linux-kernel - Re: [PATCH v4 05/12] sched/fair: Keep a fully

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Date:   Sat, 13 May 2023 00:11:45 +0530
From:   Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc:     Ricardo Neri <ricardo.neri@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Len Brown <len.brown@...el.com>, Mel Gorman <mgorman@...e.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Ionela Voinescu <ionela.voinescu@....com>, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        "Tim C . Chen" <tim.c.chen@...el.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        shrikanth hegde <sshegde@...ux.vnet.ibm.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: Re: [PATCH v4 05/12] sched/fair: Keep a fully_busy SMT sched group as
 busiest



On 4/7/23 2:01 AM, Ricardo Neri wrote:
> When comparing two fully_busy scheduling groups, keep the current busiest
> group if it represents an SMT core. Tasks in such scheduling group share
> CPU resources and need more help than tasks in a non-SMT fully_busy group.
>
> Cc: Ben Segall <bsegall@...gle.com>
> Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> Cc: Ionela Voinescu <ionela.voinescu@....com>
> Cc: Len Brown <len.brown@...el.com>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> Cc: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Tim C. Chen <tim.c.chen@...el.com>
> Cc: Valentin Schneider <vschneid@...hat.com>
> Cc: x86@...nel.org
> Cc: linux-kernel@...r.kernel.org
> Tested-by: Zhang Rui <rui.zhang@...el.com>
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
> ---
> Changes since v3:
>  * None
>
> Changes since v2:
>  * Introduced this patch.
>
> Changes since v1:
>  * N/A
> ---
>  kernel/sched/fair.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index b151e93ec316..ea23a5163bfa 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9566,10 +9566,22 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>  		 * contention when accessing shared HW resources.
>  		 *
>  		 * XXX for now avg_load is not computed and always 0 so we
> -		 * select the 1st one.
> +		 * select the 1st one, except if @sg is composed of SMT
> +		 * siblings.
>  		 */
> -		if (sgs->avg_load <= busiest->avg_load)
> +
> +		if (sgs->avg_load < busiest->avg_load)
>  			return false;
> +
> +		if (sgs->avg_load == busiest->avg_load) {
> +			/*
> +			 * SMT sched groups need more help than non-SMT groups.
> +			 * If @sg happens to also be SMT, either choice is good.
> +			 */
> +			if (sds->busiest->flags & SD_SHARE_CPUCAPACITY)
> +				return false;
> +		}
> +
>  		break;

IIUC,

Earlier, we used to go to out_balanced if sgs->avg_load <= busiest->avg_load.
Now we go only if it is less. lets say sgs->avg_load == busiest->avg_load,
then we will return true in MC,DIE domain. This might end up traversing
multiple such group's and pick the last one as the busiest instead of
first. I guess eventually any load balance if exists will be fixed.  But
this might cause slight overhead. would it?



nit: There is typo in [2/12]  if the whole core is repeated.
+	 * CPUs. When done between cores, do it only if the whole core if the
+	 * whole core is idle.

Mentioning in this reply instead, to avoid sending another mail reply for this.


>  
>  	case group_has_spare: