linux-kernel - Re: [RFC PATCH 1/2] membarrier: Provide register expedited private command

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Date:   Mon, 18 Sep 2017 14:46:43 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:     Peter Zijlstra <peterz@...radead.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        <linux-kernel@...r.kernel.org>, Boqun Feng <boqun.feng@...il.com>,
        Andrew Hunter <ahh@...gle.com>,
        Maged Michael <maged.michael@...il.com>, <gromer@...gle.com>,
        Avi Kivity <avi@...lladb.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Dave Watson <davejwatson@...com>,
        Will Deacon <will.deacon@....com>,
        Andy Lutomirski <luto@...nel.org>, <linux-arch@...r.kernel.org>
Subject: Re: [RFC PATCH 1/2] membarrier: Provide register expedited private
 command

On Mon, 18 Sep 2017, Mathieu Desnoyers wrote:

> Provide a new command allowing processes to register their intent to use
> the private expedited command.
> 
> This allows PowerPC to skip the full memory barrier in switch_mm(), and
> only issue the barrier when scheduling into a task belonging to a
> process that has registered to use expedited private.
> 
> Processes are now required to register before using
> MEMBARRIER_CMD_PRIVATE_EXPEDITED, otherwise that command returns EPERM.
> 
> [ Runtime testing on the PowerPC architecture would be welcome. ]
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>

> +++ b/arch/powerpc/include/asm/membarrier.h
> @@ -0,0 +1,37 @@
> +#ifndef _ASM_POWERPC_MEMBARRIER_H
> +#define _ASM_POWERPC_MEMBARRIER_H
> +
> +static inline void membarrier_arch_sched_in(struct task_struct *prev,
> +		struct task_struct *next)
> +{
> +	/*
> +	 * Only need the full barrier when switching between processes.
> +	 */
> +	if (likely(!test_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED)
> +			|| prev->mm == next->mm))
> +		return;
> +
> +	/*
> +	 * The membarrier system call requires a full memory barrier
> +	 * after storing to rq->curr, before going back to user-space.
> +	 */
> +	smp_mb();
> +}
> +static inline void membarrier_arch_fork(struct task_struct *t,
> +		unsigned long clone_flags)
> +{
> +	/*
> +	 * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread
> +	 * fork is protected by siglock. membarrier_arch_fork is called
> +	 * with siglock held.
> +	 */
> +	if (t->mm->membarrier_private_expedited)
> +		set_ti_thread_flag(t, TIF_MEMBARRIER_PRIVATE_EXPEDITED);
> +}

Why have two separate bitflags for the same thing?  Can't you just use
the mm->membarrier_private_expedited flag everywhere and forget about 
TIF_MEMBARRIER_PRIVATE_EXPEDITED?

Alan Stern