[PATCH] powerpc64: use fixed lock token for !CONFIG_PPC_SPLPAR

[Kevin Hao]

On Tue, Mar 10, 2015 at 11:15:18AM +1100, Michael Ellerman wrote:
> On Mon, 2015-03-09 at 17:53 +1100, Benjamin Herrenschmidt wrote:
> > On Sat, 2015-03-07 at 19:19 +0800, Kevin Hao wrote:
> > > It makes no sense to use a variant lock token on a platform which
> > > doesn't support for shared-processor logical partitions. Actually we
> > > can eliminate a memory load by using a fixed lock token on these
> > > platforms.
> > 
> > Does this provide an actual measurable benefit ? I found that the lock
> > token was quite handy for debugging ...
> 
> Yeah. It can be very useful to know which cpu holds a lock when you get into a
> dead lock.
> 
> Unless you can show this is a performance boost I'm inclined to leave it as-is.

I am not sure if there is more suitable benchmark for spinlock. I tried the
perf locking benchmark got from here [1]. The test was running on a t4240rdb
board with xfs rootfs. I have run the following commands four times before and
after applying this patch.
    ./perf record ./perf bench locking vfs; ./perf report

Before:
3.06%  locking-vfs  [kernel.kallsyms]   [k] ._raw_spin_lock
3.04%  locking-vfs  [kernel.kallsyms]   [k] ._raw_spin_lock
3.03%  locking-vfs  [kernel.kallsyms]   [k] ._raw_spin_lock
3.00%  locking-vfs  [kernel.kallsyms]   [k] ._raw_spin_lock

After:
3.05%  locking-vfs  [kernel.kallsyms]   [k] ._raw_spin_lock 
2.97%  locking-vfs  [kernel.kallsyms]   [k] ._raw_spin_lock
2.96%  locking-vfs  [kernel.kallsyms]   [k] ._raw_spin_lock
2.97%  locking-vfs  [kernel.kallsyms]   [k] ._raw_spin_lock

[1] http://lkml.iu.edu/hypermail/linux/kernel/1412.1/01419.html

Thanks,
Kevin
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