linux-kernel - Re: [PATCH -mm] scheduler: fix the return of the first time

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Message-ID: <20070416114701.GB84@tv-sign.ru>
Date:	Mon, 16 Apr 2007 15:47:01 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>
Cc:	Con Kolivas <kernel@...ivas.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH -mm] scheduler: fix the return of the first time_slice

On 04/16, Oleg Nesterov wrote:
>
> On 04/16, Satoru Takeuchi wrote:
> >
> > --- linux-2.6.21-rc6-mm1.tsfix.orig/kernel/exit.c	2007-04-15 16:56:12.000000000 +0900
> > +++ linux-2.6.21-rc6-mm1.tsfix/kernel/exit.c	2007-04-15 16:56:17.000000000 +0900
> > @@ -680,10 +680,14 @@
> >  			reaper = child_reaper(father);
> >  			break;
> >  		}
> > +		if (reaper->time_slice_reaper == father)
> > +			reaper->time_slice_reaper = NULL;
> >  	} while (reaper->exit_state);
> >  
> >  	list_for_each_safe(_p, _n, &father->children) {
> >  		p = list_entry(_p, struct task_struct, sibling);
> > +		if (p->time_slice_reaper == father)
> > +			p->time_slice_reaper = NULL;
> >  		choose_new_parent(p, reaper);
> >  		reparent_thread(p, father);
> >  	}
> > Index: linux-2.6.21-rc6-mm1.tsfix/kernel/sched.c
> > ===================================================================
> > --- linux-2.6.21-rc6-mm1.tsfix.orig/kernel/sched.c	2007-04-15 16:56:12.000000000 +0900
> > +++ linux-2.6.21-rc6-mm1.tsfix/kernel/sched.c	2007-04-15 16:56:17.000000000 +0900
> > @@ -1693,7 +1693,7 @@
> >  	 * The remainder of the first timeslice might be recovered by
> >  	 * the parent if the child exits early enough.
> >  	 */
> > -	p->first_time_slice = 1;
> > +	p->time_slice_reaper = current;
> >  	p->timestamp = sched_clock();
> >  	local_irq_enable();
> 
> I am afraid this doesn't work for CLONE_THREAD. Suppose that some sub-thread (not
> main thread) T1 creates another sub-thread, T2.
> 
> 	T2->time_slice_reaper = T1.
> 
> then T1 exits, but forget_original_parent(T1) doesn't change T2->time_slice_reaper,
> because T2 is not on T1->children list. Now T2->time_slice_reaper points to an
> already freed task_struct.

In case I was not clear...

Note that "do {} while ()" loop in forget_original_parent() stops when it finds
a sub-thread with ->exit_state == 0. This means we can miss another sub-thread
which has ->time_slice_reaper == father.

To make this correct, we should iterate over all thread-group, but this can slow
down exit() when we have a lot of threads.

I guess we need Ingo's opinion on that.

Oleg.

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