On Thu, Jan 21, 2016 at 10:23:25AM +0100, Vik Heyndrickx wrote: > Systems show a minimal load average of 0.00, 0.01, 0.05 even when they have > no load at all.
Thanks, I've edited the patch Changelog to include a few extra details you mentioned in our preview correspondence. See below. Please let me know if you're OK with this. --- Subject: sched: Fix non-zero idle loadavg From: Vik Heyndrickx <[email protected]> Date: Thu, 21 Jan 2016 10:23:25 +0100 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. Once the (old) load becomes 93 or higher, it mathematically can never get lower than 93, even when the active (load) remains 0 forever. This results in the strange 0.00, 0.01, 0.05 uptime values on idle systems. Note: 93/2048 = 0.0454..., which rounds up to 0.05. It is not correct to add a 0.5 rounding (=1024/2048) here, since the result from this function is fed back into the next iteration again, so the result of that +0.5 rounding value then gets multiplied by (2048-2037), and then rounded again, so there is a virtual "ghost" load created, next to the old and active load terms. 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. Fixes: 0f004f5a696a ("sched: Cure more NO_HZ load average woes") Cc: Doug Smythies <[email protected]> Signed-off-by: Vik Heyndrickx <[email protected]> [Changelog edits] Signed-off-by: Peter Zijlstra (Intel) <[email protected]> Link: http://lkml.kernel.org/r/[email protected] --- kernel/sched/loadavg.c | 1 - 1 file changed, 1 deletion(-) --- a/kernel/sched/loadavg.c +++ b/kernel/sched/loadavg.c @@ -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; }
