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Re: [PATCH 00/36] cputime: Convert core use of cputime_t to nsecs

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On Mon, 21 Nov 2016 17:20:06 +0100
Frederic Weisbecker <fweisbec@gmail.com> wrote:

> On Mon, Nov 21, 2016 at 07:59:56AM +0100, Martin Schwidefsky wrote:
> > On Fri, 18 Nov 2016 15:47:02 +0100
> > Frederic Weisbecker <fweisbec@gmail.com> wrote:
> > > > The do_account_vtime function is called once per jiffy and once per task
> > > > switch. HZ is usually set to 100 for s390, the conversion once per jiffy
> > > > would not be so bad, but the call on the scheduling path *will* hurt.
> > > 
> > > I don't think we need to flush on task switch. If we maintain the accumulators
> > > on the task/thread struct instead of per-cpu, then the remaining time after
> > > task switch out will be accounted on next tick after after next task switch in.
> > 
> > You can not properly calculate steal time if you allow sleeping tasks to sit on
> > up to 5*HZ worth of cpu time.
> 
> Ah, you mean that when the task goes to sleep, we shouldn't miss more than one
> tick worth of system/user time but the steal time can be much higher, right?

No, it is worse than that. Consider a task going to sleep just before a tick
arrives. It will have almost a full HZ time-slice in its task specific accounting
numbers. After the switch another task with a different set of accounting numbers
is running. The tick will not push the cputime for the work done in the last
HZ period. Dependent on what the new task has in its accounting number the steal
time calculation can give you anything. Repeat the whole thing with any number
of tasks and the missing cputime can get really large. Now get one of these
processes back at the beginning of a time slice and you can get nearly 200% worth
of cputime in one tick. Switch to the next task with missing cputime at the start
of the new tick and you can get many ticks with too much cputime.

Not doing accounting on task switch is just broken.

> > I think we *have* to do accounting on task switch.
> > At least on s390, likely on powerpc as well. Why not make that an option for
> > the architecture with the yet-to-be-written accumulating code.
> 
> Ok, how about doing the accumulation and always account on task switch for now,
> we'll see later if it's worth having such an option.

I am convinced that we need it. The prototype patch does it for s390.

-- 
blue skies,
   Martin.

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

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[PATCH 00/36] cputime: Convert core use of cputime_t to nsecs Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:10 +0100
  [PATCH 17/36] cputime: Increment kcpustat directly on irqtime account Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:10 +0100
  [PATCH 25/36] cputime: Push time to account_system_time() in nsecs Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:10 +0100
  [PATCH 02/36] time: Introduce jiffies64_to_nsecs() Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:10 +0100
  [PATCH 07/36] cputime: Special API to return old-typed cputime Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:10 +0100
  [PATCH 19/36] posix-timers: Convert internals to use nsecs Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:10 +0100
  [PATCH 22/36] cputime: Push time to account_user_time() in nsecs Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:10 +0100
  [PATCH 23/36] cputime: Push time to account_steal_time() in nsecs Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 29/36] ia64: Move nsecs based cputime headers to the last arch using it Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 04/36] cputime: Convert kcpustat to nsecs Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 01/36] jiffies: Reuse TICK_NSEC instead of NSEC_PER_JIFFY Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 34/36] s390: Remove unused cputime definitions Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 30/36] ia64: Convert vtime to use nsec units directly Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 33/36] powerpc: Remove unused cputime definitions Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 14/36] delaycct: Convert obsolete cputime type to nsecs Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 09/36] alpha: Convert obsolete cputime_t to nsecs Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 31/36] ia64: Remove unused cputime definitions Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 27/36] vtime: Return nsecs instead of cputime_t to account Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 24/36] cputime: Push time to account_idle_time() in nsecs Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 36/36] cputime: Remove asm generic headers Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 05/36] macintosh/rack-meter: Remove cputime_t internal use Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 13/36] acct: Convert obsolete cputime type to nsecs Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 28/36] cputime: Remove jiffies based cputime Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 03/36] sched: Remove unused INIT_CPUTIME macro Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 26/36] cputime: Complete nsec conversion of tick based accounting Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 20/36] itimer: Convert internal cputime_t units to nsec Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 06/36] cputime: Convert guest time accounting to nsecs Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 10/36] x86: Convert obsolete cputime type to nsecs Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 35/36] cputime: Remove unused nsec_to_cputime Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 12/36] binfmt: Convert obsolete cputime type to nsecs Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 18/36] posix-timers: Use TICK_NSEC instead of a dynamically ad-hoc calculated version Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 21/36] sched: Remove temporary cputime_t accessors Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 32/36] s390: Make arch_cpu_idle_time() to return nsecs Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 15/36] tsacct: Convert obsolete cputime type to nsecs Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 16/36] signal: Convert obsolete cputime type to nsecs Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  [PATCH 08/36] cputime: Convert task/group cputime to nsecs Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-17 19:20 +0100
  Re: [PATCH 00/36] cputime: Convert core use of cputime_t to nsecs Martin Schwidefsky <schwidefsky@de.ibm.com> - 2016-11-18 13:10 +0100
    Re: [PATCH 00/36] cputime: Convert core use of cputime_t to nsecs Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-18 16:10 +0100
      Re: [PATCH 00/36] cputime: Convert core use of cputime_t to nsecs Martin Schwidefsky <schwidefsky@de.ibm.com> - 2016-11-21 08:10 +0100
        Re: [PATCH 00/36] cputime: Convert core use of cputime_t to nsecs Martin Schwidefsky <schwidefsky@de.ibm.com> - 2016-11-21 11:20 +0100
          Re: [PATCH 00/36] cputime: Convert core use of cputime_t to nsecs Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-22 14:50 +0100
            Re: [PATCH 00/36] cputime: Convert core use of cputime_t to nsecs Martin Schwidefsky <schwidefsky@de.ibm.com> - 2016-11-22 15:30 +0100
        Re: [PATCH 00/36] cputime: Convert core use of cputime_t to nsecs Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-21 17:30 +0100
          Re: [PATCH 00/36] cputime: Convert core use of cputime_t to nsecs Martin Schwidefsky <schwidefsky@de.ibm.com> - 2016-11-22 07:20 +0100
      Re: [PATCH 00/36] cputime: Convert core use of cputime_t to nsecs Ingo Molnar <mingo@kernel.org> - 2016-11-21 10:50 +0100
        Re: [PATCH 00/36] cputime: Convert core use of cputime_t to nsecs Frederic Weisbecker <fweisbec@gmail.com> - 2016-11-21 17:30 +0100

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