linux-kernel - Re: [PATCH 05/10] smp: Fast path check on IPI list

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Message-ID: <20140729120739.GX3935@laptop>
Date:	Tue, 29 Jul 2014 14:07:39 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH 05/10] smp: Fast path check on IPI list

On Mon, Jul 28, 2014 at 07:37:31PM +0200, Frederic Weisbecker wrote:
> When we enqueue a remote irq work, we trigger the same IPI as those
> raised by smp_call_function_*() family.
> 
> So when we receive such IPI, we check both irq_work and smp_call_function
> queues. Thus if we trigger a remote irq work, we'll likely find the
> smp_call_function queue empty unless we collide with concurrent enqueuers
> but the probability is low.
> 
> Meanwhile, checking the smp_call_function queue can be costly because
> we use llist_del_all() which relies on cmpxchg().

xchg()
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