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Date: Tue, 17 Mar 2015 10:55:33 -0700
From: josh@joshtriplett.org
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Nicholas Miell <nmiell@comcast.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@redhat.com>, Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
        Lai Jiangshan <laijs@cn.fujitsu.com>,
        Stephen Hemminger <stephen@networkplumber.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        David Howells <dhowells@redhat.com>
Subject: Re: [RFC PATCH] sys_membarrier(): system/process-wide memory
 barrier (x86) (v12)
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On Tue, Mar 17, 2015 at 04:36:27PM +0000, Mathieu Desnoyers wrote:
> > > On Tue, Mar 17, 2015 at 01:45:25AM +0000, Mathieu Desnoyers wrote:
> [...]
> > > 
> > > > Would you see it as acceptable if we start by implementing
> > > > only the non-expedited sys_membarrier() ?
> > > 
> > > Sure.
> > > 
> > > > Then we can add
> > > > the expedited-private implementation after rq->curr becomes
> > > > available through RCU.
> > > 
> > > Yeah, or not at all; I'm still trying to get Paul to remove the
> > > expedited nonsense from the kernel RCU bits; and now you want it in
> > > userspace too :/
> > 
> > The non-expedited case makes sense when we batch RCU work
> > with call_rcu. However, some users require to use synchronize_rcu()
> > directly after modifying their data structure. Therefore, the
> > latency associated with sys_membarrier() then becomes important,
> > hence the interest for an expedited scheme.
> > 
> > I agree that we should try to find a way to implement it with
> > low disturbance on the CPU's rq locks. I'd be certainly
> > OK with starting with just the non-expedited scheme, and add
> > the expedited scheme later on. This is why we have the flags
> > anyway.
> 
> Paul, I'm currently reworking the patch to keep only the
> non-expedited scheme. I don't need to touch the scheduler
> internals anymore, so should I move the sys_membarrier
> system call implementation into kernel/rcu/update.c ?

If you don't need access to scheduler internals, I'd suggest putting it
in its own file (something like kernel/membarrier.c), so that you can
use obj-$(CONFIG_MEMBARRIER) in a Makefile to enable/disable it rather
than an #ifdef in a .c file.

- Josh Triplett