Re: [PATCH] Avoid mask based num_possible_cpus and num_online_cpus-v5

[Srivatsa S. Bhat]

On 02/14/2012 02:13 AM, Venki Pallipadi wrote:

> On Mon, Feb 13, 2012 at 12:25 PM, Srivatsa S. Bhat
> <srivatsa.bhat@xxxxxxxxxxxxxxxxxx> wrote:
>> On 02/14/2012 01:24 AM, Tony Luck wrote:
>>
>>> On Thu, Feb 2, 2012 at 12:03 PM, Rusty Russell <rusty@xxxxxxxxxxxxxxx> wrote:
>>>> IIRC playing with 3 archs boot code seemed like a recipe for disaster.
>>>> Feel free to try to fix this in -next though, and see what breaks...
>>>
>>> ia64 is what breaks ... well not actually broken ... but some very
>>> weird delays that
>>> show up in different places depending on whether this patch is present.
>>>
>>> First linux-next kernel to be blessed with this patch was
>>> next-20120210. Booting it
>>> I see:
>>> [    7.164233] Switching to clocksource itc
>>> [  146.077315] pnp: PnP ACPI init
>>>
>>> An ugly 138.913 second delay.  Digging in the code showed that the bad bits
>>> happened inside stop_machine()
>>>
>>> Reverting just this patch makes this big delay disappear:
>>>
>>> [   32.780232] Switching to clocksource itc
>>> [   32.832100] pnp: PnP ACPI init
>>>
>>> but notice that it takes 25 extra seconds to get to this point in the
>>> boot (and while
>>> we expect to save some time by not re-computing num_online_cpus each time we
>>> need it ... this looks to be a lot more than I'd expect!)
>>>
>>
>>
>> Oh no!! ia64 directly uses cpu_set() and cpu_clear() on cpu_online_map!!
>> Grr.. It means num_online_cpus can be different from the actual number of
>> online cpus because it doesn't go through the set_cpu_online() path.. I haven't
>> yet pin-pointed the exact problem, but this definitely doesn't look good...
>>
> 
> This feels like a minefield in general. ia64, mips and um seems to
> have cpu_set and cpu_clear of cpu_online_map and/or cpu_possible_map
> in there.
> 


Since I had almost never seen code using "cpu_online_map" instead of
"cpu_online_mask", I hadn't checked it while reviewing your patch... :-(
Honestly, it is only now that I realized that there is this other variant too!

Regards,
Srivatsa S. Bhat

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