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nohz_full: Document additional restrictions

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This commit calls out the potential for slowing the tick even when there
are multiple runnable processes per CPU,  It also points out that current
mainlined version keeps the tick going on at least one CPU even when all
CPUs are otherwise idle.  Finally, it notes the need for a 1-HZ tick in
order to calculate CPU load, maintain sched average, compute CFS entity
vruntime, compute avenrun, and carry out load balancing.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
This commit is contained in:
Paul E. McKenney 2013-05-13 10:32:10 -07:00
parent 295fde89be
commit ce5f4fc861
1 changed files with 18 additions and 3 deletions
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21
Documentation/timers/NO_HZ.txt
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@ -278,6 +278,11 @@ o Adaptive-ticks does not do anything unless there is only one
single runnable SCHED_FIFO task and multiple runnable SCHED_OTHER
tasks, even though these interrupts are unnecessary.
And even when there are multiple runnable tasks on a given CPU,
there is little point in interrupting that CPU until the current
running task's timeslice expires, which is almost always way
longer than the time of the next scheduling-clock interrupt.
Better handling of these sorts of situations is future work.
o A reboot is required to reconfigure both adaptive idle and RCU
@ -308,6 +313,16 @@ o Unless all CPUs are idle, at least one CPU must keep the
scheduling-clock interrupt going in order to support accurate
timekeeping.
o If there are adaptive-ticks CPUs, there will be at least one
CPU keeping the scheduling-clock interrupt going, even if all
CPUs are otherwise idle.
o If there might potentially be some adaptive-ticks CPUs, there
will be at least one CPU keeping the scheduling-clock interrupt
going, even if all CPUs are otherwise idle.
Better handling of this situation is ongoing work.
o Some process-handling operations still require the occasional
scheduling-clock tick. These operations include calculating CPU
load, maintaining sched average, computing CFS entity vruntime,
computing avenrun, and carrying out load balancing. They are
currently accommodated by scheduling-clock tick every second
or so. On-going work will eliminate the need even for these
infrequent scheduling-clock ticks.