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

From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [RFC PATCH v2] membarrier: expedited private command
Date	2017-08-01 10:20 +0200
Message-ID	<u9Azf-Lh-13@gated-at.bofh.it> (permalink)
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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.

Thread

Re: [RFC PATCH v2] membarrier: expedited private command Michael Ellerman <mpe@ellerman.id.au> - 2017-07-31 15:30 +0200
  Re: [RFC PATCH v2] membarrier: expedited private command Peter Zijlstra <peterz@infradead.org> - 2017-07-31 15:40 +0200
  Re: [RFC PATCH v2] membarrier: expedited private command Nicholas Piggin <npiggin@gmail.com> - 2017-08-01 02:40 +0200
    Re: [RFC PATCH v2] membarrier: expedited private command Mathieu Desnoyers <mathieu.desnoyers@efficios.com> - 2017-08-01 03:40 +0200
      Re: [RFC PATCH v2] membarrier: expedited private command Nicholas Piggin <npiggin@gmail.com> - 2017-08-01 04:10 +0200
        Re: [RFC PATCH v2] membarrier: expedited private command Peter Zijlstra <peterz@infradead.org> - 2017-08-01 10:20 +0200
          Re: [RFC PATCH v2] membarrier: expedited private command Nicholas Piggin <npiggin@gmail.com> - 2017-08-01 12:00 +0200
            Re: [RFC PATCH v2] membarrier: expedited private command Peter Zijlstra <peterz@infradead.org> - 2017-08-01 12:30 +0200
              Re: [RFC PATCH v2] membarrier: expedited private command Avi Kivity <avi@scylladb.com> - 2017-08-01 12:40 +0200
                Re: [RFC PATCH v2] membarrier: expedited private command Peter Zijlstra <peterz@infradead.org> - 2017-08-01 12:50 +0200
              Re: [RFC PATCH v2] membarrier: expedited private command Nicholas Piggin <npiggin@gmail.com> - 2017-08-01 12:40 +0200
                Re: [RFC PATCH v2] membarrier: expedited private command Peter Zijlstra <peterz@infradead.org> - 2017-08-01 13:10 +0200
                Re: [RFC PATCH v2] membarrier: expedited private command Nicholas Piggin <npiggin@gmail.com> - 2017-08-01 14:00 +0200
              Re: [RFC PATCH v2] membarrier: expedited private command "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-08-01 15:30 +0200
                Re: [RFC PATCH v2] membarrier: expedited private command Peter Zijlstra <peterz@infradead.org> - 2017-08-01 16:20 +0200
                Re: [RFC PATCH v2] membarrier: expedited private command "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-08-02 01:40 +0200
                Re: [RFC PATCH v2] membarrier: expedited private command Nicholas Piggin <npiggin@gmail.com> - 2017-08-02 02:50 +0200

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