Re: [PATCH V5 2/2] sched/fair: Remove group imbalance from calculate_imbalance()

[Jeffrey Hugo]

On 7/5/2017 5:22 AM, Peter Zijlstra wrote:
> On Wed, Jun 07, 2017 at 01:18:58PM -0600, Jeffrey Hugo wrote:
> > The group_imbalance path in calculate_imbalance() made sense when it was
> > added back in 2007 with commit 908a7c1b9b80 ("sched: fix improper load
> > balance across sched domain") because busiest->load_per_task factored into
> > the amount of imbalance that was calculated. That is not the case today.
>
> It would be nice to have some more information on which patch(es)
> changed that.

From the history it looks like commit dd5feea14a7d ("sched: Fix 
SCHED_MC regression caused by change in sched cpu_power") removed 
load_per_task from imbalance calculations (was factored into max_pull). 
After this change, the group imbalance modifications to this variable 
appear to no longer work as originally intended.

> > The group_imbalance path can only affect the outcome of
> > calculate_imbalance() when the average load of the domain is less than the
> > original busiest->load_per_task. In this case, busiest->load_per_task is
> > overwritten with the scheduling domain load average. Thus
> > busiest->load_per_task no longer represents actual load that can be moved.
> >
> > At the final comparison between env->imbalance and busiest->load_per_task,
> > imbalance may be larger than the new busiest->load_per_task causing the
> > check to fail under the assumption that there is a task that could be
> > migrated to satisfy the imbalance. However env->imbalance may still be
> > smaller than the original busiest->load_per_task, thus it is unlikely that
> > there is a task that can be migrated to satisfy the imbalance.
> > Calculate_imbalance() would not choose to run fix_small_imbalance() when we
> > expect it should. In the worst case, this can result in idle cpus.
> >
> > Since the group imbalance path in calculate_imbalance() is at best a NOP
> > but otherwise harmful, remove it.
>
> load_per_task is horrible and should die. Ever since we did cgroup
> support the number is complete crap, but even before that the concept
> was dubious.
>
> Most of the logic that uses the number stems from the pre-smp-nice era.
>
> This also of course means that fix_small_imbalance() is probably a load
> of crap. Digging through all that has been on the todo list for a long
> while but somehow not something I've ever gotten to :/ 

Based on the state of the code today, our change fixes a current issue 
that we are encoutering. If we add the above history to the commit text, 
is our change sufficent as a short term solution between now and 
whenever the load_per_task path is rearchitected?

--
Jeffrey Hugo
Qualcomm Datacenter Technologies as an affiliate of Qualcomm 
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.