Messages in this thread | | | Subject | Re: [PATCH] virtual sched_clock() for s390 | From | Martin Schwidefsky <> | Date | Mon, 23 Jul 2007 15:24:08 +0200 |
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On Mon, 2007-07-23 at 09:15 +0000, Jan Glauber 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.
At least for s390 and powerpc the utime and stime already contain a very precise value how much time was spent in the user and system context. For s390 the granularity is a microsecond. The other values nice, idle, iowait, irq, softirq and steal are precise as well.
> > > 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.]
With the exact accounting of utime and stime that would mean that p->se.sum_exec_runtime is utime + stime, no? Precise Accounting at every cpu context switch has some cost, but for s390 it is not as bad as it sounds. We do 2 store-cpu-timer (STPT) instructions, 2 64 bit adds and 2 64 bit subtracts. In terms of cycles it is less than 30 cycles on each system entry on the latest machine.
> 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?
Imho, we just have to update utime and stime when the process accounting values are requested and set se.sum_exec_runtime to the sum of utime and stime for CONFIG_VIRT_CPU_ACCOUNTING=y.
-- blue skies, Martin.
"Reality continues to ruin my life." - Calvin.
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