Re: [RFC PATCH v2] membarrier: expedited private command

From: Peter Zijlstra
Date: Tue Aug 01 2017 - 04:13:34 EST

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.