Date: Tue, 1 Aug 2017 10:12:30 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Nicholas Piggin <npiggin@gmail.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>, Andrew Hunter <ahh@google.com>,
        maged michael <maged.michael@gmail.com>, gromer <gromer@google.com>,
        Avi Kivity <avi@scylladb.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Palmer Dabbelt <palmer@dabbelt.com>, Dave Watson <davejwatson@fb.com>
Subject: Re: [RFC PATCH v2] membarrier: expedited private command
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On Tue, Aug 01, 2017 at 12:00:47PM +1000, Nicholas Piggin wrote:
> Thanks for this, I'll take a look. This should be a good start as a stress
> test, but I'd also be interested in some application. The reason being that
> for example using runqueue locks may give reasonable maximum throughput
> numbers, but could cause some latency or slowdown when it's used in more
> realistic scenario.

Given this is an unprivileged interface we have to consider DoS and
other such lovely things.  And since we cannot use mm_cpumask() we're
stuck with for_each_online_cpu().

Combined that means that using rq->lock is completely out of the
question, some numbnut doing 'for (;;) sys_membarrier();' can
completely wreck the system.

Yes, it might work for 'normal' workloads, but the interference
potential is just too big.

I have the same problem with Paul's synchronize_rcu_expedited() patch,
that is a machine wide IPI spray and will interfere with unrelated work.