Re: [RFC][PATCH] sched/rt: Use IPI to trigger RT task push migration instead of pulling

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Feb 04, 2015 at 02:39:06PM -0500, Steven Rostedt wrote:
> The problem is that if you have 100 CPUs having an RT task schedule out
> (just like cyclictest -d0 -t -i100 does a lot), and if there's one rq
> that has more than one RT task scheduled on it (overloaded), each
> of those 100 CPUs are going to try to get that second RT task on that
> lonely rq, and each of those 100 CPUs are going to take that lonely
> rq's lock! Then what happens if the task running on that rq wants to
> schedule? Well, it needs to wait behind 100 CPUs contending for its rq
> lock, and we see a HUGE latency (1ms or more).
> 
> What does this patch do instead? Instead of trying to fight for the rq,
> if it sees that there's an overloaded rq, it simply sends an IPI to
> that CPU to have it push the overloaded RT task off to another CPU.

So can't we flip the problem around; 99 overloaded cpus and 1 going
'low', then we IPI the 99, and they're all going to try and push their
tasks on the one (now) sad cpu?