linux-kernel - Re: [PATCH v5 4/5] sched/fair: Skip SIS domain scan if fully busy

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Date:   Wed, 14 Sep 2022 17:22:09 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Abel Wu <wuyun.abel@...edance.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Josh Don <joshdon@...gle.com>, Chen Yu <yu.c.chen@...el.com>,
        K Prateek Nayak <kprateek.nayak@....com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 4/5] sched/fair: Skip SIS domain scan if fully busy

On Fri, 2022-09-09 at 13:53 +0800, Abel Wu wrote:
> If a full domain scan failed, then no unoccupied cpus available
> and the LLC is fully busy.  In this case we'd better use cpus
> more wisely, rather than wasting it trying to find an idle cpu
> that probably not exist. The fully busy status will be cleared
> when any cpu of that LLC goes idle and everything goes back to
> normal again.
> 
> Make the has_idle_cores boolean hint more rich by turning it
> into a state machine.
> 
> Signed-off-by: Abel Wu <wuyun.abel@...edance.com>
> ---
>  include/linux/sched/topology.h | 35 +++++++++++++++++-
>  kernel/sched/fair.c            | 67 ++++++++++++++++++++++++++++------
>  2 files changed, 89 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
> index 816df6cc444e..cc6089765b64 100644
> --- a/include/linux/sched/topology.h
> +++ b/include/linux/sched/topology.h
> @@ -77,10 +77,43 @@ extern int sched_domain_level_max;
>  
>  struct sched_group;
>  
> +/*
> + * States of the sched-domain
> + *
> + * - sd_has_icores
> + *	This state is only used in LLC domains to indicate worthy
> + *	of a full scan in SIS due to idle cores available.
> + *
> + * - sd_has_icpus
> + *	This state indicates that unoccupied (sched-idle/idle) cpus
> + *	might exist in this domain. For the LLC domains it is the
> + *	default state since these cpus are the main targets of SIS
> + *	search, and is also used as a fallback state of the other
> + *	states.
> + *
> + * - sd_is_busy
> + *	This state indicates there are no unoccupied cpus in this

Suggest reword to

.. indicates that all cpus are occupied in this ...

> + *	domain. So for LLC domains, it gives the hint on whether
> + *	we should put efforts on the SIS search or not.
> + *
> 

Tim