Commit-ID: ce5f4fc861e84739289c187f147040e6af6599b2 Gitweb: http://git.kernel.org/tip/ce5f4fc861e84739289c187f147040e6af6599b2 Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> AuthorDate: Mon, 13 May 2013 10:32:10 -0700 Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> CommitDate: Mon, 10 Jun 2013 13:42:38 -0700 nohz_full: Document additional restrictions 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@xxxxxxxxxxxxxxxxxx> Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx> --- Documentation/timers/NO_HZ.txt | 21 ++++++++++++++++++--- 1 file changed, 18 insertions(+), 3 deletions(-) diff --git a/Documentation/timers/NO_HZ.txt b/Documentation/timers/NO_HZ.txt index d5323e0..8869758 100644 --- a/Documentation/timers/NO_HZ.txt +++ b/Documentation/timers/NO_HZ.txt @@ -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. -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
|