linux-kernel - Re: Rough notes from sys_membarrier() lightning BoF

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Message-ID: <20170921130950.nwfbyiil34psoyua@hirez.programming.kicks-ass.net>
Date:   Thu, 21 Sep 2017 15:09:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Will Deacon <will.deacon@....com>,
        Alan Stern <stern@...land.harvard.edu>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Dave Watson <davejwatson@...com>,
        maged michael <maged.michael@...il.com>
Subject: Re: Rough notes from sys_membarrier() lightning BoF

On Wed, Sep 20, 2017 at 06:13:50PM +0000, Mathieu Desnoyers wrote:

> > Also, can you elaborate on the PPC issue?  PPC appears to track
> > mm_cpumask more or less just like x86.  Is the issue just that this
> > tracking has no implied barriers?  If so, how does TLB flush on ppc
> > work?  It really does seem impressive to me that an architecture can
> > efficiently support munmap() but not an expedited private membarrier.
> 
> I'll leave this question to the PPC experts :)

IIRC PPC does not keep a tight mm_cpumask, it only sets bit, it never
clears bits. The atomic op required to set bits does not imply any
memory barrier on PPC.

TLB invalidation is a TLBI instruction, it sends TLBI broadcast packets
over the interconnect, it doesn't require IPIs like x86.

The only optimization PPC does is that if the mm_cpumask has only a
single bit set, it uses a TLBI instruction without broadcast, which is
cheaper.