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Date: Tue, 19 Jan 2016 14:17:08 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Byungchul Park <byungchul.park@lge.com>,
        Chris Metcalf <cmetcalf@ezchip.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Luiz Capitulino <lcapitulino@redhat.com>,
        Christoph Lameter <cl@linux.com>,
        "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
        Mike Galbraith <efault@gmx.de>, Rik van Riel <riel@redhat.com>
Subject: Re: [RFC PATCH 4/4] sched: Upload nohz full CPU load on task
 enqueue/dequeue
Message-ID: <20160119131708.GF6344@twins.programming.kicks-ass.net>
References: <1452700891-21807-1-git-send-email-fweisbec@gmail.com>
 <1452700891-21807-5-git-send-email-fweisbec@gmail.com>
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On Wed, Jan 13, 2016 at 05:01:31PM +0100, Frederic Weisbecker wrote:
> The full nohz CPU load is currently accounted on tick restart only.
> But there are a few issues with this model:
> 
> _ On tick restart, if cpu_load[0] doesn't contain the load of the actual
>   tickless load that just ran, we are going to account a wrong value.
>   And it is very likely to be so given that cpu_load[0] doesn't have
>   an opportunity to be updated between tick stop and tick restart.
> 
> _ If the runqueue had updates that didn't trigger a tick restart, we
>   are going to miss those CPU load changes.
> 
> A solution to fix this is to update the CPU load everytime we enqueue
> or dequeue a task in the fair runqueue and more than a jiffy occured
> since the last update.

Would not a much better solution be to do this remotely instead of from
one of the hottest functions in the scheduler?