linux-kernel - Re: [patch v8 4/9] sched: fix slept time double counting in enqueue entity

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Date:	Mon, 17 Jun 2013 23:41:43 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Paul Turner <pjt@...gle.com>
CC:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Namhyung Kim <namhyung@...nel.org>,
	Mike Galbraith <efault@....de>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Michael Wang <wangyun@...ux.vnet.ibm.com>,
	Jason Low <jason.low2@...com>,
	Changlong Xie <changlongx.xie@...el.com>, sgruszka@...hat.com,
	Frédéric Weisbecker <fweisbec@...il.com>
Subject: Re: [patch v8 4/9] sched: fix slept time double counting in enqueue
 entity

On 06/17/2013 07:51 PM, Paul Turner wrote:
> Can you add something like:
> 
> +               /*
> +                * Task re-woke on same cpu (or else
> migrate_task_rq_fair()
> +                * would have made count negative); we must be careful
> to avoid
> +                * double-accounting blocked time after synchronizing
> decays.
> +                */
> 
> 
> Reviewed-by: Paul Turner <pjt@...gle.com>

thanks for review!
---

>From 24d9b43e7a269e6ffee5b874d39812b83812a809 Mon Sep 17 00:00:00 2001
From: Alex Shi <alex.shi@...el.com>
Date: Thu, 9 May 2013 10:54:13 +0800
Subject: [PATCH] sched: fix slept time double counting in enqueue entity

The wakeuped migrated task will __synchronize_entity_decay(se); in
migrate_task_fair, then it needs to set
`se->avg.last_runnable_update -= (-se->avg.decay_count) << 20'
before update_entity_load_avg, in order to avoid slept time is updated
twice for se.avg.load_avg_contrib in both __syncchronize and
update_entity_load_avg.

but if the slept task is waked up from self cpu, it miss the
last_runnable_update before update_entity_load_avg(se, 0, 1), then the
slept time was used twice in both functions.
So we need to remove the double slept time counting.

Signed-off-by: Alex Shi <alex.shi@...el.com>
Reviewed-by: Paul Turner <pjt@...gle.com>
---
 kernel/sched/fair.c |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index df5b8a9..1e5a5e6 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1571,7 +1571,13 @@ static inline void enqueue_entity_load_avg(struct cfs_rq *cfs_rq,
 		}
 		wakeup = 0;
 	} else {
-		__synchronize_entity_decay(se);
+		/*
+		 * Task re-woke on same cpu (or else migrate_task_rq_fair()
+		 * would have made count negative); we must be careful to avoid
+		 * double-accounting blocked time after synchronizing decays.
+		 */
+		se->avg.last_runnable_update += __synchronize_entity_decay(se)
+							<< 20;
 	}
 
 	/* migrated tasks did not contribute to our blocked load */
-- 
1.7.5.4


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