Re: [PATCH] virtual sched_clock() for s390

[Jan Glauber <jang@linux.vnet.ibm.com>]

On Fri, 2007-07-20 at 09:22 +0200, Ingo Molnar wrote:
> * Paul Mackerras <paulus@samba.org> wrote:
> > As with s390, 64-bit PowerPC also uses CONFIG_VIRT_CPU_ACCOUNTING. 
> > That affects how tsk->utime and tsk->stime are accumulated (we call 
> > account_user_time and account_system_time directly rather than calling 
> > update_process_times) as well as the system hardirq/softirq time, idle 
> > time, and stolen time.
> 
> tsk->utime and tsk->stime is only used for a single purpose: to 
> determine the 'split' factor of how to split up the precise total time 
> between user and system time.
> 
> > When you say "precise task statistics for /proc", where are they 
> > accumulated?  I don't see any changes to the way that tsk->utime and 
> > ctime are computed.
> 
> we now use p->se.sum_exec_runtime that measures (in nanoseconds) the 
> precise amount of time spent executing (sum of system and user time) - 
> and ->stime and ->utime is used to determine the 'split'. [this allows 
> us to gather ->stime and ->utime via low-resolution sampling, while 
> keeping the 'total' precise. Accounting at every system entry point 
> would be quite expensive on most platforms.]

Using se.sum_exec_runtime to generate ->utime and ->stime breaks
the process accounting we have on s390 (and probably on PowerPC too).
With CONFIG_VIRT_CPU_ACCOUNTING we already have precise values in
->utime and ->stime. Can we make the calculation of the CFS-based time
values conditional by CONFIG_VIRT_CPU_ACCOUNTING?

Jan

> 	Ingo
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