linux-kernel - Re: [PATCH] Fix: disable sys_membarrier when nohz

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Message-ID: <20161117095011.1857bca5@gandalf.local.home>
Date:   Thu, 17 Nov 2016 09:50:11 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Josh Triplett <josh@...htriplett.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH] Fix: disable sys_membarrier when nohz_full is enabled

On Thu, 17 Nov 2016 13:54:27 +0000 (UTC)
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

 
> >> > 
> >> > Acked-by: Lai Jiangshan <jiangshanlai@...il.com>
> >> > 
> >> > But I'm afraid, in the future, tick_nohz_full will become a default y
> >> > feature. thus it makes sys_membarrier() always disabled. we might
> >> > need a new MEMBARRIER_CMD_XXX to handle it?  
> >> 
> >> This may require that we send an IPI to nohz_full CPUs, which will
> >> disturb them real-time wise. Any better ideas ?  
> > 
> > Restrict the IPIs to CPUs running the process executing the
> > sys_membarrier() system call.  This would mean that CPUs only
> > are interrupted by their own application's request.  
> 
> This would break use-cases of cross-process shared memory. :-(

Perhaps make this an opt in. That is, all processes that want to be
affected by this can call this function with some flag that sets a flag
in tasks struct. And have that process get an IPI even in no-hz-full
mode if it asked to do it.

-- Steve