Re: [RFC PATCH] Dynamic sched domains (v0.5)

Nick Piggin
2005-05-02T23:23:55Z

Dinakar Guniguntala wrote:
> On Mon, May 02, 2005 at 07:44:05PM +1000, Nick Piggin wrote:
>
>>Dinakar Guniguntala wrote:
>>
>>
>>>+void rebuild_sched_domains(cpumask_t span1, cpumask_t span2)
>>>+{
>>>+ cpumask_t change_map;
>>>+
>>>+ cpus_or(change_map, span1, span2);
>>>+
>>>+ preempt_disable();
>>
>>Oh, you can't do this here, attach_domains does a synchronize_kernel.
>>So take it out, it doesn't do anything anyway, does it?
>
>
> I put that in to prevent hangs with CONFIG_PREEMPT turned on, but
> clearly didn't test it with preempt turned on. Looks like all I need to
> do here is a local_irq_disable
>

What are you protecting against, though? synchroinze_kernel can
sleep, so local_irq_disable is probably the wrong thing to do as well.

AFAIKS, you don't need anything here - so long as you have mutual
exclusion from other sched-domain building then this can take as long
as it wants / be preempted as many times as we like.

>
>>I suggest you also use some sort of locking to prevent concurrent rebuilds
>>and rebuilds racing with cpu hotplug. You could probably have a static
>>semaphore around rebuild_sched_domains, and take lock_cpu_hotplug here too.
>
>
> I already do a lock_cpu_hotplug() in cpuset.c before calling
> rebuild_sched_domains and also am holding cpuset_sem, so that should take
> care of both hotplug and concurrent rebuilds
>

OK.

But if we want this to be a respectable interface (possibly for more than
just cpusets) then it should probably do some locking itself. It isn't
performance critical, so I think taking a semaphore wouldn't hurt.

--
SUSE Labs, Novell Inc.