[PATCH 0/2] sched,time: reduce nohz_full syscall overhead 40%

[riel]

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 a 10% win.

Not calling __acct_update_integrals and related code unless
jiffies changed (__acct_update_integrals does not do anything
with smaller time intervals anyway) shaves off a further 30%.

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 + both patches			2.3 seconds