Re: [PATCH] sched/rt: don't try to balance rt_runtime when it is futile

[Paul Gortmaker <paul.gortmaker@windriver.com>]

On 14-05-14 10:49 PM, Mike Galbraith wrote:
> On Wed, 2014-05-14 at 15:11 -0400, Paul Gortmaker wrote:
> 
>> Given that, perhaps a separate change to sched_rt_runtime_exceeded()
>> that works out the CPU from the rt_rq, and returns zero if it is a
>> nohz_full cpu?  Does that make sense?  Then the nohz_full people won't
>> get the throttling message even if they go 100%.
> 
> I don't get it.  What reason would there be to run a hog on a dedicated
> core as realtime policy/priority?  Given no competition, there's nothing
> to prioritize, you could just as well run a critical task as SCHED_IDLE.

Well, as per the original commit log, we acknowledge that people will do
stupid things that don't make 100% sense, and when they do, we should
ideally behave in a sane fashion in response to that.  And I don't think
that "no competition" is a given for most folks.  They see all these
internal threads running and just figure they can chrt their way to a
solution, vs. taking the time to clean up, enable RCU_NOCB etc etc.
Don't get me wrong; I'm not defending such behaviour...

> 
> I would also expect that anyone wanting bare metal will have all of
> their critical cores isolated from the scheduler, watchdogs turned off
> as well as that noisy throttle, the whole point being to make as much
> silent as possible.  Seems to me tick_nohz_full_cpu(cpu) should be
> predicated by that cpu being isolated from the #1 noise source, the
> scheduler and its load balancing.  There's just no point to nohz_full
> without that, or if there is, I sure don't see it.

An interesting point.  One could argue that the default for the nohz_full
cores should be to be isolated from the scheduler, vs needing to be
manually excluded.

P.
--

> 
> When I see people trying to run a hog as a realtime task, it's because
> they are trying in vain to keep competition away from precious cores..
> and one mlockall with a realtime hog blocking flush_work() gives them a
> wakeup call.
> 
> -Mike
>