Messages in this thread Patch in this message | | | From | Gregory Haskins <> | Subject | [PATCH] keep rd->online and cpu_online_map in sync | Date | Mon, 10 Mar 2008 09:39:34 -0400 |
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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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