Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752318AbdIRSqq (ORCPT ); Mon, 18 Sep 2017 14:46:46 -0400 Received: from iolanthe.rowland.org ([192.131.102.54]:51196 "HELO iolanthe.rowland.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1750998AbdIRSqo (ORCPT ); Mon, 18 Sep 2017 14:46:44 -0400 Date: Mon, 18 Sep 2017 14:46:43 -0400 (EDT) From: Alan Stern X-X-Sender: stern@iolanthe.rowland.org To: Mathieu Desnoyers cc: Peter Zijlstra , "Paul E . McKenney" , , Boqun Feng , Andrew Hunter , Maged Michael , , Avi Kivity , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Dave Watson , Will Deacon , Andy Lutomirski , Subject: Re: [RFC PATCH 1/2] membarrier: Provide register expedited private command In-Reply-To: <20170918165201.16264-1-mathieu.desnoyers@efficios.com> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1828 Lines: 54 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 > +++ 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