linux-kernel - Re: [PATCH 2/3] sched/fair: Correctly insert cfs

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Date:   Fri, 28 May 2021 17:06:28 +0200
From:   Odin Ugedal <odin@...d.al>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Odin Ugedal <odin@...d.al>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        "open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] sched/fair: Correctly insert cfs_rq's to list on unthrottle

Hi,

> What would be the other condition in addition to the current one
> :cfs_rq->nr_running >= 1 ?

The condition is that if it has load, we should add it (I don't have
100% control on util_avg and runnable_avg tho.). Using
"!cfs_rq_is_decayed()" is another way, but imo. that is a bit
overkill.

> We need to add a cfs_rq in the list if it still contributes to the
> tg->load_avg and the split of the share. Can't we add a condition for
> this instead of adding a new field ?

Yes, using cfs_rq->tg_load_avg_contrib as below would also work the
same way. I still think being explicit that we insert it if we have
removed it is cleaner in a way, as it makes it consistent with the
other use of list_add_leaf_cfs_rq() and list_del_leaf_cfs_rq(), but
that is about preference I guess. I do however think that using
tg_load_avg_contrib will work just fine, as it should always be
positive in case the cfs_rq has some load. I can resent v2 of this
patch using this instead;

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ad7556f99b4a..969ae7f930f5 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4720,7 +4720,7 @@ static int tg_unthrottle_up(struct task_group
*tg, void *data)
                                             cfs_rq->throttled_clock_task;

                /* Add cfs_rq with already running entity in the list */
-               if (cfs_rq->nr_running >= 1)
+               if (cfs_rq->tg_load_avg_contrib)
                        list_add_leaf_cfs_rq(cfs_rq);
        }