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

[Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>]

On Wed, 2015-11-04 at 08:58 -0800, Jacob Pan wrote:
> 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.
I think we should have cooling device as we are already using this
cooling device. Once it pass RFC stage,I think we should consider add
this.
Thanks,
Srinivas
> 
> 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]