linux-kernel - [PATCH v2 0/3] sched/fair: NOHZ cleanups and misfit improvement

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Date:   Mon, 11 Feb 2019 17:59:43 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, peterz@...radead.org, vincent.guittot@...aro.org,
        morten.rasmussen@....com, Dietmar.Eggemann@....com
Subject: [PATCH v2 0/3] sched/fair: NOHZ cleanups and misfit improvement

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