linux-kernel - [PATCH 0/2] Per-task load tracking errors

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Date:	Mon,  9 Dec 2013 12:59:08 +0000
From:	Chris Redpath <chris.redpath@....com>
To:	pjt@...gle.com, mingo@...hat.com, peterz@...radead.org,
	alex.shi@...aro.org, morten.rasmussen@....com,
	dietmar.eggemann@....com
Cc:	linux-kernel@...r.kernel.org, Chris Redpath <chris.redpath@....com>
Subject: [PATCH 0/2] Per-task load tracking errors

Hi Paul, Peter etc.

I've found a couple of bugs in the load tracking code and here
is my attempt to fix them. I have some test code available which
can trigger the issues if anyone is interested.

The first one is straightforward. We can leave a number in 
se.avg.decay_count after a short sleep. If that task is later
migrated while runnable, then the left-over decay looks like
unaccounted sleep time so the load is decayed.

The second one is similar. Here we are losing sleep time for a
task if it is migrated while sleeping and the CPU it previously
ran on has entered nohz mode.

I don't really like this fix much, but the root of the problem
is that load tracking more-or-less expects the runqueue's
decay_counter to be up to date, and when nohz is in use it is
not. The fix demonstrates the issue anyway, I haven't seen
other occasions where nohz CPUs distort the tracked load.

Chris Redpath (2):
  sched: reset blocked load decay_count during synchronization
  sched: update runqueue clock before migrations away

 kernel/sched/fair.c |   38 +++++++++++++++++++++++++++++++++-----
 1 file changed, 33 insertions(+), 5 deletions(-)

-- 
1.7.9.5

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