linux-kernel - [PATCH] keep rd->online and cpu_online

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Date:	Mon, 10 Mar 2008 09:39:34 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	ego@...ibm.com
Cc:	suresh.b.siddha@...el.com, rjw@...k.pl, akpm@...ux-foundation.org,
	dmitry.adamushko@...il.com, ego@...ibm.com, mingo@...e.hu,
	oleg@...sign.ru, yi.y.yang@...el.com, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, ghaskins@...ell.com
Subject: [PATCH] keep rd->online and cpu_online_map in sync

>>> On Mon, Mar 10, 2008 at  4:14 AM, in message
<20080310081425.GA11031@...ibm.com>, Gautham R Shenoy <ego@...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@...ell.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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