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

From: Arun Srinivas
Date: Mon Nov 08 2004 - 21:29:32 EST


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.


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

Hash: SHA1

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

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