[RFC PATCH 0/2] sched: Load Balancing using Per-entity-Load-tracking - linaro-dev

17 Oct 2012

      Hi Guys,
Can you please have a look at the below patchset? Your review comments
are very necessary and valuable.Thanks in advance.
...
This patchset uses the per-entity-load-tracking patchset which will soon be
available in the kernel.It is based on the tip/master tree and the first 8
latest patches of sched:per-entity-load-tracking alone have been imported to
the tree to avoid the complexities of task groups and to hold back the
optimizations of this patch for now.
This patchset is an attempt to begin the integration of Per-entity-load-
metric for the cfs_rq,henceforth referred to as PJT's metric,with the load
balancer in a step wise fashion,and progress based on the consequences.
The following issues have been considered towards this:
[NOTE:an x% task referred to in the logs and below is calculated over a
duty cycle of 10ms.]
1.Consider a scenario,where there are two 10% tasks running on a cpu.The
  present code will consider the load on this queue to be 2048,while
  using PJT's metric the load is calculated to be <1000,rarely exceeding this
  limit.Although the tasks are not contributing much to the cpu load,they are
  decided to be moved by the scheduler.
But one could argue that 'not moving one of these tasks could throttle
  them.If there was an idle cpu,perhaps we could have moved them'.While the
  power save mode would have been fine with not moving the task,the
  performance mode would prefer not to throttle the tasks.We could strive
  to strike a balance by making this decision tunable with certain parameters.
  This patchset includes such tunables.This issue is addressed in Patch[1/2].
2.We need to be able to do this cautiously,as the scheduler code is too
  complex.This patchset is an attempt to begin the integration of PJT's
  metric with the load balancer in a step wise fashion,and progress based on
  the consequences.
  I dont intend to vary the parameters used by the load balancer.Some
  parameters are however included anew to make decisions about including a
  sched group as a candidate for load balancing.
This patchset therefore has two primary aims.
         Patch[1/2]: This patch aims at detecting short running tasks and
    prevent their movement.In update_sg_lb_stats,dismiss a sched group
    as a candidate for load balancing,if load calculated by PJT's metric
    says that the average load on the sched_group <= 1024+(.15*1024).
    This is a tunable,which can be varied after sufficient experiments.
     Patch[2/2]:In the current scheduler greater load would be analogous
     to more number of tasks.Therefore when the busiest group is picked
     from the sched domain in update_sd_lb_stats,only the loads of the
     groups are compared between them.If we were to use PJT's metric,a
     higher load does not necessarily mean more number of tasks.This
patch addresses this issue.

3.The next step towards integration should be in using the PJT's metric for
  comparison between the loads of the busy sched group and the sched
  group which has to pull the tasks,which happens in find_busiest_group.

Preeti U Murthy (2):
      sched:Prevent movement of short running tasks during load balancing
      sched:Pick the apt busy sched group during load balancing
kernel/sched/fair.c |   38 +++++++++++++++++++++++++++++++++++---
 1 file changed, 35 insertions(+), 3 deletions(-)
--
The links to PATCH[1/2] https://lkml.org/lkml/2012/10/12/13
             PATCH[2/2] https://lkml.org/lkml/2012/10/12/11
Regards
Preeti U Murthy