Re: [RFC PATCH 1/7] sched/pelt: Add a new function to approximate the future util_avg value

Qais Yousef
2023-09-16T19:50:08Z

On 09/13/23 19:22, Dietmar Eggemann wrote:
> On 10/09/2023 21:58, Qais Yousef wrote:
> > On 09/07/23 13:12, Dietmar Eggemann wrote:
> >> On 06/09/2023 23:19, Qais Yousef wrote:
> >>> On 09/06/23 14:56, Dietmar Eggemann wrote:
> >>>> On 28/08/2023 01:31, Qais Yousef wrote:
>
> [...]
>
> >> Another thing ... I guess if you call accumulate_sum with delta the PELT
> >> machinery assumes `delta = now - sa->last_update_time` which means you
> >> would have to use `clock_pelt + TICK_USEC` as delta.
> >
> > Right.
> >
> > The way I understood it is that at TICK we should do update_load_avg() which
> > would call __update_load_sum() which uses
> >
> > delta = now - sa->last_update_time
> >
> > which passes this delta to accumulate_sum()
> >
> > I can see we are not very accurate since there will be a small additional time
> > besides TICK_USEC that we are not accounting for. But I can't see how this can
> > cause a big error.
> >
> > predicted (assumed) tick time/delta
> >
> > sa->last_update_time = now
> > tick_time = TICK_USEC + now
> >
> > delta = tick_time - sa->last_update_time
> > delta = TICK_USEC + now - now
> > delta = TICK_USEC
> >
> > but actual tick time/delta
> >
> > sa->last_update_time = now - x
> > tick_time = TICK_USEC + now
> >
> > delta = tick_time - sa->last_update_time
> > delta = TICK_USEC + now - (now - x)
> > delta = TICK_USEC + x
> >
> > So the delta I am using might be slightly shorter than it should be.
> >
> > IIUC, what you're saying that the `x` in my equation above is clock_pelt,
> > right?
>
> No, I was wrong here. Calling accumulate_sum with `delta = TICK_USEC` is
> fine.
>
> accumulate_sum() will accrue `sa->util.sum` and ___update_load_avg()
> will then adjust `sa->util_avg` accordingly.
>
> delta should be 4000 on Arm64 boards so you will cross period
> boundaries. In case `delta < 1024` you might want to not call
> ___update_load_avg() to be in pair with __update_load_avg_cfs_rq().

You mean *not* call, or actually *do* call ___update_load_avg() if delta
< 1024? I am certainly not calling it now and I think you're suggesting to
actually call it when period is less than 1024.

This area is not my strength, so I do sure appreciate any suggestion to make it
better! :-) I will look into that for next version.

Many thanks!

--
Qais Yousef