linux-kernel - Re: [PATCH v3] sched/deadline: Fix bad accounting of nr

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Date:	Wed, 19 Feb 2014 09:46:18 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Juri Lelli <juri.lelli@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3] sched/deadline: Fix bad accounting of nr_running

On Tue, Feb 18, 2014 at 09:50:12PM -0500, Steven Rostedt wrote:
>  
> > Rationale for this odd behavior is that, when a task is throttled, it
> > is removed only from the dl_rq, but we keep it on_rq (as this is not
> > a "full dequeue", that is the task is not actually sleeping). But, it
> > is also true that, while throttled a task behaves like it is sleeping
> > (e.g., its timer will fire on a new CPU if the old one is dead). So,
> > Steven's fix sounds also semantically correct.
> 
> Actually, it seems that I was hitting it again, but this time getting a
> negative number. OK, after looking at the code a bit more, I think we
> should update the runqueue nr_running only when the task is officially
> enqueued and dequeued, and all accounting within, will not touch that
> number.

But if the task is throttled it should still very much decrement the
number. There's places that very much rely on nr_running be exactly the
number of runnable tasks.
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