Re: [patch 0/4] [RFC] true vs. system idle cputime

[Martin Schwidefsky <schwidefsky@de.ibm.com>]

On Wed, 2008-10-08 at 18:19 +0200, Martin Schwidefsky wrote:
> Greetings,
> while working on the analysis of a mismatch between the cputime accounting
> numbers of z/VM as the host and Linux as the guest I started to wonder
> about the accounting of idle time. z/VM showed more cpu time for the guest
> as the guest itself. With the current code everything that the idle process
> does is accounted as idle time. If idle is sleeping that is fine, but if
> idle is actually using cpu cycles this is wrong.
> 
> The question is how wrong? To find out I've implemented really precise
> accounting of true idle vs. system idle cputime for s390. A really simple
> test that wakes up 100 times per second to do some minimal work before
> going back to sleep showed 0.35% of system idle time. If you are dealing
> with lots of virtual penguins this quickly becomes significant.
> 
> There are four patches in this series:
> Patch #1: Cleanup scaled / unscaled cputime accounting
> Patch #2: Change the accounting interface to allow the architectures to do
>           precise idle time accounting
> Patch #3: s390 patch to improve the precision of the idle_time_us value
> Patch #4: s390 patch to implement improved idle time accounting
> 
> There is one change in patch #2 that might require a change on powerpc
> and/or ia64. The generic TICK_ONESHOT/NO_HZ code calculates the number
> of ticks spent with a disabled HZ timer and accounts this as idle time.
> For a configuration for VIRT_CPU_ACCOUNTING=y this is horribly wrong.
> Either you have precise accounting or you don't. Patch #2 just removes
> the calculation for VIRT_CPU_ACCOUNTING=y. The architectures which support
> precise accounting have to deal with it on their own. This is where the
> powerpc and ia64 maintainer come into play. Would you look at patch #2
> please ?
> 
> To make it clearer what happens in tick_nohz_restart_sched_tick I've added
> a new function account_idle_ticks(). And for good measure another one named
> account_steal_ticks() for xen where "interesting" things have been done
> with the account_steal_time interface.

Any news about powerpc? Do these patches break anything or does it work?

-- 
blue skies,
  Martin.

"Reality continues to ruin my life." - Calvin.