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

[Srivatsa S. Bhat]

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...
 
Regards,
Srivatsa S. Bhat

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/