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 S1754511AbbKJRAT (ORCPT <rfc822;w@1wt.eu>);
	Tue, 10 Nov 2015 12:00:19 -0500
Received: from bombadil.infradead.org ([198.137.202.9]:45801 "EHLO
	bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751202AbbKJRAS (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Tue, 10 Nov 2015 12:00:18 -0500
Date: Tue, 10 Nov 2015 18:00:10 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Rafael Wysocki <rafael.j.wysocki@intel.com>,
        Len Brown <len.brown@intel.com>, Andi Kleen <andi.kleen@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>, Paul Turner <pjt@google.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Eduardo Valentin <edubezval@gmail.com>,
        Punit Agrawal <punit.agrawal@arm.com>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Subject: Re: [RFC PATCH v2 3/3] sched: introduce synchronized idle injection
Message-ID: <20151110170010.GH17308@twins.programming.kicks-ass.net>
References: <1447114883-23851-1-git-send-email-jacob.jun.pan@linux.intel.com>
 <1447114883-23851-4-git-send-email-jacob.jun.pan@linux.intel.com>
 <20151110132324.GC17308@twins.programming.kicks-ass.net>
 <20151110060116.26cd5ff8@yairi>
 <20151110145823.GD17308@twins.programming.kicks-ass.net>
 <20151110082859.1493f106@icelake>
 <20151110163646.GG17308@twins.programming.kicks-ass.net>
 <20151110085055.67bdb1fa@icelake>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20151110085055.67bdb1fa@icelake>
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 Tue, Nov 10, 2015 at 08:50:55AM -0800, Jacob Pan wrote:
> On Tue, 10 Nov 2015 17:36:46 +0100
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > > The downside is that we need to restart the timers every time if
> > > user were to change injection parameters, i.e. duration and percent.
> > > Or do locking which might be too expensive. In the previous
> > > approach, it will naturally catch up the parameter change.  
> > 
> > Why? the timer will fire and observe the new value for reprogramming
> > the next period. All you need to do is to ensure whole values are
> > written/read -- ie. avoid load/store tearing.
> Different per CPU timer may intercept parameter changes at slightly
> different time, so there is a race condition such that some CPUs may
> catch the period change later by one period, which results in a correct
> period change but at a different time, i.e. out of sync.

Ah yes. So if the locking hurts I can come up with a lockless algorithm
for this. Shouldn't be too hard.