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

[Oleg Nesterov]

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.

Oleg.

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