linux-kernel - Re: [PATCH] docs/scheduler: Change unit of cpu_time and rq

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Message-ID: <YlA8AEfqK6eRnAPO@hirez.programming.kicks-ass.net>
Date:   Fri, 8 Apr 2022 15:43:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     "brookxu.cn" <brookxu.cn@...il.com>, corbet@....net,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH] docs/scheduler: Change unit of cpu_time and rq_time to
 nanoseconds

On Fri, Apr 08, 2022 at 01:45:35PM +0100, Matthew Wilcox wrote:
> On Fri, Apr 08, 2022 at 05:31:06PM +0800, brookxu.cn wrote:
> > From: Chunguang Xu <brookxu@...cent.com>
> > 
> > In the current implementation, cpu_time and rq_time should be
> > in nanoseconds, so this document needs to be modified.
> 
> I agree that this is wrong, but we shouldn't be changing the units
> of measurement reported to userspace without changing the schedstats
> version.  I suspect this was an inadvertent change, and we should be
> converting from sched_clock() time (~= ns) to jiffies in show_schedstat().
> Adding scheduler experts.

Seems to have happend at 425e0968a25f ("sched: move code into
kernel/sched_stats.h").

Before that we have:

-static inline void
-rq_sched_info_depart(struct rq *rq, unsigned long delta_jiffies)
-{
-       if (rq)
-               rq->rq_sched_info.cpu_time += delta_jiffies;
-}

-static inline void sched_info_depart(struct task_struct *t)
-{
-       unsigned long delta_jiffies = jiffies - t->sched_info.last_arrival;
-
-       t->sched_info.cpu_time += delta_jiffies;
-       rq_sched_info_depart(task_rq(t), delta_jiffies);
-}

afterwards we have:

+static inline void sched_info_depart(struct task_struct *t)
+{
+       unsigned long long delta = sched_clock() - t->sched_info.last_arrival;
+
+       t->sched_info.cpu_time += delta;
+       rq_sched_info_depart(task_rq(t), delta);
+}

And that's 15 years and at least one SCHEDSTAT_VERSION ago (although
this change itself didn't bump the version).

So I'm thinking we can update the documentation and forget about this.