linux-kernel - Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option

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Date:   Wed, 26 Jul 2017 08:42:29 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        dipankar <dipankar@...ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        rostedt <rostedt@...dmis.org>,
        David Howells <dhowells@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        fweisbec <fweisbec@...il.com>, Oleg Nesterov <oleg@...hat.com>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option

On Wed, Jul 26, 2017 at 09:46:56AM +0200, Peter Zijlstra wrote:
> On Tue, Jul 25, 2017 at 10:50:13PM +0000, Mathieu Desnoyers wrote:
> > This would implement a MEMBARRIER_CMD_PRIVATE_EXPEDITED (or such) flag
> > for expedited process-local effect. This differs from the "SHARED" flag,
> > since the SHARED flag affects threads accessing memory mappings shared
> > across processes as well.
> > 
> > I wonder if we could create a MEMBARRIER_CMD_SHARED_EXPEDITED behavior
> > by iterating on all memory mappings mapped into the current process,
> > and build a cpumask based on the union of all mm masks encountered ?
> > Then we could send the IPI to all cpus belonging to that cpumask. Or
> > am I missing something obvious ?
> 
> I would readily object to such a beast. You far too quickly end up
> having to IPI everybody because of some stupid shared map or something
> (yes I know, normal DSOs are mapped private).

Agreed, we should keep things simple to start with.  The user can always
invoke sys_membarrier() from each process.

							Thanx, Paul

> The whole private_expedited is only palatable because we can only hinder
> our own threads (much).
>