Re: [PATCH 1/13] timestamp fixes

[Andrew Theurer <habanero@us.ibm.com>]

Nick, can you describe the system you run the DB tests on?  Do you have 
any cpu idle time stats and hopefully some context switch rate stats? 

I think I understand the concern [patch 6] of stealing a task from one 
node to an idle cpu in another node, but I wonder if we can have some 
sort of check for idle balance: if the domain/node to steal from has 
some idle cpu somewhere, we do not steal, period.  To do this we have a 
cpu_idle bitmask, we update as cpus go idle/busy, and we reference this 
cpu_idle & sd->cpu_mask to see if there's at least one cpu that's idle.

> Ingo wrote:
>
> But i expect fork/clone balancing to be almost certainly a problem. (We
> didnt get it right for all workloads in 2.6.7, and i think it cannot be
> gotten right currently either, without userspace API help - but i'd be
> happy to be proven wrong.)

Perhaps initially one could balance on fork up to the domain level which 
has task_hot_time=0, up to a shared cache by default.  Anything above 
that could require a numactl like preference from userspace.

-Andrew Theurer