linux-kernel - Re: [PATCH 0/2] sched: tasklist_lock cleanups (Was: don't use while_each

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Date:	Mon, 18 Aug 2014 01:14:45 +0400
From:	Kirill Tkhai <tkhai@...dex.ru>
To:	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	Rik van Riel <riel@...hat.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Frank Mayhar <fmayhar@...gle.com>,
	Frederic Weisbecker <fweisbec@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sanjay Rao <srao@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] sched: tasklist_lock cleanups (Was: don't use while_each_thread())

On 17.08.2014 19:25, Oleg Nesterov wrote:
> On 08/13, Oleg Nesterov wrote:
>>
>> Peter, could you take these simple patches ?
>>
>> Better later than never... per-file, but please feel free to join
>> them in a single patch.
>>
>> read_lock_irq*(tasklist_lock) in kernel/sched/ files looks strange.
>> Why? I'll recheck, but this looks unneeded.
> 
> Yes, please consider these minor cleanups on top of for_each_thread
> conversions.
> 
> read_lock_irq(tasklist) in normalize_rt_tasks() doesn't really hurt,
> but it looks confusing. If we really have a reason to disable irqs
> this (subtle) reason should be documented.
> 
> And I can't understand tg_has_rt_tasks(). Don't we need something
> like the patch below? If not, please do not ask me why I think so,
> I don't understand this black magic ;) But the usage of the global
> "runqueues" array looks suspicious.

This function searches RT task which is related to this tg. It's
opaquely because it looks that there is an error.

task_rq(p)->rt.tg is a task group of a top rt_rq, while the task may
be queued on a child rt_rq instead of this. So, your patch is a BUGFIX,
not a cleanup.

> --- x/kernel/sched/core.c
> +++ x/kernel/sched/core.c
> @@ -7354,7 +7354,7 @@ static inline int tg_has_rt_tasks(struct
>  	struct task_struct *g, *p;
>  
>  	for_each_process_thread(g, p) {
> -		if (rt_task(p) && task_rq(p)->rt.tg == tg)
> +		if (rt_task(p) && task_group(p) == tg)
>  			return 1;
>  	}
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