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Subject[PATCH v2 0/3] sched/fair: NOHZ cleanups and misfit improvement
Date
In

commit 5fbdfae5221a ("sched/fair: Kick nohz balance if rq->misfit_task_load")

was added a trigger for nohz kicks, which is required to offload misfit tasks
from LITTLE to big CPUs. However, those kicks could be issued a lot more
frequently than what is strictly needed.

This patch-set tunes down unneeded nohz kicks.

- Patch 1 adds some more comments to nohz_balancer_kick()
- Patches [2-3] tweak the nohz kick conditions for asymmetric systems

* Changes since v1

- Patches 1-3 from v1 are in tip/sched/core and thus not included
tip HEAD is 1b5500d73466 ("sched/fair: Remove unused 'sd' parameter from select_idle_smt()")
- Patch 1 from v2 is new (Peter)
- Patch 3 from v2 (5 from v1) now shuffles conditions to avoid a goto (Peter)

* nohz_balancer_kick() shuffling impact

The ASYM_PACKING loop used to be towards the end of nohz_balancer_kick(),
and the LLC condition was higher up. Since the LLC condition is very
often true, we probably were avoiding the loop most of the time on systems
that use ASYM_PACKING. However, I don't have one at hand and I'm not sure
hacking up a kernel to enable ASYM_PACKING on a system that doesn't need it
would be truly relevant.

I ran 20 iterations of

'hackbench -g 1 -l 100000'

on a 2-sockets Xeon E5 (40 logical cores, no ASYM_PACKING) but the difference
(hackbench duration & nohz_balancer_kick() FTrace profiling) lies in the noise.

--------------------------------------------------------------------------------
* Testing
** kick_ilb() hits
This causes a large reduction in calls to kick_ilb() (and thus subsequent
rescheduling interrupts & useless nohz balance calls) in most scenarios.

The "best case" one is running NR_BIG_CPUS big tasks, which I tested with
4 50% periodic tasks running for 5 seconds on my HiKey960 (4x4 big.LITTLE):

| CPU | hits (baseline) | hits (patchset) |
|-----+-----------------+-----------------|
| 0 | 31 | 41 |
| 1 | 21 | 3 |
| 2 | 35 | 2 |
| 3 | 9 | 4 |
|-----+-----------------+-----------------|
| 4 | 170 | 4 |
| 5 | 573 | 4 |
| 6 | 544 | 4 |
| 7 | 579 | 4 |


Something a bit less idealistic with NR_CPUS-1 big tasks still shows some
improvements (7 100% tasks running for 5 seconds on my HiKey960):

| CPU | hits (baseline) | hits (patchset) |
|-----+-----------------+-----------------|
| 0 | 14 | 122 |
| 1 | 47 | 162 |
| 2 | 11 | 156 |
| 3 | 9 | 3 |
|-----+-----------------+-----------------|
| 4 | 53 | 6 |
| 5 | 276 | 13 |
| 6 | 312 | 7 |
| 7 | 250 | 11 |

I was surprised to see such an increase in calls to kick_ilb() from LITTLE
CPUs ([0-3]), but after a bit of investigation it turns out that the big
CPUs would always run nohz_balancer_kick() a jiffy before the LITTLEs, so
the LITTLEs would always bail out because nohz.next_balance had just been
updated before they called nohz_balancer_kick(). IOW,

time_before(now, nohz.next_balance)

would always be true on CPUs [0-3] during my workload. Quieting the kicks
issued by the big CPUs allowed the LITTLEs to execute nohz_balancer_kick()
past that condition, explaining the higher number of kicks issued from LITTLE
CPUs.

** misfit behaviour
For good measure I also ran the usual misfit tests [1] which showed no
particular change.

[1]: https://github.com/ARM-software/lisa/blob/next/lisa/tests/kernel/scheduler/misfit.py

Valentin Schneider (3):
sched/fair: Comment some nohz_balancer_kick() kick conditions
sched/fair: Tune down misfit nohz kicks
sched/fair: Skip LLC nohz logic for asymmetric systems

kernel/sched/fair.c | 84 +++++++++++++++++++++++++++++++++------------
1 file changed, 63 insertions(+), 21 deletions(-)

--
2.20.1

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