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

From: [email-masked] (Wanpeng Li)
Date: Mon, 03 Nov 2014 21:03:54 +0800

On 14/11/3 ??6:55, Vincent Guittot wrote:
> On 3 November 2014 03:12, Wanpeng Li <[email-masked]> wrote:
>> Hi Vincent,
>> On 14/10/31 ??4:47, 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]