Messages in this thread | | | Date | Thu, 13 Dec 2012 00:00:42 +0530 | From | "Srivatsa S. Bhat" <> | Subject | Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context |
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On 12/12/2012 11:32 PM, 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 ;)
OK :-)
> >>> 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); > } >
Sorry, may be I'm too blind to see, but I didn't understand the logic of how the mask helps us avoid disabling interrupts.. Can you kindly elaborate?
> 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(); > } >
Regards, Srivatsa S. Bhat
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