linux-kernel - [PATCH] sched: loadavg 0.00, 0.01, 0.05 on idle

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Message-ID: <56A0A38D.4040900@veribox.net>
Date:	Thu, 21 Jan 2016 10:23:25 +0100
From:	Vik Heyndrickx <vik.heyndrickx@...ibox.net>
To:	a.p.zijlstra@...llo.nl
Cc:	linux-kernel@...r.kernel.org
Subject: [PATCH] sched: loadavg 0.00, 0.01, 0.05 on idle

Systems show a minimal load average of 0.00, 0.01, 0.05 even when they 
have no load at all.

Uptime and /proc/loadavg on all systems with kernels released during the 
last five years up until kernel version 4.4, show a 5- and 15-minute 
minimum loadavg of 0.01 and 0.05 respectively. This should be 0.00 on 
idle systems, but the way the kernel calculates this value prevents it 
from getting lower than the mentioned values. Likewise but not as 
obviously noticeable, a fully loaded system with no processes waiting, 
shows a maximum 1/5/15 loadavg of 1.00, 0.99, 0.95 (multiplied by number 
of cores).
By removing the single code line that performed a rounding on the 
internally kept load value, effectively returning this function 
calc_load to its state it had before, the visualization problem is 
completely fixed.
The modified code was tested on nohz=off and nohz kernels. It was tested 
on vanilla kernel 4.4 and on centos 7.1 kernel 3.10.0-327. It was tested 
on single, dual, and octal cores system. It was tested on virtual hosts 
and bare hardware. No unwanted effects have been observed, and the 
problems that the patch intended to fix were indeed gone.

The following patch is for kernel version 4.x . In kernel 3.x, the 
affected code was in core.c instead of loadavg.c

Signed-off-by: Vik Heyndrickx <vik.heyndrickx@...ibox.net>

--- linux-4.4-org/kernel/sched/loadavg.c 2016-01-21 09:11:15 +0100
+++ linux-4.4/kernel/sched/loadavg.c 2016-01-21 09:11:31 +0100
@@ -101,7 +101,6 @@ calc_load(unsigned long load, unsigned l
  {
         load *= exp;
         load += active * (FIXED_1 - exp);
-       load += 1UL << (FSHIFT - 1);
         return load >> FSHIFT;
  }