Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

On 12/20/2012 07:12 PM, Oleg Nesterov wrote:
> On 12/20, Srivatsa S. Bhat wrote:
>>
>> On 12/20/2012 12:44 AM, Oleg Nesterov wrote:
>>>
>>> We need 2 helpers for writer, the 1st one does synchronize_sched() and the
>>> 2nd one takes rwlock. A generic percpu_write_lock() simply calls them both.
>>>
>>
>> Ah, that's the problem no? Users of reader-writer locks expect to run in
>> atomic context (ie., they don't want to sleep).
> 
> Ah, I misunderstood.
> 
> Sure, percpu_write_lock() should be might_sleep(), and this is not
> symmetric to percpu_read_lock().
> 
>> We can't expose an API that
>> can make the task go to sleep under the covers!
> 
> Why? Just this should be documented. However I would not worry until we
> find another user. Until then we do not even need to add percpu_write_lock
> or try to generalize this code too much.
> 

Hmm.. But considering the disable_nonboot_cpus() case you mentioned below, I'm
only getting farther away from using synchronize_sched() ;-) And that also makes
it easier to expose a generic percpu rwlock API, like Tejun was suggesting.
So I'll give it a shot.

>>> To me, the main question is: can we use synchronize_sched() in cpu_down?
>>> It is slow.
>>>
>>
>> Haha :-) So we don't want smp_mb() in the reader,
> 
> We need mb() + rmb(). Plust cli/sti unless this arch has optimized
> this_cpu_add() like x86 (as you pointed out).
> 
>> *and* also don't want
>> synchronize_sched() in the writer! Sounds like saying we want to have the cake
>> and eat it too ;-) :P
> 
> Personally I'd vote for synchronize_sched() but I am not sure. And I do
> not really understand the problem space.
> 
>> And moreover, since I'm still not convinced about the writer API part if use
>> synchronize_sched(), I'd rather avoid synchronize_sched().)
> 
> Understand.
> 
> And yes, synchronize_sched() adds more problems. For example, where should
> we call it? I do not this _cpu_down() should do this, in this case, say,
> disable_nonboot_cpus() needs num_online_cpus() synchronize_sched's.
> 

Ouch! I should have seen that coming!

> So probably cpu_down() should call it before cpu_maps_update_begin(), this
> makes the locking even less obvious.
> 

True.

> In short. What I am trying to say is, don't ask me I do not know ;)
>

OK then, I'll go with what I believe is a reasonably good way (not necessarily
the best way) to deal with this:

I'll avoid the use of synchronize_sched(), expose a decent-looking percpu
rwlock implementation, use it in CPU hotplug and get rid of stop_machine().
That would certainly be a good starting base, IMHO.

Regards,
Srivatsa S. Bhat