linux-kernel - Re: [RFC PATCH 0/4] sched: Improve cpu load accounting with nohz

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Date:	Tue, 19 Jan 2016 14:04:57 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Dietmar Eggemann <dietmar.eggemann@....com>
Cc:	Byungchul Park <byungchul.park@....com>, perterz@...radead.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>, Rik van Riel <riel@...hat.com>
Subject: Re: [RFC PATCH 0/4] sched: Improve cpu load accounting with nohz

On Fri, Jan 15, 2016 at 04:56:36PM +0000, Dietmar Eggemann wrote:
> Couldn't we set tickless_load only in case:
> 
> unsigned long tickless_load = (active && pending_updates > 1) ?
> this_rq->cpu_load[0] : 0;
> 
> Even though update_cpu_load_nohz() can call with pending_updates=1 and
> active=1 but then we don't have to decay.

decay_load_missed() has an early bail for !missed, which will be tickled
with pending_updates == 1.

What I was thinking of doing however is:

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4445,13 +4445,15 @@ static void __update_cpu_load(struct rq
 
 		old_load = this_rq->cpu_load[i];
 		old_load = decay_load_missed(old_load, pending_updates - 1, i);
-		old_load -= decay_load_missed(tickless_load, pending_updates - 1, i);
-		/*
-		 * old_load can never be a negative value because a decayed
-		 * tickless_load cannot be greater than the original
-		 * tickless_load.
-		 */
-		old_load += tickless_load;
+		if (tickless_load) {
+			old_load -= decay_load_missed(tickless_load, pending_updates - 1, i);
+			/*
+			 * old_load can never be a negative value because a
+			 * decayed tickless_load cannot be greater than the
+			 * original tickless_load.
+			 */
+			old_load += tickless_load;
+		}
 		new_load = this_load;
 		/*
 		 * Round up the averaging division if load is increasing. This


Since regardless of the pending_updates, none of that makes sense if
!tickless_load.