Re: [RFC][PATCH] CPUSets: Move most calls to rebuild_sched_domains() to the workqueue

[Max Krasnyansky <maxk@qualcomm.com>]

Paul Menage wrote:
> On Thu, Jun 26, 2008 at 1:34 PM, Paul Menage <menage@google.com> wrote:
>> void get_online_cpus(void)
>> {
>>        might_sleep();
>>        if (cpu_hotplug.active_writer == current)
>>                return;
>>        down_read(&cpu_hotplug.lock);
>> }
>>
>> void put_online_cpus(void)
>> {
>>        if (cpu_hotplug.active_writer == current)
>>                return;
>>        up_read(&cpu_hotplug.lock);
>> }
>>
>> static void cpu_hotplug_begin(void)
>> {
>>        down_write(&cpu_hotplug.lock);
>>        cpu_hotplug.active_writer = current;
>> }
>>
>> static void cpu_hotplug_done(void)
>> {
>>        cpu_hotplug.active_writer = NULL;
>>        up_write(&cpu_hotplug.lock);
>> }
>>
>> I think that combined with moving the async rebuild_sched_domains to a
>> separate thread should solve the problem, but I'm wondering why
>> cpu_hotplug.lock was implemented this way in the first place.
> 
> Oh, I guess that doesn't work because of recursive calls to
> get_online_cpus(). Maybe we need a down_read_recursive() that skips
> ahead of waiting writers if the lock is already held in read mode?

Instead of changing cpu_hotplug locking should we maybe try to avoid using
cgroup_lock in rebuild_sched_domains() ?
There is a comment in cpuset.c that says
 * If a task is only holding callback_mutex, then it has read-only
 * access to cpusets.

I'm not sure if it's still valid. rebuild_sched_domains() only needs read only
access, it does not really modify any cpuset structures.

Max