sar: Compute global CPU stats as the sum of individual ones

sar: Compute global CPU stats as the sum of individual ones

sar used to get statistics for CPU "all" from the first line of the
/proc/stat file giving global CPU utilization.

There are several problems with this:

1) With recent kernels (problem detected on a 4.4.14 kernel), the number
of jiffies spent in idle and iowait modes given by this file for global
CPU utilization goes crazy when a CPU is set offline or comes back
online. These counters may even not be monotonic, resulting in wrong
results being displayed by sar.
E.g.:

cat /proc/stat |grep "cpu "
cpu  8123 235 3359 1099139 15985 0 14 0 0 0

(Set a CPU offline)

cat /proc/stat |grep "cpu "
cpu  8146 235 3374 1168377 18919 0 14 0 0 0

(Set the CPU back online)

cat /proc/stat |grep "cpu "
cpu  8169 236 3391 1033989 15978 0 14 0 0 0

2) The updating of the /proc/stat global and individual values is not
done atomically. As a result there can be skew between the global and
individual values reported by sar.
E.g.:

01:46:12   CPU   %user   %nice  %system  %iowait  %steal  %idle
01:46:13   all    0.25    0.00    10.89     0.00    0.00  88.86
01:46:13     0    0.00    0.00    84.80     0.00    0.00  15.20
01:46:13     1    0.00    0.00    89.65     0.00    0.00  10.35
01:46:13     2    0.17    0.00    83.33     0.00    0.00  16.50
01:46:13     3    0.00    0.00    83.64     0.00    0.00  16.36

In the above case, the %system and %idle for "all" is wrongly
reported as 10.89%.

This patch fixes those problems by calculating the statistics for CPU
"all" (global CPU utilization) as the sum of each individual CPU.

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>