[lkml]   [2004]   [Apr]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: CPU Hotplug broken -mm5 onwards
Srivatsa Vaddagiri wrote:
> Hi,
> I found that I can't boot with CONFIG_HOTPLUG_CPU defined in both
> mm5 and mm6. Debugging this revealed it to be because exec path can now require
> cpu hotplug sem (sched_migrate_task) and this has lead to a deadlock between
> flush_workqueue and __call_usermodehelper.
> flush_workqueue takes cpu hotplug sem and blocks until workqueue is flushed.
> __call_usermodehelper, one of the queued work function, blocks because it
> also needs cpu hotplug sem during exec. As of result of this, exec does not
> progress and system does not boot.
> I feel we can fix this by converting cpucontrol to a reader-writer semaphore or
> big-reader-lock(?). One problem with reader-writer semaphore is there does not
> seem to be any down_write_interruptible, which is needed by cpu_down/up.
> Comments?

You are right, but it wasn't introduced in -mm or sched-domains
patches. However, one of Ingo's recent patches does balance on
exec for SMP, not just NUMA so it will make this more common.

So, Rusty has to fix it ;)

I think a rwsem might be a good idea anyway, because
sched_migrate_task can end up being called pretty often with
balance on exec and balance on clone. The semaphore could easily
place undue serialisation on that path.

> BTW, I think a cpu_is_offline check is needed in sched_migrate_task, since
> dest_cpu could have been downed by the time it has acquired the semaphore.
> In which case, we could end up adding the task to dead cpu's runqueue?
> An alternate solution would be to put the same check in __migrate_task.

Yes you are correct.

Can we arrange some of these checks to disappear when HOTPLUG_CPU
is not set? For example, make cpu_is_offline only valid to call for
CPUs that have been online sometime, and can evaluate to 0 if
HOTPLUG_CPU is not set?
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

 \ /
  Last update: 2005-03-22 14:02    [W:0.067 / U:1.216 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site