Re: problem with printk on SMP-- somebody please help

[Arun Srinivas]

hi

I really appreciate your suggestions and as a newcomer eager to learn more 
from you people.

As I said I was able to do printk anywhere in the sched.c (including 
_activate_task ) on a non-smp kernel and on a smp-kernel I can do it only on 
the main schedule() function.
Also, I would like to add that I am not able to do the macro rdtsc() for 
reading the timestamp counter in the same function.When I compile the kernel 
it dosent show any error, but just the printk's and rdtsc()'s get subdued!

Well,  with reference to your reply, I have some basic questions:
1) on a non-smp kernel will the _activate_task not lock the given runqueue?
2) where is the best place I can do the rdtsc() and printk to read the value 
as to when a task is being scheduled for execution, on a SMP kernel?

Please help.

Thanks
Arun

> From: Mike Waychison <Michael.Waychison@xxxxxxx>
> To: Arun Srinivas <getarunsri@xxxxxxxxxxx>
> CC: linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: problem with printk--  somebody please help
> Date: Mon, 08 Nov 2004 14:47:59 -0500
>
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> Arun Srinivas wrote:
> > hi
> >
> > I am new to the kernel world and would be very glad if somebody could
> > help me with this problem.
> >
> > I am unable to do printk or even a macro call like rdtsc()...(for
> > reading the time stamp counter) from within the "activate_task"
> > function on a kernel with smp support.But these work under the main
> > schedule() function.
> >
> > I was able to do all these i.e., inside "activate_task" on a kernel
> > without smp support.Can anybody suggest a solution as to what could be
> > the problem??
> >
> > somebody please help.
>
> I seem to recall that doing so would deadlock your machine.
> activate_task is called with the given cpu's runqueue locked.  printk
> eventually calls release_console_sem, which will wake_up_interruptible,
> which will eventually call try_to_wake_up which grabs the same lock.
>
> I don't think I've ever seen printk work while task_rq_lock'ed.
>
> Someone correct me if I'm wrong, this is the best I was able to figure
> out before giving up.
>
> - --
> Mike Waychison
> Sun Microsystems, Inc.
> 1 (650) 352-5299 voice
> 1 (416) 202-8336 voice
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> NOTICE:  The opinions expressed in this email are held by me,
> and may not represent the views of Sun Microsystems, Inc.
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