Re: [PATCH v8 00/10] sched: consolidation of CPU capacity and usage

[Wanpeng Li]

On 14/11/3 äå6:55, Vincent Guittot wrote:
> On 3 November 2014 03:12, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
> > Hi Vincent,
> > On 14/10/31 äå4:47, 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
> >
> > I don't know the history, could you explain what's the meaning of 'ghost
> > cores' ?
> The capacity_factor gives the number of tasks that can be handled by a
> group of CPUs by dividing the group's capacity by SCHED_CAPACITY_SCALE
>
> For a system with SMT, the default capacity of a core is 1178 so the
> capacity of each CPU for a dual threads per core is 589.
>
> At CPU level we have a capacity_factor of 1  = div_round_closest(589, 1024)
> At core level we still have a capacity_factor of 1  =
> div_round_closest(1178, 1024).  This is a intended behavior to promote
> 1 task per core
> Then, if we have 4 cores in a node, the capacity_factor is 5 =
> div_round_closest(4712, 1024) whereas we should have 4. So a 5th ghost
> core has appeared in the group and the load balancer will not
> considered the group as overloaded if there is 5 tasks whereas it
> should in order to try to move this 5th task on an idle core (if there
> is one)
> Patch [0] solves some use cases by ensuring that we will not have more
> cores than possible so we can't have more than 4 core for the previous
> example.
> Now, if some RT tasks are running and using almost 1 core (1024 as an
> example), the capacity_factor is still 4 = div_round_closest(3688,
> 1024) whereas a core is nearly fully used and the capacity_factor
> should be 3
>
> [0] https://lkml.org/lkml/2013/8/28/194

Got it, thanks for your great explanation.

Regards,
Wanpeng Li

> Regards,
> Vincent
>
> > Regards,
> > Wanpeng Li
> >
> >
> > > 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.
> [snip]

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