Re: [PATCH v2 1/1] doc: Add remote CPU access details and others to this_cpu_ops.txt

[Pranith Kumar <bobby.prani@gmail.com>]

On 07/17/2014 10:55 AM, Christoph Lameter wrote:
> On Thu, 17 Jul 2014, Pranith Kumar wrote:
> 
>>> The RCU code has .... ummmm... some issues with percpu usage and should
>>> not be taken as a good example. If you look at the RCU code it looks
>>> horrible with numerous barriers around remote percpu read/wrirte
>>> accesses and one wonders if that code is actually ok.
>>
>> Well, it is running in all our kernels with not many reported issues, isn't it ;)
>> And yes, that is one of the extra-ordinary situations where we use per-cpu data.
>> Once you've extracted a pointer to the per-cpu area -and- ensure that concurrent
>> accesses do not happen(or happen with enough guarantees using barriers), what is
>> the case against remote accesses? I am asking from a correctness and a
>> performance point of view, not style/aesthetics.
> 
> Could be working but I do not want it to be mentioned in the
> documentation given the problems it causes. IPI is preferable.

I can mention that IPI is preferable. What is that you don't want mentioned? atomic_t?

> 
>>>> If data needs to be modified from multiple cpus only very rarely, doesn't it
>>>> make sense to use per-cpu areas?
>>>
>>> I would suggest that this should not occur. You can always "modify" remote
>>> percpu areas by generating an IPI on that cpu to make that processor
>>> update its own per cpu data.
>>>
>>
>> The case against doing that is not to wake up CPUs which are in idle/sleep
>> states. I think mentioning it here that remote accesses are strongly discouraged
>> with a reasonable explanation of the implications should be enough. There might
>> always be rare situations where remote accesses might be necessary.
> 
> Remote percpu updates are extremely rare events. If the cpu is idle/asleep
> then usually no updates are needed because no activity is occurring on
> that cpu.
> 

Yes, -usually- that is the case. But we are talking about the extreme rare event
where we need to update some remote CPU`s per-cpu data without waking it up from
sleep/idle. How do you suggest we handle this? I don't think suggesting not to
use per-cpu areas because of this is a good idea, since we lose a lot of
performance in the most common cases.

Thoughts?

--
Pranith