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From:   Aubrey Li <aubrey.intel@gmail.com>
Date:   Thu, 4 Apr 2019 16:31:35 +0800
Message-ID: <CAERHkrv1W4vC9h1g1iifcdLeMAHar3KiHS=hP=Y2iUyM2wswhg@mail.gmail.com>
Subject: Re: [RFC][PATCH 15/16] sched: Trivial forced-newidle balancer
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Valentin Schneider <valentin.schneider@arm.com>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paul Turner <pjt@google.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Subhra Mazumdar <subhra.mazumdar@oracle.com>,
        =?UTF-8?B?RnLDqWTDqXJpYyBXZWlzYmVja2Vy?= <fweisbec@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Greg Kerr <kerrnel@google.com>
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Sender: linux-kernel-owner@vger.kernel.org
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On Fri, Feb 22, 2019 at 12:42 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Feb 21, 2019 at 04:19:46PM +0000, Valentin Schneider wrote:
> > Hi,
> >
> > On 18/02/2019 16:56, Peter Zijlstra wrote:
> > [...]
> > > +static bool try_steal_cookie(int this, int that)
> > > +{
> > > +   struct rq *dst = cpu_rq(this), *src = cpu_rq(that);
> > > +   struct task_struct *p;
> > > +   unsigned long cookie;
> > > +   bool success = false;
> > > +
> > > +   local_irq_disable();
> > > +   double_rq_lock(dst, src);

Here, should we check dst and src's rq status before lock their rq?
if src is idle, it could be in the progress of load balance already?

Thanks,
-Aubrey

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3e3162f..a1e0a6f 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3861,6 +3861,13 @@ static bool try_steal_cookie(int this, int that)
        unsigned long cookie;
        bool success = false;

+       /*
+        * Don't steal if src is idle or has only one runnable task,
+        * or dst has more than one runnable task
+        */
+       if (src->nr_running <= 1 || unlikely(dst->nr_running >= 1))
+               return false;
+
        local_irq_disable();
        double_rq_lock(dst, src);