linux-kernel - Re: [RFC PATCH v13] sys_membarrier(): system/process-wide memory barrier (x86)

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Message-ID: <1338809369.22714.1426618883816.JavaMail.zimbra@efficios.com>
Date:	Tue, 17 Mar 2015 19:01:23 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	josh@...htriplett.org, linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Howells <dhowells@...hat.com>
Subject: Re: [RFC PATCH v13] sys_membarrier(): system/process-wide memory
 barrier (x86)

----- Original Message -----
> On Tue, Mar 17, 2015 at 10:57:50AM -0700, josh@...htriplett.org wrote:
> > On Tue, Mar 17, 2015 at 06:30:35PM +0100, Peter Zijlstra wrote:
> > > On Tue, Mar 17, 2015 at 01:22:02PM -0400, Mathieu Desnoyers wrote:
> > > > Here is an implementation of a new system call, sys_membarrier(), which
> > > > executes a memory barrier on either all running threads of the current
> > > > process (MEMBARRIER_PRIVATE) issues a memory barrier on all threads
> > > > running on the system (~MEMBARRIER_PRIVATE). Both are currently
> > > > implemented by calling synchronize_sched().
> > > 
> > > Then why bother with the flag?
> > 
> > Semantically, MEMBARRIER_PRIVATE is allowed to avoid issuing a barrier
> > on CPUs not running the current process if it can, while
> > ~MEMBARRIER_PRIVATE may not.  (The latter would be useful for
> > applications such as system-wide tracing.)  That they're currently both
> > implemented the same way doesn't mean they're semantically equivalent.
> 
> Sure; but why bother with pointless fluff like that? We can always
> introduce the private flag if and when it starts to make sense having
> it.

Without the expedited implementation, the only usefulness of the
private flag is to skip synchronize_sched() if called from a
single-threaded process.

We could easily argue that if a process is using sys_membarrier in
the first place, it's very likely that it is multithreaded. So I
agree that we can drop the flag for now, and add it later on,
e.g. when adding the expedited mode.

I am tempted to leave the "flags" argument in place though, along
with the "MEMBARRIER_QUERY" flag. Thoughts ?

Thanks,

Mathieu


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