linux-kernel - Re: [PATCH v4 for 4.14 1/3] membarrier: Provide register expedited private command

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Date:   Thu, 28 Sep 2017 17:51:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-arch <linux-arch@...r.kernel.org>,
        Avi Kivity <avi@...lladb.com>,
        maged michael <maged.michael@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Dave Watson <davejwatson@...com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Andrew Hunter <ahh@...gle.com>,
        Paul Mackerras <paulus@...ba.org>,
        Andy Lutomirski <luto@...nel.org>,
        Alan Stern <stern@...land.harvard.edu>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        gromer <gromer@...gle.com>
Subject: Re: [PATCH v4 for 4.14 1/3] membarrier: Provide register expedited
 private command

On Fri, Sep 29, 2017 at 01:01:12AM +1000, Nicholas Piggin wrote:
> That's fine. If a user is not bound to a subset of CPUs, they could
> also cause disturbances with other syscalls and faults, taking locks,
> causing tlb flushes and IPIs and things.

So on the big SGI class machines we've had trouble with
for_each_cpu() loops before, and IIRC the biggest Power box is not too
far from that 1-2K CPUs IIRC.

Bouncing that lock across the machine is *painful*, I have vague
memories of cases where the lock ping-pong was most the time spend.

But only Power needs this, all the other architectures are fine with the
lockless approach for MEMBAR_EXPEDITED_PRIVATE.

The ISYNC variant of the same however appears to want TIF flags or
something to aid a number of archs, the rq->lock will not help there.