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Re: Tracking down exception in sched.c

To: Rojhalat Ibrahim <imr@rtschenk.de>
Subject: Re: Tracking down exception in sched.c
From: "Kevin D. Kissell" <kevink@mips.com>
Date: Mon, 20 Feb 2006 14:35:10 +0100
Cc: Mark E Mason <mark.e.mason@broadcom.com>, linux-mips@linux-mips.org
In-reply-to: <43F9B168.6090105@rtschenk.de>
Original-recipient: rfc822;linux-mips@linux-mips.org
References: <7E000E7F06B05C49BDBB769ADAF44D0773A636@NT-SJCA-0750.brcm.ad.broadcom.com> <43F9B168.6090105@rtschenk.de>
Sender: linux-mips-bounce@linux-mips.org
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The "for" loop is what used to be there, but if you use it in
a system with "hot-pluggable" CPUs, I could imagine that there
would be problems.  While for_each_cpu is pathetically inefficient
as it gets expanded and compiled for MIPS, if your phys_cpu_present_map
(which is by default what gets used in MIPS as cpu_possible_map
for the purposes of sched.c) is being properly initialized and
maintained, the behavior of the two loops should be the same.
Have you double-checked that?  Secondary CPUs turn generally
set their bits in that mask in prom_build_cpu_map().

                Regards,

                Kevin K.

Rojhalat Ibrahim wrote:
Hi,

I tracked this one down to 88a2a4ac6b671a4b0dd5d2d762418904c05f4104
(percpu data: only iterate over possible CPUs). I don't know if this
is the correct way to fix this, but the following patch makes the
problem go away for me.

--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -6021,7 +6021,7 @@ void __init sched_init(void)
        runqueue_t *rq;
        int i, j, k;

-       for_each_cpu(i) {
+       for (i = 0; i < NR_CPUS; i++) {
                prio_array_t *array;

                rq = cpu_rq(i);

Any other suggestions, how to fix this?

Thanks,
Rojhalat Ibrahim


Mark E Mason wrote:
[Cross-posted from LKML]
Hello all, Working from the linux-mip.org repository (which just recently merged
from the kernel.org repository), we've been getting exceptions on
several different processors due to NULL pointer dereferences in
sched.c.  These happen on SMP systems only (but both 32 and 64-bit
systems trigger this problem).
The Oops output and surrounding text (w/ backtrace) is below. What I've
traced is down to so far is that enqueue_task() gets called with a ready
queue (rq) where (rq->active == NULL).

Backtracing a bit, the following patch triggers an earlier, slightly
more controlled failure:

[mason@hawaii linux.git]$ git diff kernel/sched.c diff --git
a/kernel/sched.c b/kernel/sched.c
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1264,6 +1264,7 @@ static int try_to_wake_up(task_t *p, uns  #endif

        rq = task_rq_lock(p, &flags);
+       BUG_ON(rq->active == NULL);
        old_state = p->state;
        if (!(old_state & state))
                goto out;


My question is, is the above assert valid (ie. Should rq->active always
be non-NULL at this point)?  It seems like it should be, but I'm pretty
new to this code, and thought I should double-check before going off
into the weeds.

If anyone has any ideas about where specifically to look for the
underlying problem, I'd appreciate it.

Thanks (very much) in advance,
Mark Mason
mason@broadcom.com
Newberg, Oregon



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