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Subject: Re: [PATCH v3 4/7] sched/fair: Introduce a CPU capacity comparison
 helper
To:     Valentin Schneider <valentin.schneider@arm.com>,
        linux-kernel@vger.kernel.org
Cc:     Qais Yousef <qais.yousef@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Quentin Perret <qperret@google.com>,
        Pavan Kondeti <pkondeti@codeaurora.org>,
        Rik van Riel <riel@surriel.com>
References: <20210311120527.167870-1-valentin.schneider@arm.com>
 <20210311120527.167870-5-valentin.schneider@arm.com>
From:   Chandra Sekhar Lingutla <clingutla@codeaurora.org>
Message-ID: <c833d09c-d99f-7541-f8cb-5202dc5e7243@codeaurora.org>
Date:   Wed, 31 Mar 2021 17:04:47 +0530
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Hi Valentin,

The reduced margin is helping our platforms, Please feel free to add my 
tested-by tag:

Tested-by:  Lingutla Chandrasekhar <clingutla@codeaurora.org>

On 3/11/2021 5:35 PM, Valentin Schneider wrote:
> During load-balance, groups classified as group_misfit_task are filtered
> out if they do not pass
>
>    group_smaller_max_cpu_capacity(<candidate group>, <local group>);
>
> which itself employs fits_capacity() to compare the sgc->max_capacity of
> both groups.
>
> Due to the underlying margin, fits_capacity(X, 1024) will return false for
> any X > 819. Tough luck, the capacity_orig's on e.g. the Pixel 4 are
> {261, 871, 1024}. If a CPU-bound task ends up on one of those "medium"
> CPUs, misfit migration will never intentionally upmigrate it to a CPU of
> higher capacity due to the aforementioned margin.
>
> One may argue the 20% margin of fits_capacity() is excessive in the advent
> of counter-enhanced load tracking (APERF/MPERF, AMUs), but one point here
> is that fits_capacity() is meant to compare a utilization value to a
> capacity value, whereas here it is being used to compare two capacity
> values. As CPU capacity and task utilization have different dynamics, a
> sensible approach here would be to add a new helper dedicated to comparing
> CPU capacities.
>
> Reviewed-by: Qais Yousef <qais.yousef@arm.com>
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> ---
>   kernel/sched/fair.c | 7 +++++++
>   1 file changed, 7 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index db892f6e222f..ddb2ab3edf6d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -113,6 +113,13 @@ int __weak arch_asym_cpu_priority(int cpu)
>    */
>   #define fits_capacity(cap, max)	((cap) * 1280 < (max) * 1024)
>   
> +/*
> + * The margin used when comparing CPU capacities.
> + * is 'cap1' noticeably greater than 'cap2'
> + *
> + * (default: ~5%)
> + */
> +#define capacity_greater(cap1, cap2) ((cap1) * 1024 > (cap2) * 1078)
>   #endif
>   
>   #ifdef CONFIG_CFS_BANDWIDTH