From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S932733AbbLBPJS (ORCPT <rfc822;w@1wt.eu>);
	Wed, 2 Dec 2015 10:09:18 -0500
Received: from casper.infradead.org ([85.118.1.10]:57884 "EHLO
	casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S932425AbbLBPJP (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Wed, 2 Dec 2015 10:09:15 -0500
Date: Wed, 2 Dec 2015 16:09:08 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>, Chris Metcalf <cmetcalf@ezchip.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Luiz Capitulino <lcapitulino@redhat.com>,
        Christoph Lameter <cl@linux.com>, Ingo Molnar <mingo@kernel.org>,
        Viresh Kumar <viresh.kumar@linaro.org>, Rik van Riel <riel@redhat.com>
Subject: Re: [PATCH 2/7] nohz: New tick dependency mask
Message-ID: <20151202150908.GR3816@twins.programming.kicks-ass.net>
References: <1447424529-13671-1-git-send-email-fweisbec@gmail.com>
 <1447424529-13671-3-git-send-email-fweisbec@gmail.com>
 <20151201204109.GN17308@twins.programming.kicks-ass.net>
 <20151201222025.GA31973@lerouge>
 <20151202105633.GQ3816@twins.programming.kicks-ass.net>
 <20151202140815.GB14766@lerouge>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20151202140815.GB14766@lerouge>
User-Agent: Mutt/1.5.21 (2012-12-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, Dec 02, 2015 at 03:08:16PM +0100, Frederic Weisbecker wrote:
> On Wed, Dec 02, 2015 at 11:56:33AM +0100, Peter Zijlstra wrote:
> > On Tue, Dec 01, 2015 at 11:20:28PM +0100, Frederic Weisbecker wrote:
> > > > > +	prev = fetch_or(dep, BIT_MASK(bit));
> > > > > +	if (!prev) {
> > > > > +		/*
> > > > > +		* We need the IPIs to be sent from sane process context.
> > > > 
> > > > Why ?
> > > 
> > > Because posix timers code is all called with interrupts disabled and we can't
> > > send IPIs then.
> > > 
> > > > 
> > > > > +		* The posix cpu timers are always set with irqs disabled.
> > > > > +		*/
> > > > > +		schedule_work(&kick_all_work);
> > > > > +	}
> > > > > +}
> > > > > +
> > > > > +/*
> > > > > + * Set a global tick dependency. Lets do the wide IPI kick asynchronously
> > > > > + * for callers with irqs disabled.
> > > > 
> > > > This seems to suggest you can call this with IRQs disabled
> > > 
> > > Ah right, that's a misleading comment. We need to use the _delayed() version
> > > when interrupts are disabled.
> > 
> > Why can't you use tick_nohz_full_kick_cpu() for all that, which is
> > usable from IRQ context and avoid all that delayed muck?
> 
> Because I need to kick all the CPUs where the task/signal is running on. That's
> a bit difficult to do though. I think we had something to try to send an IPI to a task,
> but I can't retrieve it. Looks easy to do anyway. But in the signal case I'd need to do
> that for all tasks in the group. That sounds like a costly loop.
> 
> So I simplify that with a global IPI.

Sure, but what I meant was that:

	for_each_cpu(cpu, mask)
		tick_nohz_full_kick_cpu(cpu);

is IRQ safe, whereas the current thingy is not. Sure, its a wee bit less
efficient, but do we really care?