lkml.org 
[lkml]   [2007]   [Apr]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC] Cpu-hotplug: Using the Process Freezer (try2)
On Mon, Apr 02, 2007 at 08:16:12AM +0200, Ingo Molnar wrote:
>
> * Gautham R Shenoy <ego@in.ibm.com> wrote:
>
> > Hello Everybody,
> >
> > This is another attempt towards process-freezer based cpu-hotplug.
> > This patchset covers just about everything that was discussed on the
> > LKML with respect to the freezer-based cpu-hotplug.
>
> wow - you have made really nice progress!

The discussions on the list helped clear up a lot of issues.

>
> > I believe that the reasons for freezer failing as N increases are :
> > - 'make -jN' keeps forking new tasks every now and then, thereby resulting
> > in a never-ending catching up game in the do_while loop inside
> > try_to_freeze_tasks (kernel/power/process.c)
>
> hm, shouldnt the make be frozen immediately?
>
> doesnt the 'please freeze ASAP' flag get propagated to all tasks,
> immediately? After that point any cloning activity should duplicate that
> flag too, resulting in any new child freezing immediately too.
>
> > Instead of waiting for all the tasks to call try_to_freeze in the
> > above mentioned do_while loop, I wonder if we can put some hooks in
> > sched.c so asto not schedule the task marked PF_FREEZING/PF_FROZEN.
>
> we could definitely do that - but i think it should be unnecessary: if
> we mark all tasks as PF_FREEZING atomically, that should result in
> _every_ task immediately dropping dead (once they get back from
> TASK_UNINTERRUPTIBLE). No excuses. If there's some longer delay then
> that can only be explained by some new cloned task/thread slipping
> through the net somehow. (i.e. the PF_FREEZING flag not being duplicated
> across fork?)
>

I will try again Vatsa's suggestion of having a

if (freezing(current))
freeze_process(p);
in copy processes() and check if we can do away with the fork race.
That sounds lot simpler than the scheduler hooks.

> i'm wondering about how TASK_UNINTERRUPTIBLE tasks are handled by the
> freezer: are they assumed frozen immediately, or do we wait until they
> notice their PF_FREEZING and go into try_to_freeze()? I'd expect
> TASK_UNINTERRUPTIBLE to be the largest source of latency. (and hence be
> the primary source for freezing 'failures')

From what I can make out, we fail to freeze if we have some task in
the TASK_UNINTERRUPTIBLE state for more than the timeout period.

The kernel threads have to call try_to_freeze() explicitly and for the
userspace tasks, try_to_freeze() is called in get_signal_to_deliver().
The system is considered frozen only when *all* the freezeable tasks
call try_to_freeze() one way or the other. This is unlikely in case of
a TASK_UNINTERRUPTIBLE task.

Question is can we have some task in TASK_UNINTERRUPTIBLE state for such
a long duration (20sec) ??

>
> Ingo

Thanks and Regards
gautham.
--
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2007-04-02 13:23    [from the cache]
©2003-2011 Jasper Spaans