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Date: Tue, 1 Aug 2017 16:16:54 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Nicholas Piggin <npiggin@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
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        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
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Subject: Re: [RFC PATCH v2] membarrier: expedited private command
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On Tue, Aug 01, 2017 at 06:23:09AM -0700, Paul E. McKenney wrote:
> On Tue, Aug 01, 2017 at 12:22:03PM +0200, Peter Zijlstra wrote:
> 
> [ . . . ]
> 
> > As to scheduler IPIs, those are limited to the CPUs the user is limited
> > to and are rate limited by the wakeup-latency of the tasks. After all,
> > all the time a task is runnable but not running, wakeups are no-ops.
> 
> Can't that wakeup-latency limitation be overcome by a normal user simply
> by having lots of tasks to wake up, which then go back to sleep almost
> immediately?  Coupled with very a low-priority CPU-bound task on each CPU?

Let me put it like this; there is no way to cause more interference
using IPIs then there is simply running while(1) loops ;-)