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

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Date:   Fri, 29 Sep 2017 13:45:18 +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 09:38:53PM +1000, Nicholas Piggin wrote:

> Not really. There is some ability to hold onto a line for a time, but
> there is no way to starve them, let alone starve hundreds of other
> CPUs. They will request the cacheline exclusive and eventually get it.

OK, hardware fairness there is nice.

> I would really prefer to go this way on powerpc first. We could add the
> the registration APIs as basically no-ops, but which would allow the
> locking approach to be changed if we find it causes issues. I'll try to
> find some time and a big system when I can.

Fair enough I suppose.

> > A semi related issue; I suppose we can do a arch upcall to flush_tlb_mm
> > and reset the mm_cpumask when we change cpuset groups.
> 
> For powerpc we have been looking at how mm_cpumask can be improved.
> It has real drawbacks even when you don't consider this new syscall.

What else do you use mm_cpumask for?