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Date:   Thu, 4 Feb 2021 15:05:36 +0000
From:   Qais Yousef <qais.yousef@arm.com>
To:     Valentin Schneider <valentin.schneider@arm.com>
Cc:     linux-kernel@vger.kernel.org,
        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>
Subject: Re: [PATCH 1/8] sched/fair: Clean up active balance
 nr_balance_failed trickery
Message-ID: <20210204150536.2d26y7zrp7vhmn53@e107158-lin>
References: <20210128183141.28097-1-valentin.schneider@arm.com>
 <20210128183141.28097-2-valentin.schneider@arm.com>
 <20210203151429.rnbdgt7wyoaz2vui@e107158-lin>
 <jhjo8h12dsn.mognet@arm.com>
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On 02/03/21 18:42, Valentin Schneider wrote:
> >> @@ -9805,9 +9810,6 @@ static int load_balance(int this_cpu, struct rq *this_rq,
> >>  					active_load_balance_cpu_stop, busiest,
> >>  					&busiest->active_balance_work);
> >>  			}
> >> -
> >> -			/* We've kicked active balancing, force task migration. */
> >> -			sd->nr_balance_failed = sd->cache_nice_tries+1;
> >
> > This has an impact on future calls to need_active_balance() too, no? We enter
> > this path because need_active_balance() returned true; one of the conditions it
> > checks for is
> >
> > 	return unlikely(sd->nr_balance_failed > sd->cache_nice_tries+2);
> >
> > So since we used to reset nr_balanced_failed to cache_nice_tries+1, the above
> > condition would be false in the next call or two IIUC. But since we remove
> > that, we could end up here again soon.
> >
> > Was this intentional?
> >
> 
> Partially, I'd say :-)
> 
> If you look at active_load_balance_cpu_stop(), it does
> 
>   sd->nr_balance_failed = 0;
> 
> when it successfully pulls a task. So we get a reset of the failed counter
> on pull, which I've preserved. As for interactions with later
> need_active_balance(), the commit that introduced the current counter write
> (which is over 15 years old!):  
> 
>   3950745131e2 ("[PATCH] sched: fix SMT scheduling problems")
> 
> only states the task_hot() issue; thus I'm doubtful whether said
> interaction was intentional.

The '+1' was added in that comment. 'Original' code was just resetting the
nr_balance_failed cache_nice_tries, so that we don't do another one too soon
I think.

With this change, no active balance is allowed until later. Which makes sense.
I can't see why we would have allowed another kick sooner tbh. But as you say,
this is ancient piece of logic.

I agree I can't see a reason to worry about this (potential) change of
behavior.

Thanks

--
Qais Yousef