From: Michael Ellerman <mpe@ellerman.id.au>
To: Peter Zijlstra <peterz@infradead.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org, Boqun Feng <boqun.feng@gmail.com>,
        Andrew Hunter <ahh@google.com>,
        Maged Michael <maged.michael@gmail.com>, gromer@google.com,
        Avi Kivity <avi@scylladb.com>, Nicholas Piggin <npiggin@gmail.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Palmer Dabbelt <palmer@dabbelt.com>
Subject: Re: [RFC PATCH v2] membarrier: expedited private command
In-Reply-To: <20170728115702.5vgnvwhmbbmyrxbf@hirez.programming.kicks-ass.net>
References: <20170727211314.32666-1-mathieu.desnoyers@efficios.com> <20170728085532.ylhuz2irwmgpmejv@hirez.programming.kicks-ass.net> <20170728115702.5vgnvwhmbbmyrxbf@hirez.programming.kicks-ass.net>
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Date: Mon, 31 Jul 2017 23:20:59 +1000
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Peter Zijlstra <peterz@infradead.org> writes:

> On Fri, Jul 28, 2017 at 10:55:32AM +0200, Peter Zijlstra wrote:
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index e9785f7aed75..33f34a201255 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2641,8 +2641,18 @@ static struct rq *finish_task_switch(struct task_struct *prev)
>>  	finish_arch_post_lock_switch();
>>  
>>  	fire_sched_in_preempt_notifiers(current);
>> +
>> +	/*
>> +	 * For CONFIG_MEMBARRIER we need a full memory barrier after the
>> +	 * rq->curr assignment. Not all architectures have one in either
>> +	 * switch_to() or switch_mm() so we use (and complement) the one
>> +	 * implied by mmdrop()'s atomic_dec_and_test().
>> +	 */
>>  	if (mm)
>>  		mmdrop(mm);
>> +	else if (IS_ENABLED(CONFIG_MEMBARRIER))
>> +		smp_mb();
>> +
>>  	if (unlikely(prev_state == TASK_DEAD)) {
>>  		if (prev->sched_class->task_dead)
>>  			prev->sched_class->task_dead(prev);
>> 
>> 
>
>> a whole bunch of architectures don't in fact need this extra barrier at all.
>
> In fact, I'm fairly sure its only PPC.
>
> Because only ARM64 and PPC actually implement ACQUIRE/RELEASE with
> anything other than smp_mb() (for now, Risc-V is in this same boat and
> MIPS could be if they ever sort out their fancy barriers).
>
> TSO archs use a regular STORE for RELEASE, but all their atomics imply a
> smp_mb() and there are enough around to make one happen (typically
> mm_cpumask updates).
>
> Everybody else, aside from ARM64 and PPC must use smp_mb() for
> ACQUIRE/RELEASE.
>
> ARM64 has a super duper barrier in switch_to().
>
> Which only leaves PPC stranded.. but the 'good' news is that mpe says
> they'll probably need a barrier in switch_mm() in any case.

I may have been sleep deprived. We have a patch, probably soon to be
merged, which will add a smp_mb() in switch_mm() but *only* when we add
a CPU to mm_cpumask, ie. when we run on a CPU we haven't run on before.

I'm not across membarrier enough to know if that's sufficient, but it
seems unlikely?

cheers