Re: [PATCH] docs/scheduler: Change unit of cpu_time and rq_time to nanoseconds

Re: [PATCH] docs/scheduler: Change unit of cpu_time and rq_time to nanoseconds

[Matthew Wilcox]

On Fri, Apr 08, 2022 at 05:31:06PM +0800, brookxu.cn wrote:
> From: Chunguang Xu <brookxu@tencent.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.

> Signed-off-by: Chunguang Xu <brookxu@tencent.com>
> ---
>  Documentation/scheduler/sched-stats.rst | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/scheduler/sched-stats.rst b/Documentation/scheduler/sched-stats.rst
> index dd9b99a..9078a27 100644
> --- a/Documentation/scheduler/sched-stats.rst
> +++ b/Documentation/scheduler/sched-stats.rst
> @@ -56,9 +56,9 @@ Next two are try_to_wake_up() statistics:
>  
>  Next three are statistics describing scheduling latency:
>  
> -     7) sum of all time spent running by tasks on this processor (in jiffies)
> +     7) sum of all time spent running by tasks on this processor (in nanoseconds)
>       8) sum of all time spent waiting to run by tasks on this processor (in
> -        jiffies)
> +        nanoseconds)
>       9) # of timeslices run on this cpu
>  
>  
> -- 
> 1.8.3.1
> 

Re: [PATCH] docs/scheduler: Change unit of cpu_time and rq_time to nanoseconds

[Peter Zijlstra]

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@tencent.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.

Re: [PATCH] docs/scheduler: Change unit of cpu_time and rq_time to nanoseconds

[Jonathan Corbet]

Peter Zijlstra <peterz@infradead.org> writes:

> 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.

OK, I've applied the patch, thanks.

jon