Date: Thu, 2 Jul 2015 09:05:39 +0800
From: Yuyang Du <yuyang.du@intel.com>
To: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
        Rabin Vincent <rabin.vincent@axis.com>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Paul Turner <pjt@google.com>, Ben Segall <bsegall@google.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>
Subject: Re: [PATCH?] Livelock in pick_next_task_fair() / idle_balance()
Message-ID: <20150702010539.GB5197@intel.com>
References: <20150630143057.GA31689@axis.com>
 <1435728995.9397.7.camel@gmail.com>
 <20150701145551.GA15690@axis.com>
 <20150701204404.GH25159@twins.programming.kicks-ass.net>
 <20150701232511.GA5197@intel.com>
 <1435824347.5351.18.camel@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1435824347.5351.18.camel@gmail.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1305
Lines: 31

Hi Mike,

On Thu, Jul 02, 2015 at 10:05:47AM +0200, Mike Galbraith wrote:
> On Thu, 2015-07-02 at 07:25 +0800, Yuyang Du wrote:
> 
> > That being said, it is also obvious to prevent the livelock from happening:
> > idle pulling until the source rq's nr_running is 1, becuase otherwise we
> > just avoid idleness by making another idleness.
> 
> Yeah, but that's just the symptom, not the disease.  Better for the idle
> balance symptom may actually be to only pull one when idle balancing.
> After all, the immediate goal is to find something better to do than
> idle, not to achieve continual perfect (is the enemy of good) balance.
> 
Symptom? :)

You mean "pull one and stop, can't be greedy"? Right, but still need to
assure you don't make another idle CPU (meaning until nr_running == 1), which
is the cure to disease.

I am ok with at most "pull one", but probably we stick to the load_balance()
by pulling an fair amount, assuming load_balance() magically computes the
right imbalance, otherwise you may have to do multiple "pull one"s.

Thanks,
Yuyang
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/