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Subject[PATCH] keep rd->online and cpu_online_map in sync
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>>> On Mon, Mar 10, 2008 at  4:14 AM, in message
<20080310081425.GA11031@in.ibm.com>, Gautham R Shenoy <ego@in.ibm.com> wrote:

> On Sat, Mar 08, 2008 at 12:10:15AM -0500, Gregory Haskins wrote:

[ snip ]

>> @@ -5813,6 +5813,13 @@ migration_call(struct notifier_block *nfb, unsigned
> long action, void *hcpu)
>> /* Must be high prio: stop_machine expects to yield to it. */
>> rq = task_rq_lock(p, &flags);
>> __setscheduler(rq, p, SCHED_FIFO, MAX_RT_PRIO-1);
>> +
>> + /* Update our root-domain */
>> + if (rq->rd) {
>> + BUG_ON(!cpu_isset(cpu, rq->rd->span));
>> + cpu_set(cpu, rq->rd->online);
>> + }
>
> Hi Greg,
>
> Suppose someone issues a wakeup at some point between CPU_UP_PREPARE
> and CPU_ONLINE, then isn't there a possibility that the task could
> be woken up on the cpu which has not yet come online ?
> Because at this point in find_lowest_cpus()
>
> cpus_and(*lowest_mask, task_rq(task)->rd->online, task->cpus_allowed);
>
> the cpu which has not yet come online is set in both rd->online map
> and the task->cpus_allowed.

Hi Gautham,
This is a good point. I think I got it backwards (or rather, I had the ordering more correct before the patch): I really want to keep the set() on ONLINE as I had it originally. Its the clear() that is misplaced, as DOWN_PREPARE is too early in the chain. I probably should have used DYING to defer the clear out to the point right before the cpu_online_map is updated. Thanks to Dmitry for highlighting the smpboot path which helped me to find the proper notifier symbol.

>
> I wonder if assigning a priority to the update_sched_domains() notifier
> so that it's called immediately after migration_call() would solve the
> problem.

Possibly (and I just saw your patch). But note that migration_call was previously declared as "should run first" so I am not sure if moving update_sched_domain() above it will not have a ripple effect in some other area. If that is "ok", then I agree your solution solves the ordering problem, albeit in a bit heavy handed manner. Otherwise s/CPU_DOWN_PREPARE/CPU_DYING in migrate call will fix the issue as well, I believe. This will push the update of rd->online to be much more tightly coupled with updates to cpu_online_map.

Ingo/Andrew: Please drop the earlier fix I submitted. I don't think it is correct on several fronts. Please try the one below. As before, I have tested that I can offline/online CPUs, but I dont have s2ram capability here.

Regards,
-Greg

---------------------------------------------
keep rd->online and cpu_online_map in sync

It is possible to allow the root-domain cache of online cpus to
become out of sync with the global cpu_online_map. This is because we
currently trigger removal of cpus too early in the notifier chain.
Other DOWN_PREPARE handlers may in fact run and reconfigure the
root-domain topology, thereby stomping on our own offline handling.

The end result is that rd->online may become out of sync with
cpu_online_map, which results in potential task misrouting.

So change the offline handling to be more tightly coupled with the
global offline process by triggering on CPU_DYING intead of
CPU_DOWN_PREPARE.

Signed-off-by: Gregory Haskins <ghaskins@novell.com>
---

kernel/sched.c | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 52b9867..a616fa1 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -5881,7 +5881,7 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
spin_unlock_irq(&rq->lock);
break;

- case CPU_DOWN_PREPARE:
+ case CPU_DYING:
/* Update our root-domain */
rq = cpu_rq(cpu);
spin_lock_irqsave(&rq->lock, flags);


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