linux-kernel - Re: [RFC PATCH v2] membarrier: expedited private command

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Date:   Tue, 1 Aug 2017 16:32:03 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Nicholas Piggin <npiggin@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Andrew Hunter <ahh@...gle.com>,
        maged michael <maged.michael@...il.com>,
        gromer <gromer@...gle.com>, Avi Kivity <avi@...lladb.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        Dave Watson <davejwatson@...com>
Subject: Re: [RFC PATCH v2] membarrier: expedited private command

On Tue, Aug 01, 2017 at 04:16:54PM +0200, Peter Zijlstra wrote:
> 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 ;-)

Very good, that does give us some guidance, give or take context switches
happening during the IPI latency window.  ;-)

							Thanx, Paul