Re: [PATCH v2 2/2] sched/fair: Fix negative energy delta in find_energy_efficient_cpu()

[Lukasz Luba <lukasz.luba@arm.com>]

On 4/29/21 11:19 AM, Pierre.Gondois@arm.com wrote:
> From: Pierre Gondois <Pierre.Gondois@arm.com>
> 
> find_energy_efficient_cpu() (feec()) searches the best energy CPU
> to place a task on. To do so, compute_energy() estimates the energy
> impact of placing the task on a CPU, based on CPU and task utilization
> signals.
> 
> Utilization signals can be concurrently updated while evaluating a
> performance domain (pd). In some cases, this leads to having a
> 'negative delta', i.e. placing the task in the pd is seen as an
> energy gain. Thus, any further energy comparison is biased.
> 
> In case of a 'negative delta', return prev_cpu since:
> 1. a 'negative delta' happens in less than 0.5% of feec() calls,
>     on a Juno with 6 CPUs (4 little, 2 big)
> 2. it is unlikely to have two consecutive 'negative delta' for
>     a task, so if the first call fails, feec() will correctly
>     place the task in the next feec() call
> 3. EAS current behavior tends to select prev_cpu if the task
>     doesn't raise the OPP of its current pd. prev_cpu is EAS's
>     generic decision
> 4. prev_cpu should be preferred to returning an error code.
>     In the latter case, select_idle_sibling() would do the placement,
>     selecting a big (and not energy efficient) CPU. As 3., the task
>     would potentially reside on the big CPU for a long time
> 
> Reported-by: Xuewen Yan <xuewen.yan@unisoc.com>
> Suggested-by: Xuewen Yan <xuewen.yan@unisoc.com>
> Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com>
> ---
>   kernel/sched/fair.c | 28 ++++++++++++++++------------
>   1 file changed, 16 insertions(+), 12 deletions(-)
> 


Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>

Regards,
Lukasz