Re: [PATCH v9 00/10] sched: consolidation of CPU capacity and usage

From: Morten Rasmussen
Date: Fri Nov 21 2014 - 07:33:45 EST

On Mon, Nov 03, 2014 at 04:54:37PM +0000, Vincent Guittot wrote:
> This patchset consolidates several changes in the capacity and the usage
> tracking of the CPU. It provides a frequency invariant metric of the usage of
> CPUs and generally improves the accuracy of load/usage tracking in the
> scheduler. The frequency invariant metric is the foundation required for the
> consolidation of cpufreq and implementation of a fully invariant load tracking.
> These are currently WIP and require several changes to the load balancer
> (including how it will use and interprets load and capacity metrics) and
> extensive validation. The frequency invariance is done with
> arch_scale_freq_capacity and this patchset doesn't provide the backends of
> the function which are architecture dependent.
> As discussed at LPC14, Morten and I have consolidated our changes into a single
> patchset to make it easier to review and merge.
> During load balance, the scheduler evaluates the number of tasks that a group
> of CPUs can handle. The current method assumes that tasks have a fix load of
> SCHED_LOAD_SCALE and CPUs have a default capacity of SCHED_CAPACITY_SCALE.
> This assumption generates wrong decision by creating ghost cores or by
> removing real ones when the original capacity of CPUs is different from the
> default SCHED_CAPACITY_SCALE. With this patch set, we don't try anymore to
> evaluate the number of available cores based on the group_capacity but instead
> we evaluate the usage of a group and compare it with its capacity.
> This patchset mainly replaces the old capacity_factor method by a new one and
> keeps the general policy almost unchanged. These new metrics will be also used
> in later patches.
> The CPU usage is based on a running time tracking version of the current
> implementation of the load average tracking. I also have a version that is
> based on the new implementation proposal [1] but I haven't provide the patches
> and results as [1] is still under review. I can provide change above [1] to
> change how CPU usage is computed and to adapt to new mecanism.
> Change since V8
> - reorder patches

I think the patch set is in a good shape. I have got few comments and
suggestions mostly minor things and things related to naming to the
first 7 patches. I haven't found any problems with the code itself. I
haven't tested it yet though. The last three patches I have a few
questions for.

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