Re: [PATCH 9/7] sched,numa: remove task_h_load from task_numa_compare

[Rik van Riel <riel@redhat.com>]

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On 06/25/2014 01:31 AM, Peter Zijlstra wrote:
> On Wed, Jun 25, 2014 at 01:25:00AM -0400, Rik van Riel wrote:
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>> 
>> On 06/25/2014 01:21 AM, Peter Zijlstra wrote:
>>> On Wed, Jun 25, 2014 at 07:07:35AM +0200, Peter Zijlstra
>>> wrote:
>>>> Shall I merge this into patch 3?
>>> 
>>> Which gets me the below; which is has a wrong changelog.
>>> 
>>> task_h_load() already computes the load as seen from the root 
>>> group. effective_load() just does a better (and more expensive)
>>> job of computing the task movement implications of a move.
>>> 
>>> So the total effect of this patch shouldn't be very big;
>>> regular load balancing also only uses task_h_load(), see
>>> move_tasks().
>>> 
>>> Now, we don't run with preemption disabled, don't run as
>>> often, etc.., so maybe we can indeed use the more expensive
>>> variant just fine, but does it really matter?
>> 
>> In my testing, it appears to make a difference between workloads 
>> converging, and workloads sitting with one last thread stuck on 
>> another node that never gets moved...
> 
> Fair enough; can you provide a new Changelog that I can paste in?

Here it goes:

When CONFIG_FAIR_GROUP_SCHED is enabled, the load that a task places
on a CPU is determined by the group the task is in. The active groups
on the source and destination CPU can be different, resulting in a
different load contribution by the same task at its source and at its
destination. As a result, the load needs to be calculated separately
for each CPU, instead of estimated once with task_h_load.

Getting this calculation right allows some workloads to converge,
where previously the last thread could get stuck on another node,
without being able to migrate to its final destination.




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