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

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Date:	Fri, 06 Feb 2015 16:58:08 +0100
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Clark Williams <williams@...hat.com>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC][PATCH] sched/rt: Use IPI to trigger RT task push
 migration instead of pulling

On Fri, 2015-02-06 at 13:40 +0100, Peter Zijlstra wrote:
> On Thu, Feb 05, 2015 at 11:04:20AM -0500, Steven Rostedt wrote:

> Well, the thing is, with or without this patch, the worst case is still
> the very same. The likelihood of actually triggering it goes down
> dramatically [*], but any spin_lock() has that O(nr_cpus) worst case, and
> that spinlock op isn't going away.
> 
> Now, lowering avg/median cases is good, but don't kid yourself that the
> worst case is going to be better.
> 
> People running -RT on big boxes had better be aware of this.

The folks I know of running -RT on big boxen isolate cores and nail
absolutely everything to the floor.  The only way to survive in a big
box is to chop the thing into tiny pieces.  A plasma cutter would be
even better.. 'course you could just _buy smaller boxen_ and connect
what you want connected, take the lazy path to big box solution ;-)
  
	-Mike

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