linux-kernel - Re: [PATCH 5/5] nohz: Use IPI implicit full barrier against rq->nr

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Date:	Tue, 3 Jun 2014 17:02:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kevin Hilman <khilman@...aro.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH 5/5] nohz: Use IPI implicit full barrier against
 rq->nr_running r/w

On Tue, Jun 03, 2014 at 04:40:20PM +0200, Frederic Weisbecker wrote:
> A full dynticks CPU is allowed to stop its tick when a single task runs.
> Meanwhile when a new task gets enqueued, the CPU must be notified so that
> it can restart its tick to maintain local fairness and other accounting
> details.
> 
> This notification is performed by way of an IPI. Then when the target
> receives the IPI, we expect it to see the new value of rq->nr_running.
> 
> Hence the following ordering scenario:
> 
>    CPU 0                   CPU 1
> 
>    write rq->running       get IPI
>    smp_wmb()               smp_rmb()
>    send IPI                read rq->nr_running
> 
> But Paul Mckenney says that nowadays IPIs imply a full barrier on
> all architectures. So we can safely remove this pair and rely on the
> implicit barriers that come along IPI send/receive. Lets
> just comment on this new assumption.
> 
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Kevin Hilman <khilman@...aro.org>
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Acked-by: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Viresh Kumar <viresh.kumar@...aro.org>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>

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