Re: [PATCH v3] sched/cputime: let ktimers align with ksoftirqd in accounting CPUTIME_SOFTIRQ

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2023-12-07 13:18:11 [-0500], Steven Rostedt wrote:
> On Thu, 7 Dec 2023 12:19:28 -0500
> Yuanhan Zhang <zyhtheonly@gmail.com> wrote:
> 
> In irqtime_account_process_tick() there's:
> 
> 	if (this_cpu_ksoftirqd() == p) {
> 		/*
> 		 * ksoftirqd time do not get accounted in cpu_softirq_time.
> 		 * So, we have to handle it separately here.
> 		 * Also, p->stime needs to be updated for ksoftirqd.
> 		 */
> 		account_system_index_time(p, cputime, CPUTIME_SOFTIRQ);
> 
> Which to me looks like it is counting ksoftirqd for SOFTIRQ time. But
> honestly, why do we care about that? What's the difference if ksoftirqd
> were to run or softirqd were to pass work off to a workqueue?
> 
> ksoftirqd runs in vanilla Linux as SCHED_OTHER. The work it does doesn't
> interrupt processes any more than any other kernel thread. I don't know why
> we make it "special".

The special part is that it runs with disabled preemption the whole time
and the scheduler can't do a thing about it. This is different on
PREEMPT_RT where the softirq is preemptible and scheduler can replace it
with another task if suited. It still runs as SCHED_OTHER. The ktimers/
thread runs as SCHED_FIFO 1. So accounting it (incl. ksoftirqd) on
SYSTEM is fine IMHO. 

> I guess the better question I need to ask is, what is this information used
> for? I thought it was how much time was take away from tasks. As current
> would be a task, and we do care if a real softirq is running, as we do not
> want to add that to the current task accounting.
> 
> But for ksoftirqd, that's not the case, and I don't really care if it's
> running a softirq or not. As that time isn't interrupting actual tasks. Not
> to mention, one could simply look at the ksoftirqd tasks to see how much
> time they take up.

The original argument was to have the softirq counters right in
/proc/stat. This is what I remember from the trip to the museum.

> -- Steve

Sebastian