Re: [PATCH v6 2/3] sched/rt: Fix wrong SMP scheduler behavior for equal prio cases

[Mike Galbraith <umgwanakikbuti@gmail.com>]

On Tue, 2015-04-28 at 12:19 +0200, Peter Zijlstra wrote:
> On Sun, Apr 26, 2015 at 11:58:51AM -0400, Steven Rostedt wrote:
> > I think what Xunlei is trying to say, is that we don't currently keep
> > FIFO when preemption or migration is involved. If a task is currently
> > running, strict FIFO denotes that it should run ahead of all other
> > tasks queued at its priority or less until it decides to schedule out.
> > But the issue is, if it gets preempted or migrates, it gets placed
> > behind other tasks of the same priority as itself, but it never
> > voluntarily relinquished the CPU.
> 
> So 1) FIFO is only defined for UP, anything SMP is well outside of the
> FIFO spec and therefore we cannot break it.
> 
> 2) The 'head' of the queue only has meaning on UP, with SMP there's 'n'
> heads, which of those heads is is the foremost head? That is, we're
> already lost order, you cannot reconstruct. This cannot be done without
> first defining order and then implementing that.

Good luck with that :)  Trying to preserve run order across the box led
me to a seemingly _endless_ supply of deadlocks while piddling with
preemptible spinning locks in rt.  Maybe you can pull that off, me it
gave serious headaches.

	-Mike