Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1752729AbbKPWcS (ORCPT <rfc822;w@1wt.eu>);
	Mon, 16 Nov 2015 17:32:18 -0500
Received: from relay3-d.mail.gandi.net ([217.70.183.195]:32919 "EHLO
	relay3-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751432AbbKPWcR (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Mon, 16 Nov 2015 17:32:17 -0500
Date: Mon, 16 Nov 2015 14:32:11 -0800
From: Josh Triplett <josh@joshtriplett.org>
To: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>,
        John Stultz <john.stultz@linaro.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        Len Brown <len.brown@intel.com>,
        Rafael Wysocki <rafael.j.wysocki@intel.com>,
        Eduardo Valentin <edubezval@gmail.com>, Paul Turner <pjt@google.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: [PATCH 2/4] timer: relax tick stop in idle entry
Message-ID: <20151116223210.GA21522@cloud>
References: <1447444387-23525-1-git-send-email-jacob.jun.pan@linux.intel.com>
 <1447444387-23525-3-git-send-email-jacob.jun.pan@linux.intel.com>
 <alpine.DEB.2.11.1511131516180.3987@nanos>
 <20151113142438.3144d47d@icelake>
 <alpine.DEB.2.11.1511161558550.3761@nanos>
 <20151116135126.5a50e45d@icelake>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20151116135126.5a50e45d@icelake>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 2556
Lines: 54

On Mon, Nov 16, 2015 at 01:51:26PM -0800, Jacob Pan wrote:
> On Mon, 16 Nov 2015 16:06:57 +0100 (CET)
> Thomas Gleixner <tglx@linutronix.de> wrote:
> 
> > >           <idle>-0     [000]    30.093474: bprint:
> > > __tick_nohz_idle_enter: JPAN: tick_nohz_stop_sched_tick 609 delta
> > > 1000000 [JP] but sees delta is exactly 1 tick away. didn't stop
> > > tick.  
> > 
> > If the delta is 1 tick then it is not supposed to stop it. Did you
> > ever try to figure out WHY it is 1 tick?
> > 
> > There are two code pathes which can set it to basemono + TICK_NSEC:
> > 
> >         if (rcu_needs_cpu(basemono, &next_rcu) ||
> >             arch_needs_cpu() || irq_work_needs_cpu()) {
> >                 next_tick = basemono + TICK_NSEC;
> >         } else {
> >                 next_tmr = get_next_timer_interrupt(basejiff,
> > basemono); ts->next_timer = next_tmr;
> >                 /* Take the next rcu event into account */
> >                 next_tick = next_rcu < next_tmr ? next_rcu : next_tmr;
> >         }
> > 
> > Can you please figure out WHY the tick is requested to continue
> > instead of blindly wreckaging the logic in that code?
> 
> Looks like the it hits in both cases during forced idle.
> + Josh
> + Paul
> 
> For the first case, it is always related to RCU. I found there are two
> CONFIG options to avoid this undesired tick in idle loop.
> 1. enable CONFIG_RCU_NOCB_CPU_ALL, offload to orcu kthreads
> 2. or enable CONFIG_RCU_FAST_NO_HZ (enter dytick idle w/ rcu callback)
> 
> Either one works but my concern is that users may not realize the
> intricate CONFIG_ options and how they translate into energy savings.
> Consulted with Josh, it seems we could add a check here to recognize
> the forced idle state and relax rcu_needs_cpu() to return false even it
> has callbacks. Since we are blocking everybody for a short time (5 ticks
> default). It should not impact synchronize and kfree rcu.

Right; as long as you're blocking *everybody*, and RCU priority boosting
doesn't come into play (meaning a real-time task is waiting on RCU
callbacks), then I don't see any harm in blocking RCU callbacks for a
while.  You'd block completion of synchronize_rcu() and similar, as well
as memory reclamation, but since you've blocked *every* CPU systemwide
then that doesn't cause a problem.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/