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Date:   Wed, 16 Sep 2020 11:42:03 +0200
From:   Juri Lelli <juri.lelli@redhat.com>
To:     Peng Liu <iwtbavbm@gmail.com>
Cc:     mingo@redhat.com, peterz@infradead.org, vincent.guittot@linaro.org,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
        mgorman@suse.de, bristot@redhat.com, valentin.schneider@arm.com,
        raistlin@linux.it, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sched/deadline: Fix sched_dl_global_validate()
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References: <20200915152048.GA25835@iZj6chx1xj0e0buvshuecpZ>
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Hi,

On 15/09/20 23:20, Peng Liu wrote:
> When user changes sched_rt_{runtime, period}_us, then
> 
>   sched_rt_handler()
>     -->	sched_dl_bandwidth_validate()
> 	{
> 		new_bw = global_rt_runtime()/global_rt_period();
> 
> 		for_each_possible_cpu(cpu) {
> 			dl_b = dl_bw_of(cpu);
> 			if (new_bw < dl_b->total_bw)
> 				ret = -EBUSY;
> 		}
> 	}
> 
> Under CONFIG_SMP, dl_bw is per root domain , but not per CPU,
> dl_b->total_bw is the allocated bandwidth of the whole root domain.
> we should compare dl_b->total_bw against cpus*new_bw, where 'cpus'
> is the number of CPUs of the root domain.

Hummm, I think you are right. Guess nobody played a lot with changing
global values. :-/

> Also, below annotation(in kernel/sched/sched.h) implied implementation
> only appeared in SCHED_DEADLINE v2[1], then deadline scheduler kept
> evolving till got merged(v9), but the annotation remains unchanged,
> meaningless and misleading, correct it.
> 
> * With respect to SMP, the bandwidth is given on a per-CPU basis,
> * meaning that:
> *  - dl_bw (< 100%) is the bandwidth of the system (group) on each CPU;
> *  - dl_total_bw array contains, in the i-eth element, the currently
> *    allocated bandwidth on the i-eth CPU.
> 
> [1] https://lkml.org/lkml/2010/2/28/119
> 
> Signed-off-by: Peng Liu <iwtbavbm@gmail.com>
> ---
> In fact, I'm not 100% sure that's a bug, since it's too 'obvious' and
> not newly introduced code.
> 
> Also, the introduced #ifdef...#endif pairs look ugly, I have no idea
> how to eliminate them. Ideas and comments are welcome. Thanks.

Can't we just use dl_bw_cpus() ?

Thanks,
Juri