On Mon, May 13, 2013 at 04:03:13PM +0800, Li Zhong wrote:

On Mon, 2013-05-13 at 15:51 +1000, Benjamin Herrenschmidt wrote:

quoted

On Mon, 2013-05-13 at 13:21 +0800, Li Zhong wrote:

quoted

These patches try to support context tracking for Power arch, beginning with
64-bit pSeries. The codes are ported from that of the x86_64, and in each
patch, I listed the corresponding patch for x86.

So that's yet another pile of bloat on all syscall entry/exit and
exception entry/exit. What is it used for ? (I haven't followed on
x86_64 side).

To my understanding, it is used to enable RCU user extended quiescent
state, so RCU on that cpu doesn't need scheduler ticks. And together
with some other code(already in 3.10), we are able to remove the ticks
in some cases (e.g. only 1 task running on the cpu, with some other
limitations).

Maybe Paul, or Frederic could give some better descriptions. 

That's pretty much it. It helps RCU and cputime accounting infrastructure
to know when we enter/exit userspace. This way we can:

* Consider the CPU as idle from an RCU point of view when we run in userspace,
so RCU won't need the tick to stay alive.

* Account cputime (utime/stime/...) without using the tick. Ok powerpc already
has CONFIG_VIRT_CPU_ACCOUNTING_NATIVE but making it working with full dynticks
would require some tweaks.

All in one this is to support full dynticks.

Thanks.