Re: [RFC PATCH 0/3] CFS idle injection

[Jacob Pan <jacob.jun.pan@linux.intel.com>]

On Tue, 3 Nov 2015 22:06:55 -0800
Eduardo Valentin <edubezval@gmail.com> wrote:

> Hello Jacob,
> 
> On Mon, Nov 02, 2015 at 04:10:25PM -0800, Jacob Pan wrote:
> > Hi Peter and all,
> > 
> > A while ago, we had discussion about how powerclamp is broken in the
> > sense of turning off idle ticks in the forced idle period.
> > https://lkml.org/lkml/2014/12/18/369
> > 
> > It was suggested to replace the current kthread play idle loop with
> > a timer based runqueue throttling scheme. I finally got around to
> > implement this and code is much simpler. I also have good test
> > results in terms of efficiency, scalability, etc.
> > http://events.linuxfoundation.org/sites/events/files/slides/LinuxCon_Japan_2015_idle_injection1_0.pdf
> > slide #18+ shows the data on client and server.
> > 
> > I have two choices for this code:
> > 1) be part of existing powerclamp driver but require exporting some
> >    sched APIs.
> > 2) be part of sched since the genernal rule applies when it comes
> > down to sycnhronized idle time for best power savings.
> > 
> > The patches below are for #2. There is a known problem with LOW RES
> > timer mode that I am working on. But I am hoping to get review
> > earlier.
> > 
> 
> I also like #2 too. Specially now that it is not limited to a specific
> platform. One question though, could you still keep the cooling device
> support of it? In some systems, it might make sense to enable /
> disable idle injections based on temperature.
> 
One of the key difference between 1 and 2 is that #2 is open loop
control, since we don't have CPU c-states info baked into scheduler. To
close the loop, perhaps we can export some internal APIs to the thermal
subsystem then the thermal governors can pick the condition to inject
idle.
> Was there any particular reason you dropped the cooling device
> support?
> 
I did sysctl instead of thermal sysfs to conform the rest of the sched
tuning knobs. We could also have a proxy cooling device to call
internal APIs mentioned above.

Another reason is that, I intend to extend beyond thermal. Where we can
consolidate/sync idle work in semi-active and balanced workload.

Thanks for the suggestions,

Jacob
> BR,
> 
> Eduardo Valentin
> 
> 
> > We are entering a very power limited environment on client side,
> > frequency scaling can only be efficient at certain range. e.g. on
> > SKL, upto ~900MHz, anything below, it is increasingly more
> > efficient to do C-states insertion if coordinated.
> > 
> > Looking forward, there are use case beyond thermal/power capping. I
> > think we can consolidate ballanced partial busy workload that are
> > evenly distributed among CPUs.
> > 
> > Please let me know what you think.
> > 
> > Thanks,
> > 
> > 
> > Jacob Pan (3):
> >   ktime: add a roundup function
> >   timer: relax tick stop in idle entry
> >   sched: introduce synchronized idle injection
> > 
> >  include/linux/ktime.h        |  10 ++
> >  include/linux/sched.h        |  12 ++
> >  include/linux/sched/sysctl.h |   5 +
> >  include/trace/events/sched.h |  23 +++
> >  init/Kconfig                 |   8 +
> >  kernel/sched/fair.c          | 345
> > +++++++++++++++++++++++++++++++++++++++++++
> > kernel/sched/sched.h         |   3 + kernel/sysctl.c
> > |  20 +++ kernel/time/tick-sched.c     |   2 +-
> >  9 files changed, 427 insertions(+), 1 deletion(-)
> > 
> > -- 
> > 1.9.1
> > 
> > --
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[Jacob Pan]