sched/cputime: Fix ksoftirqd cputime accounting regression

sched/cputime: Fix ksoftirqd cputime accounting regression

irq_time_read() returns the irqtime minus the ksoftirqd time. This
is necessary because irq_time_read() is used to substract the IRQ time
from the sum_exec_runtime of a task. If we were to include the softirq
time of ksoftirqd, this task would substract its own CPU time everytime
it updates ksoftirqd->sum_exec_runtime which would therefore never
progress.

But this behaviour got broken by:

  a499a5a14db ("sched/cputime: Increment kcpustat directly on irqtime account")

... which now includes ksoftirqd softirq time in the time returned by
irq_time_read().

This has resulted in wrong ksoftirqd cputime reported to userspace
through /proc/stat and thus "top" not showing ksoftirqd when it should
after intense networking load.

ksoftirqd->stime happens to be correct but it gets scaled down by
sum_exec_runtime through task_cputime_adjusted().

To fix this, just account the strict IRQ time in a separate counter and
use it to report the IRQ time.

Reported-and-tested-by: Jesper Dangaard Brouer <[email protected]>
Signed-off-by: Frederic Weisbecker <[email protected]>
Reviewed-by: Rik van Riel <[email protected]>
Acked-by: Jesper Dangaard Brouer <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stanislaw Gruszka <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Wanpeng Li <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>