lkml.org 
[lkml]   [2012]   [Dec]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context
On 12/13/2012 01:06 AM, Oleg Nesterov wrote:
> On 12/12, Oleg Nesterov wrote:
>>
>> On 12/12, Srivatsa S. Bhat wrote:
>>>
>>> On 12/12/2012 10:47 PM, Oleg Nesterov wrote:
>>>>
>>>> Why it needs to be per-cpu? It can be global and __read_mostly to avoid
>>>> the false-sharing. OK, perhaps to put reader_percpu_refcnt/writer_signal
>>>> into a single cacheline...
>>>
>>> Even I realized this (that we could use a global) after posting out the
>>> series.. But do you think that it would be better to retain the per-cpu
>>> variant itself, due to the cache effects?
>>
>> I don't really know, up to you. This was the question ;)
>
> But perhaps there is another reason to make it per-cpu...
>
> It seems we can avoid cpu_hotplug.active_writer == current check in
> get/put.
>
> take_cpu_down() can clear this_cpu(writer_signal) right after it takes
> hotplug_rwlock for writing. It runs with irqs and preemption disabled,
> nobody else will ever look at writer_signal on its CPU.
>

Hmm.. And then the get/put_ on that CPU will increment/decrement the per-cpu
refcount, but we don't care.. because we only need to ensure that they don't
deadlock by taking the rwlock for read.

This sounds great!

Regards,
Srivatsa S. Bhat



\
 
 \ /
  Last update: 2012-12-12 21:01    [W:0.081 / U:0.700 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site