linux-kernel - [PATCH v3 0/5] Improve newidle lb cost tracking and early abort

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Message-Id: <20211019123537.17146-1-vincent.guittot@linaro.org>
Date:   Tue, 19 Oct 2021 14:35:32 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, linux-kernel@...r.kernel.org,
        tim.c.chen@...ux.intel.com
Cc:     Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH v3 0/5] Improve newidle lb cost tracking and early abort

This patchset updates newidle lb cost tracking and early abort:

The time spent running update_blocked_averages is now accounted in the 1st
sched_domain level. This time can be significant and move the cost of
newidle lb above the avg_idle time.

The decay of max_newidle_lb_cost is modified to start only when the field
has not been updated for a while. Recent update will not be decayed
immediatlybut only after a while.

The condition of an avg_idle lower than sysctl_sched_migration_cost has
been removed as the 500us value is quite large and prevent opportunity to
pull task on the newly idle CPU for at least 1st domain levels.

Monitoring sd->max_newidle_lb_cost on cpu0 of a Arm64 system
THX2 (2 nodes * 28 cores * 4 cpus) during the benchmarks gives the
following results:
       min    avg   max
SMT:   1us   33us  273us - this one includes the update of blocked load
MC:    7us   49us  398us
NUMA: 10us   45us  158us


Some results for hackbench -l $LOOPS -g $group :
group      tip/sched/core     + this patchset
1           15.189(+/- 2%)       14.987(+/- 2%)  +1%
4            4.336(+/- 3%)        4.322(+/- 5%)  +0%
16           3.654(+/- 1%)        2.922(+/- 3%) +20%
32           3.209(+/- 1%)        2.919(+/- 3%)  +9%
64           2.965(+/- 1%)        2.826(+/- 1%)  +4%
128          2.954(+/- 1%)        2.993(+/- 8%)  -1%
256          2.951(+/- 1%)        2.894(+/- 1%)  +2%

tbench and reaim have not shown any difference

Change since v2:
- Update and decay of sd->last_decay_max_lb_cost are gathered in
  update_newidle_cost(). The behavior remains almost the same except that
  the decay can happen during newidle_balance now.

  Tests results haven't shown any differences
  
  I haven't modified rq->max_idle_balance_cost. It acts as the max value
  for avg_idle and prevents the latter to reach high value during long
  idle phase. Moving on an IIR filter instead, could delay the convergence
  of avg_idle to a reasonnable value that reflect current situation.

- Added a minor cleanup of newidle_balance

Change since v1:
- account the time spent in update_blocked_averages() in the 1st domain

- reduce number of call of sched_clock_cpu() 

- change the way max_newidle_lb_cost is decayed. Peter suggested to use a
  IIR but keeping a track of the current max value gave the best result

- removed the condition (this_rq->avg_idle < sysctl_sched_migration_cost)
  as suggested by Peter

Vincent Guittot (5):
  sched/fair: Account update_blocked_averages in newidle_balance cost
  sched/fair: Skip update_blocked_averages if we are defering load
    balance
  sched/fair: Wait before decaying max_newidle_lb_cost
  sched/fair: Remove sysctl_sched_migration_cost condition
  sched/fair: cleanup newidle_balance

 include/linux/sched/topology.h |  2 +-
 kernel/sched/fair.c            | 65 ++++++++++++++++++++++------------
 kernel/sched/topology.c        |  2 +-
 3 files changed, 45 insertions(+), 24 deletions(-)

-- 
2.17.1