linux-kernel - false nr_running check in load balance?

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Date:	Tue, 6 Aug 2013 21:23:46 +0800
From:	Lei Wen <adrian.wenl@...il.com>
To:	Paul Turner <pjt@...gle.com>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, leiwen@...vell.com
Subject: false nr_running check in load balance?

Hi Paul,

I notice in load_balance function, it would check busiest->nr_running
to decide whether to perform the real task movement.

But in some case, I saw the nr_running is not matching with
the task in the queue, which seems make scheduler to do many redundant
checking.
What I means is like there is only one task in the queue, but nr_running
shows it has two. So if that task cannot be moved, it would be still checked
for twice.

With further checking, I find there is one patch you submit before:
commit 953bfcd10e6f3697233e8e5128c611d275da39c1
Author: Paul Turner <pjt@...gle.com>
Date:   Thu Jul 21 09:43:27 2011 -0700

    sched: Implement hierarchical task accounting for SCHED_OTHER

In this patch, you increase nr_running when enqueue enqueue_task_stop,
which is the reason nr_running is increase while task not be increased.
It is true at that time, the stopper has been waken up and enqueue again
into cpu, and do the migration job. So the logic should be right there.

My question is whether we could change the judgment into cfs_rq->nr_running?
Since the load_balance is only for cfs, right?

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index bb456f4..ffc0d35 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5096,7 +5096,7 @@ redo:
        schedstat_add(sd, lb_imbalance[idle], env.imbalance);

        ld_moved = 0;
-       if (busiest->nr_running > 1) {
+       if (busiest->cfs.nr_running > 1) {
                /*
                 * Attempt to move tasks. If find_busiest_group has found
                 * an imbalance but busiest->nr_running <= 1, the group is

Thanks,
Lei
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