linux-kernel - weakness of runnable load tracking?

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Message-ID: <CAGjg+kFZ2+tzxOS4SVo3PTzEDJJ=B0gZJ0aEOhNHvTycFuVT6Q@mail.gmail.com>
Date:	Wed, 5 Dec 2012 23:19:37 +0800
From:	Alex Shi <lkml.alex@...il.com>
To:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Turner <pjt@...gle.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Alex Shi <alex.shi@...el.com>
Subject: weakness of runnable load tracking?

Hi Paul&Ingo:

Runnable load tracking patch set introduce a good way to tracking each
entity/rq's running time.
But when I try to enable it in load balance, I found burst forking
many new tasks will make just few cpu heavy while other cpu has no
much task assigned. That is due to the new forked task's
load_avg_contrib is zero after just created. then no matter how many
tasks assigned to a CPU can not increase the cfs_rq->runnable_load_avg
or rq->avg.load_avg_contrib if this cpu idle.
Actually, if just for new task issue, we can set new task's initial
load_avg same as load_weight. but if we want to burst wake up many
long time sleeping tasks, it has the same issue here since their were
decayed to zero. So what solution I can thought is recording the se's
load_avg_contrib just before dequeue, and don't decay the value, when
it was waken up, add this value to new cfs_rq. but if so, the runnable
load tracking is total meaningless.
So do you have some idea of burst wakeup balancing with runnable load tracking?

-- 
Thanks
    Alex
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