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

> > Do we really need local_irq_save/restore in put_ ?
> >
>
> Hmm.. good point! I don't think we need it.

And _perhaps_ get_ can avoid it too?

I didn't really try to think, probably this is not right, but can't
something like this work?

#define XXXX (1 << 16)
#define MASK (XXXX -1)

void get_online_cpus_atomic(void)
{
preempt_disable();

// only for writer
__this_cpu_add(reader_percpu_refcnt, XXXX);

if (__this_cpu_read(reader_percpu_refcnt) & MASK) {
__this_cpu_inc(reader_percpu_refcnt);
} else {
smp_wmb();
if (writer_active()) {
...
}
}

__this_cpu_dec(reader_percpu_refcnt, XXXX);
}

void put_online_cpus_atomic(void)
{
if (__this_cpu_read(reader_percpu_refcnt) & MASK)
__this_cpu_dec(reader_percpu_refcnt);
else
read_unlock(&hotplug_rwlock);
preempt_enable();
}

Oleg.



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