[BUG] sched: clock wrap bug in 2.6.35-stable kills scheduling

[Thomas Lange]

Commit 305e683 introduced a wrap bug that causes task scheduling to fail
after sched_clock() wrap. On a 1000 HZ system with 32bit jiffies, this
occurs after 49.7 days.

Bug was introduced in 2.6.35.12 and is still present in linux-2.6.35.y HEAD.

Symptoms include one task getting all available cpu time while others get
_none_. Setting niceness seems to make things even worse. Running this code
in a new process after wrap completely lock up user space, thus triggering a
watchdog reboot:
{ nice(1); while(1); }

To reproduce bug in reasonable time, one can up HZ. With 16000 HZ, bug occurs
after 3.1 days.
Modifying sched_clock() to wrap when jiffies does triggers bug after 5 mins.

The basic problem seems to be that rq->clock_task get stuck forever with a
really high value when rq->clock starts over from 0.

This fix solves that problem:

diff --git a/kernel/sched.c b/kernel/sched.c
index d40d662..883448f 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -657,6 +657,8 @@ inline void update_rq_clock(struct rq *rq)
        if (!rq->skip_clock_update)
                rq->clock = sched_clock_cpu(cpu_of(rq));
        irq_time = irq_time_cpu(cpu);
+       if (rq->clock < rq->clock_task)
+               rq->clock_task = 0;
        if (rq->clock - irq_time > rq->clock_task)
                rq->clock_task = rq->clock - irq_time;

I can create a proper patch if the above is acceptable.

A more appropriate solution would perhaps be to pull some additional sched
commits into stable branch, like fe44d62 and friends. I don't know enough
about scheduler internals to tell.

All tests were performed on mips32 systems, but all systems with 32bit
jiffies should be affected.

/Thomas
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