Re: [PATCH v5 01/10] sched/pelt: Move pelt related code in a dedicated file

Quentin Perret
2018-05-29T14:55:13Z

On Friday 25 May 2018 at 19:04:55 (+0100), Patrick Bellasi wrote:
> On 25-May 15:26, Quentin Perret wrote:
> > And also, I understand these functions are large, but if we _really_
> > want to inline them even though they're big, why not putting them in
> > sched-pelt.h ?
>
> Had the same tought at first... but then I recalled that header is
> generated from a script. Thus, eventually, it should be a different one.

Ah, good point. This patch already introduces a pelt.h so I guess that
could work as well.

>
> > We probably wouldn't accept that for everything, but
> > those PELT functions are used all over the place, including latency
> > sensitive code paths (e.g. task wake-up).
>
> We should better measure the overheads, if any, and check what
> (a modern) compiler does. Maybe some hackbench run could help on the
> first point.

FWIW, I ran a few hackbench tests today on my Intel box:
- Intel i7-6700 (4 cores / 8 threads) @ 3.40GHz
- Base kernel: today's tip/sched/core "2539fc82aa9b sched/fair: Update
util_est before updating schedutil"
- Compiler: GCC 7.3.0

The tables below summarize the results for:
perf stat --repeat 10 perf bench sched messaging --pipe --thread -l 50000 --group G

Without patch:
+---+-------+----------+---------+
| G | Tasks | Duration | Stddev |
+---+-------+----------+---------+
| 1 | 40 | 3.906 | +-0.84% |
| 2 | 80 | 8.569 | +-0.77% |
| 4 | 160 | 16.384 | +-0.46% |
| 8 | 320 | 33.686 | +-0.42% |
+---+-------+----------+---------+

With patch:
+---+-------+----------------+---------+
| G | Tasks | Duration | Stddev |
+---+-------+----------------+---------+
| 1 | 40 | 3.953 (+1.2%) | +-1.43% |
| 2 | 80 | 8.646 (+0.9%) | +-0.32% |
| 4 | 160 | 16.390 (+0.0%) | +-0.38% |
| 8 | 320 | 33.992 (+0.9%) | +-0.27% |
+---+-------+----------------+---------+

So there is (maybe) a little something on my box, but not so significant
IMHO ... :)

Thanks,
Quentin