linux-kernel - Re: [RFC PATCH v2] membarrier: expedited private command

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Date:   Tue, 1 Aug 2017 12:46:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Avi Kivity <avi@...lladb.com>
Cc:     Nicholas Piggin <npiggin@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Andrew Hunter <ahh@...gle.com>,
        maged michael <maged.michael@...il.com>,
        gromer <gromer@...gle.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        Dave Watson <davejwatson@...com>
Subject: Re: [RFC PATCH v2] membarrier: expedited private command

On Tue, Aug 01, 2017 at 01:32:43PM +0300, Avi Kivity wrote:
> I hate to propose a way to make this more complicated, but this could be
> fixed by a process first declaring its intent to use expedited process-wide
> membarrier; if it does, then every context switch updates a process-wide
> cpumask indicating which cpus are currently running threads of that process:
> 
>   if (prev->mm != next->mm)
>       if (prev->mm->running_cpumask)
>              cpumask_clear(...);
>       else if (next->mm->running_cpumask)
>              cpumask_set(...);
> 
> now only processes that want expedited process-wide membarrier pay for it
> (in other than some predictable branches). You can even have threads opt-in,
> so unrelated threads that don't participate in the party don't cause those
> bits to be set.

Either that or conditionally put in a smp_mb in switch_mm() for that
process. But yes, once we advertise intent (either explicit or implicit
on first use), there's various things that can be done.