Profiling reveals that a lot of the overhead from the nohz_full
accounting seems to come not from the accounting itself, but from
disabling and re-enabling interrupts.
This patch series removes the interrupt disabling & re-enabling
from __acct_update_integrals, which is called on both syscall
entry and exit, as well as from the user_exit called on syscall
entry.
Together they speed up a benchmark that calls getpriority in
a row 10 million times by about 10%:
run time system time
vanilla 5.49s 2.08s
__acct patch 5.21s 1.92s
both patches 4.88s 1.71s
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