()
> is optimized toward single CPU affinity.

I guess it was idle load balancing, but I don't remember because this
was a few years ago. I think this might be reproducible without using
isolcpus=. I'll give it a try shortly and let you know.

> Is there another source I'm overlooking?
> 
> > If there's a way to "disable" load balancing from user-space, say
> > with cpusets, then I think we should keep the isolated CPUs attached
> > to a domain as you suggest.  
> 
> I'm not sure such a solution would be accepted. The most sensible way
> to disable load balancing is still to tune the affinity of tasks. If there
> is an off-case overhead with load balancing (ie: when no more than one
> task is affine to that CPU) then we should solve that with a fast path.

OK, I'll take a look.

> > Another detail about isolcpus= is that it doesn't isolate the CPU
> > from kernel threads. That is, unpinned kernel threads are allowed
> > to run on CPUs not isolated with isolcpus=. We might consider changing
> > that for a new isolation option.  
> 
> You mean unpinned kernel threads are allowed to run on isolcpus, right?

Exactly.

> That definetly can be solved.
> 
> > 
> > I know that there are many arguments against isolcpus= and some people
> > advice using cpusets. The problem with that advice is that isolcpus=
> > goes a bit beyond isolating a CPU from user-space tasks. One additional
> > thing is does for example, is pinning the kernel_init() thread to
> > housekeeping CPUs. This is key, because that thread will create timers
> > at early boot that will pin themselves to the CPU they run.  
> 
> Right, but also unbound timers are affine to housekeepers, we needed that for
> nohz_full.
> 
> > Finally, I'm wondering how all this will fit together with TASK_ISOLATION.
> > One of the questions I ask myself is: can/should the things TASK_ISOLATION
> > does be done by a kernel command-line parameter instead? Or should we
> > try to come up with a list of global things to control (eg. the tick,
> > kernel thread affinity, etc) and per-task controls?  
> 
> So I've been thinking a lot about that lately. I told Chris that TASK_ISOLATION
> shouldn't be a CPU feature but a task feature. Then I realized that it doesn't work
> either, my bad :-)  In the end I think that the most part of it must be a CPU
> property: nohz, task isolation, timers and workqueue affinity, etc... Then what's
> left for the per task thing is to tell it when it is unexpectingly interrupted by noise.
> 
> Therefore I think most of the isolation features should be controlled by
> command line and cpusets (through a new cpuset subsystem maybe) then TASK_ISOLATION
> through prtcl() for the noise monitoring.

I agree.