Messages in this thread | | | Date | Wed, 12 Dec 2012 19:02:48 +0100 | From | Oleg Nesterov <> | Subject | Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context |
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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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