'\" t
.\"     Title: cpu_load_update
.\"    Author: [FIXME: author] [see http://docbook.sf.net/el/author]
.\" Generator: DocBook XSL Stylesheets v1.79.1 <http://docbook.sf.net/>
.\"      Date: January 2017
.\"    Manual: Driver Basics
.\"    Source: Kernel Hackers Manual 4.8.15
.\"  Language: English
.\"
.TH "CPU_LOAD_UPDATE" "9" "January 2017" "Kernel Hackers Manual 4\&.8\&." "Driver Basics"
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.SH "NAME"
cpu_load_update \- update the rq\->cpu_load[] statistics
.SH "SYNOPSIS"
.HP \w'void\ cpu_load_update('u
.BI "void cpu_load_update(struct\ rq\ *\ " "this_rq" ", unsigned\ long\ " "this_load" ", unsigned\ long\ " "pending_updates" ");"
.SH "ARGUMENTS"
.PP
\fIthis_rq\fR
.RS 4
The rq to update statistics for
.RE
.PP
\fIthis_load\fR
.RS 4
The current load
.RE
.PP
\fIpending_updates\fR
.RS 4
The number of missed updates
.RE
.SH "DESCRIPTION"
.PP
Update rq\->cpu_load[] statistics\&. This function is usually called every scheduler tick (TICK_NSEC)\&.
.PP
This function computes a decaying average:
.PP
load[i]\*(Aq = (1 \- 1/2^i) * load[i] + (1/2^i) * load
.PP
Because of NOHZ it might not get called on every tick which gives need for the
\fIpending_updates\fR
argument\&.
.PP
load[i]_n = (1 \- 1/2^i) * load[i]_n\-1 + (1/2^i) * load_n\-1 = A * load[i]_n\-1 + B ; A := (1 \- 1/2^i), B := (1/2^i) * load = A * (A * load[i]_n\-2 + B) + B = A * (A * (A * load[i]_n\-3 + B) + B) + B = A^3 * load[i]_n\-3 + (A^2 + A + 1) * B = A^n * load[i]_0 + (A^(n\-1) + A^(n\-2) + \&.\&.\&. + 1) * B = A^n * load[i]_0 + ((1 \- A^n) / (1 \- A)) * B = (1 \- 1/2^i)^n * (load[i]_0 \- load) + load
.PP
In the above we\*(Aqve assumed load_n := load, which is true for NOHZ_FULL as any change in load would have resulted in the tick being turned back on\&.
.PP
For regular NOHZ, this reduces to:
.PP
load[i]_n = (1 \- 1/2^i)^n * load[i]_0
.PP
see
\fBdecay_load_misses\fR\&. For NOHZ_FULL we get to subtract and add the extra term\&.
.SH "COPYRIGHT"
.br