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Subject: Re: [PATCH v2 2/6] sched/rt: Re-instate old behavior in
 select_task_rq_rt
To:     Qais Yousef <qais.yousef@arm.com>, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Pavan Kondeti <pkondeti@codeaurora.org>
Cc:     Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        linux-kernel@vger.kernel.org
References: <20200223184001.14248-1-qais.yousef@arm.com>
 <20200223184001.14248-3-qais.yousef@arm.com>
From:   Dietmar Eggemann <dietmar.eggemann@arm.com>
Message-ID: <01313581-0c60-8b4c-ceb3-e23554a600ed@arm.com>
Date:   Tue, 25 Feb 2020 15:21:16 +0000
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On 23.02.20 18:39, Qais Yousef wrote:

[...]

> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 4043abe45459..2c3fae637cef 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1474,6 +1474,13 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
>  	if (test || !rt_task_fits_capacity(p, cpu)) {
>  		int target = find_lowest_rq(p);
>  
> +		/*
> +		 * Bail out if we were forcing a migration to find a better
> +		 * fitting CPU but our search failed.
> +		 */
> +		if (!test && !rt_task_fits_capacity(p, target))
> +			goto out_unlock;

Didn't you loose the 'target != -1' condition from
https://lore.kernel.org/r/20200218041620.GD28029@codeaurora.org ?

A call to rt_task_fits_capacity(p, -1) would cause issues on a
heterogeneous system.

I tried to provoke this but wasn't able to do so. find_lowest_rq()
returns -1 in 4 places. (1) lowest_mask should be there (2) if
'task->nr_cpus_allowed == 1' select_task_rq_rt() wouldn't have been
called but maybe (3) or (4) can still return -1.

[...]