linux-kernel - Re: [RFC PATCH] sys_membarrier(): system/process-wide memory barrier (x86) (v12)

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Message-ID: <1311724860.20775.1426610187883.JavaMail.zimbra@efficios.com>
Date:	Tue, 17 Mar 2015 16:36:27 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Nicholas Miell <nmiell@...cast.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Alan Cox <gnomes@...rguk.ukuu.org.uk>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Stephen Hemminger <stephen@...workplumber.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Josh Triplett <josh@...htriplett.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Howells <dhowells@...hat.com>
Subject: Re: [RFC PATCH] sys_membarrier(): system/process-wide memory
 barrier (x86) (v12)

> > 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 ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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