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[PATCH 0/4 v3] sched,time: reduce nohz_full syscall overhead 40%

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(v3: address comments raised by Frederic)

Running with nohz_full introduces a fair amount of overhead.
Specifically, various things that are usually done from the
timer interrupt are now done at syscall, irq, and guest
entry and exit times.

However, some of the code that is called every single time
has only ever worked at jiffy resolution. The code in
__acct_update_integrals was also doing some unnecessary
calculations.

Getting rid of the unnecessary calculations, without
changing any of the functionality in __acct_update_integrals
gets us about an 11% win.

Not calling the time statistics updating code more than
once per jiffy, like is done on housekeeping CPUs and on
all the CPUs of a non-nohz_full system, shaves off a
further 30%.

I tested this series with a microbenchmark calling
an invalid syscall number ten million times in a row,
on a nohz_full cpu.

    Run times for the microbenchmark:
    
4.4				3.8 seconds
4.5-rc1				3.7 seconds
4.5-rc1 + first patch		3.3 seconds
4.5-rc1 + first 3 patches	3.1 seconds
4.5-rc1 + all patches		2.3 seconds

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[PATCH 0/4 v3] sched,time: reduce nohz_full syscall overhead 40% riel@redhat.com - 2016-02-01 03:20 +0100
  [PATCH 2/4] acct,time: change indentation in __acct_update_integrals riel@redhat.com - 2016-02-01 03:20 +0100
  [PATCH 4/4] sched,time: only call account_{user,sys,guest,idle}_time once a jiffy riel@redhat.com - 2016-02-01 03:20 +0100
    Re: [PATCH 4/4] sched,time: only call  account_{user,sys,guest,idle}_time once a jiffy Peter Zijlstra <peterz@infradead.org> - 2016-02-01 10:30 +0100
      Re: [PATCH 4/4] sched,time: only call  account_{user,sys,guest,idle}_time once a jiffy Rik van Riel <riel@redhat.com> - 2016-02-01 20:30 +0100
  [PATCH] perf tooling: Simplify 'perf bench syscall' Ingo Molnar <mingo@kernel.org> - 2016-02-01 08:50 +0100
  Re: [PATCH] perf tooling: Add 'perf bench syscall' benchmark Andy Lutomirski <luto@amacapital.net> - 2016-02-01 16:50 +0100
    Re: [PATCH] perf tooling: Add 'perf bench syscall' benchmark Ingo Molnar <mingo@kernel.org> - 2016-02-03 11:30 +0100

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