linux-kernel - Re: [RFC,v2 1/3] sched: set loop

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Message-ID: <20170209121458.GC6500@twins.programming.kicks-ass.net>
Date:   Thu, 9 Feb 2017 13:14:58 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Uladzislau Rezki <urezki@...il.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Uladzislau 2 Rezki <uladzislau2.rezki@...ymobile.com>
Subject: Re: [RFC,v2 1/3] sched: set loop_max after rq lock is taken

On Wed, Feb 08, 2017 at 09:43:27AM +0100, Uladzislau Rezki wrote:
> From: Uladzislau 2 Rezki <uladzislau2.rezki@...ymobile.com>
> 
> While doing a load balance there is a race in setting
> loop_max variable since nr_running can be changed causing
> incorect iteration loops.
> 
> As a result we may skip some candidates or check the same
> tasks again.

When doing the actual migration we'll drop this lock again and
nr_running can change again.

This cannot be done perfectly, all of load-balancing is riddled with
races like this, nobody cares.