linux-kernel - Re: [git pull] scheduler updates for v2.6.24

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Date:	Wed, 17 Oct 2007 00:38:07 +0200
From:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	"Andrew Morton" <akpm@...ux-foundation.org>,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	"Paul Jackson" <pj@....com>
Subject: Re: [git pull] scheduler updates for v2.6.24

On 15/10/2007, Ingo Molnar <mingo@...e.hu> wrote:
>
> * Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> > On Mon, 15 Oct 2007 16:17:23 +0200
> > Ingo Molnar <mingo@...e.hu> wrote:
> >
> > > Linus, please pull the latest scheduler git tree from:
> > >
> > >    git://git.kernel.org/pub/scm/linux/kernel/git/mingo/linux-2.6-sched.git
> >
> > Did Paul Jackson's crash get fixed?
>
> yes - that crash was a showstopper that was holding up the pull request
> for 2 days. Paul bisected it down to the culprit and the fix was to do
> this in wake_up_new_task():
>
> -       if (!p->sched_class->task_new || !current->se.on_rq) {
> +       if (!p->sched_class->task_new || !current->se.on_rq || !rq->cfs.curr) {
>
> (during early bootup the cfs_rq has no curr pointer yet.) It's not clear
> why this race did not trigger earlier.

an update on this issue:

shortly, SD_BALANCE_FORK is required to trigger this problem and
hence, only NUMA machines could have been affected by it (and only
ia64 and x86 have SD_BALANCE_FORK in SD_NODE_INIT).

more details:

it's perfectly legitimate for 'rq->cfs.curr' to be NULL in
task_new_fair() in the case when this_cpu != task_cpu(p) (p -- is a
newly created task).

why this_cpu != task_cpu(p) :

do_fork() --> copy_process() --> sched_fork() -->
cpu = sched_balance_self(this_cpu, SD_BALANCE_FORK)

chose a different cpu for the new task and there is _no_
'class_sched_fair' task running on this cpu at the moment (that's why
rq->cfs.curr == NULL).

[ thanks a lot to Paul for providing debugging information ]

btw., it's not the 'curr->vruntime < se->vruntime' part in
task_new_fair() that gave us the oops (it's only executed in the case
of this_cpu == task_cpu(p)) _but_ it's rather:

[*] check_spread(cfs_rq, curr) which also accesses 'curr->vruntime'.

> (and the two checks can probably
> be consolidated into a single "!rq->cfs.curr" condition.)

2 checks are required as 'current' and rq->cfs.curr are not the same :-)
It also should work if we just get rid of [*] or add an adiitional
(curr != NULL) check there.

just as a additional observation:

there are lots of per-cpu threads (like events/cpu, ksoftirq/cpu,
etc.) being created on start-up (x NUMBER_OF_CPUS) and SD_SCHED_FORK
(actually, sched_balance_self() from sched_fork()) is just an overhead
in this case...
although, sched_balance_self() is likely to be responsible for a minor
% of the time taken to create a new context so optimizing it away
(esp. for some corner cases) won't improve the start-up time
noticeable.

>
>         Ingo
> -

-- 
Best regards,
Dmitry Adamushko
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