Date: Tue, 1 Aug 2017 12:46:16 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Avi Kivity <avi@scylladb.com>
Cc: Nicholas Piggin <npiggin@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>, Andrew Hunter <ahh@google.com>,
        maged michael <maged.michael@gmail.com>, gromer <gromer@google.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Palmer Dabbelt <palmer@dabbelt.com>, Dave Watson <davejwatson@fb.com>
Subject: Re: [RFC PATCH v2] membarrier: expedited private command
Message-ID: <20170801104616.fmw5zcag2ducwsn6@hirez.programming.kicks-ass.net>
References: <20170728085532.ylhuz2irwmgpmejv@hirez.programming.kicks-ass.net>
 <20170728115702.5vgnvwhmbbmyrxbf@hirez.programming.kicks-ass.net>
 <87tw1s4u9w.fsf@concordia.ellerman.id.au>
 <20170731233731.32e68f6d@roar.ozlabs.ibm.com>
 <973223324.694.1501551189603.JavaMail.zimbra@efficios.com>
 <20170801120047.61c59064@roar.ozlabs.ibm.com>
 <20170801081230.GF6524@worktop.programming.kicks-ass.net>
 <20170801195717.7a675cc2@roar.ozlabs.ibm.com>
 <20170801102203.urldoripgbh2ohun@hirez.programming.kicks-ass.net>
 <3db6b20b-df76-8284-5bc1-37a511ee0534@scylladb.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <3db6b20b-df76-8284-5bc1-37a511ee0534@scylladb.com>
User-Agent: NeoMutt/20170609 (1.8.3)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1003
Lines: 21

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.