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Date: Wed, 13 Jul 2016 09:49:37 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Anton Blanchard <anton@samba.org>, LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>, rt@linutronix.de,
        Michael Ellerman <mpe@ellerman.id.au>,
        Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>,
        shreyas@linux.vnet.ibm.com
Subject: Re: [patch 10/15] sched/migration: Move calc_load_migrate() into
 CPU_DYING
Message-ID: <20160713074937.GP30154@twins.programming.kicks-ass.net>
References: <20160310115406.940706476@linutronix.de>
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 <20160712143745.430733e7@kryten>
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On Tue, Jul 12, 2016 at 06:33:56PM +0200, Thomas Gleixner wrote:

> Subject: sched/migration: Correct off by one in load migration
> From: Thomas Gleixner <tglx@linutronix.de>
> 
> The move of calc_load_migrate() from CPU_DEAD to CPU_DYING did not take into
> account that the function is now called from a thread running on the outgoing
> CPU. As a result a cpu unplug leakes a load of 1 into the global load
> accounting mechanism.
> 
> Fix it by adjusting for the currently running thread which calls
> calc_load_migrate().
> 
> Fixes: e9cd8fa4fcfd: "sched/migration: Move calc_load_migrate() into CPU_DYING"
> Reported-by: Anton Blanchard <anton@samba.org>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

> +++ b/kernel/sched/loadavg.c
> @@ -78,11 +78,11 @@ void get_avenrun(unsigned long *loads, unsigned long offset, int shift)
>  	loads[2] = (avenrun[2] + offset) << shift;
>  }
>  
> -long calc_load_fold_active(struct rq *this_rq)
> +long calc_load_fold_active(struct rq *this_rq, long adjust)
>  {
>  	long nr_active, delta = 0;
>  
> -	nr_active = this_rq->nr_running;
> +	nr_active = this_rq->nr_running - adjust;
>  	nr_active += (long)this_rq->nr_uninterruptible;
>  
>  	if (nr_active != this_rq->calc_load_active) {

Yeah, I think this is the only sensible approach.

How do you want to route this?