Re: On migrate_disable() and latencies

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2011-07-22 at 12:19 +0200, Peter Zijlstra wrote:
> On Wed, 2011-07-20 at 02:37 +0200, Thomas Gleixner wrote:
> >    - Twist your brain around the schedulability impact of the
> >      migrate_disable() approach.
> > 
> >      A really interesting research topic for our friends from the
> >      academic universe. Relevant and conclusive (even short notice)
> >      papers and/or talks on that topic have a reserved slot in the
> >      Kernel developers track at the Realtime Linux Workshop in Prague
> >      in October this year. 
> 
> From what I can tell it can induce a latency in the order of
> max-migrate-disable-period * nr-cpus.
> 
> The scenario is on where you stack N migrate-disable tasks on a run
> queue (necessarily of increasing priority). Doing this requires all cpus
> in the system to be as busy, for otherwise the task would simply be
> moved to another cpu.

This implies it requires at least nr-cpus^2 tasks to pull this off. So
if you want to add the nr-tasks constraint the actual limit is something
like:

   max-migrate-disable-period * min(nr-cpus, nr-tasks / nr_cpus);

> Anyway, once you manage to stack these migrate-disable tasks, all other
> tasks go to sleep, leaving a vacuum. Normally we would migrate tasks to
> fill the vacuum left by the tasks going to sleep, but clearly
> migrate-disable prohibits this.
> 
> So we have this stack of migrate-disable tasks and M-1 idle cpus (loss
> of utilization). Now it takes the length of the migrate-disable region
> of the highest priority task on the stack (the one running) to complete
> and enable migration again. This will instantly move the task away to an
> idle cpu. This will then need to happen min(N-1, M-1) times before the
> lowest priority migrate_disable task can run again or all cpus are busy.
> 
> Therefore the worst case latency is in the order of
> max-migrate-disable-period * nr-cpus.
> 
> Currently we have no means of measuring these latencies, this is
> something we need to grow, I think Steven can fairly easy craft a
> migrate_disable runtime tracer -- it needs to use t->se.sum_exec_runtime
> for measure so as to only count the actual time spend on the task and
> ignore any time it was blocked.
> 
> Once we have this, its back to the old game of 'lock'-breaking.
> 
>