Re: [PATCH] sched/cpufreq: don't trigger cpufreq update w/o real rt/deadline tasks running

[Rafael J. Wysocki]

On 4/21/2016 3:09 AM, Wanpeng Li wrote:
> 2016-04-21 6:28 GMT+08:00 Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>:
> > On 4/21/2016 12:24 AM, Wanpeng Li wrote:
> > > 2016-04-20 22:01 GMT+08:00 Peter Zijlstra <peterz@xxxxxxxxxxxxx>:
> > > > On Wed, Apr 20, 2016 at 02:32:35AM +0200, Rafael J. Wysocki wrote:
> > > > > On Monday, April 18, 2016 01:51:24 PM Wanpeng Li wrote:
> > > > > > Sometimes update_curr() is called w/o tasks actually running, it is
> > > > > > captured by:
> > > > > >       u64 delta_exec = rq_clock_task(rq) - curr->se.exec_start;
> > > > > > We should not trigger cpufreq update in this case for rt/deadline
> > > > > > classes, and this patch fix it.
> > > > > >
> > > > > > Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> > > > > The signed-off-by tag should agree with the From: header.  One way to
> > > > > achieve
> > > > > that is to add an extra From: line at the start of the changelog.
> > > > >
> > > > > That said, this looks like a good catch that should go into 4.6 to me.
> > > > >
> > > > > Peter, what do you think?
> > > > I'm confused by the Changelog. *what* ?
> > > Sometimes .update_curr hook is called w/o tasks actually running, it is
> > > captured by:
> > >
> > >           u64 delta_exec = rq_clock_task(rq) - curr->se.exec_start;
> > >
> > > We should not trigger cpufreq update in this case for rt/deadline
> > > classes, and this patch fix it.
> >
> > That's what you wrote in the changelog, no need to repeat that.
> >
> > I guess Peter is asking for more details, though.  I actually would like to
> > get some more details here too.  Like an example of when the situation in
> > question actually happens.
> I add a print to print when delta_exec is zero for rt class, something
> like below:
>
>        watchdog/5-48    [005] d...   568.449095: update_curr_rt: rt
> delta_exec is zero
>        watchdog/5-48    [005] d...   568.449104: <stack trace>
>   => pick_next_task_rt
>   => __schedule
>   => schedule
>   => smpboot_thread_fn
>   => kthread
>   => ret_from_fork
>        watchdog/5-48    [005] d...   568.449105: update_curr_rt: rt
> delta_exec is zero
>        watchdog/5-48    [005] d...   568.449111: <stack trace>
>   => put_prev_task_rt
>   => pick_next_task_idle
>   => __schedule
>   => schedule
>   => smpboot_thread_fn
>   => kthread
>   => ret_from_fork
>        watchdog/6-56    [006] d...   568.510094: update_curr_rt: rt
> delta_exec is zero
>        watchdog/6-56    [006] d...   568.510103: <stack trace>
>   => pick_next_task_rt
>   => __schedule
>   => schedule
>   => smpboot_thread_fn
>   => kthread
>   => ret_from_fork
>        watchdog/6-56    [006] d...   568.510105: update_curr_rt: rt
> delta_exec is zero
>        watchdog/6-56    [006] d...   568.510111: <stack trace>
>   => put_prev_task_rt
>   => pick_next_task_idle
>   => __schedule
>   => schedule
>   => smpboot_thread_fn
>   => kthread
>   => ret_from_fork
> [...]

And the statement in your changelog follows from this I suppose. How 
does it follow, exactly?

Thanks,
Rafael