linux-kernel - Re: [BUG] scheduler: first timeslice of the exiting thread

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Message-ID: <20070409102922.GA216@tv-sign.ru>
Date:	Mon, 9 Apr 2007 14:29:22 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Galbraith <efault@....de>, linux-kernel@...r.kernel.org
Subject: Re: [BUG] scheduler: first timeslice of the exiting thread

Satoru Takeuchi wrote:
>
>   a) On sched_fork, the creator share its timeslice with new process.
>   b) On sched_exit, if the exiting process didn't exhaust its first
>      timeslice yet, it gives its timeslice to the parent.
>
> It has no problem on the process model since the creator is the parent.
> However, on the thread model, the creator is not the parent, it is same
> as the creator's parent. Hence, on this kind of program, the creator
> can't retrieve shared timeslice and exausts its timeslice at a rate of
> knots. In addition, somehow, the parent (typically shell?) gets extra
> timeslice.

Yes, this is an old oddity.

> I believe it's a bug and the exiting process should give its timeslice
> to the creator. Now I have some patch plan to fix this problem as follow:
>
>  a) Add the field for the creator to task_struct. It needs extra memory.

... and locking/complications. The creator may already exited at this time.

>  b) Doesn't add extra field and have thread's parent the creater, which is
>     same as process creation. However it has many side effects, for example,
>     we also need to change sys_getppid() implementation.

can't understand this, sorry.

Ingo, I think sched_exit() has another buglet,

	if (unlikely(p->parent->time_slice > task_timeslice(p)))
		p->parent->time_slice = task_timeslice(p);

should be

	if (unlikely(parent->time_slice > task_timeslice(parent)))
		parent->time_slice = task_timeslice(parent);

The

	[PATCH 1/2] sched_exit: fix parent->time_slice calculation
	http://marc.info/?l=linux-kernel&m=115101842505365

was dropped because "[PATCH 2/2]" was very wrong.

Could you look at it?

Oleg.

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