tree 4cf72644a4253ddde5d8695097c2fd91d4466590
parent 0d8f0c6c250b8626e1016f1fc63878cc8989a539
author Avery Pennarun <apenwarr@gmail.com> 1413943673 -0400
committer Avery Pennarun <apenwarr@gmail.com> 1414208030 -0400

printk_persist: flush the printk buffer all the way to RAM.

Previously, the buffer was getting stuck in L2 cache, which is lost on CPU
reset, so the last kernel messages before a reboot tended to be lost.  This
was mitigated somewhat since switching to userspace seems to rapidly
invalidate most/all of the L2 cache, but usually the last kernel messages
before a reboot would be missing.  This was especially true in the case of
kernel panics, unfortunately, which means we have probably been missing a
lot of kernel panics.

Performance effect on mindspeed c2000 CPU: for a loop printk'ing 1000
messages in a tight loop, flushing cache after each iteration:
	original code: 8ms
	flush_range on the entire 8MB printk_buffer: 7201ms
	flush_range twice on the buffer: 14388ms
	flush_all (no range specified): 138ms
	flush_all twice: 253ms
	flush_range only the logbits array: 8ms
	flush_range logbits + only changed section of log_buf: 7ms

So:
- flushing twice takes twice as long.
- flushing a large range is slower than flushing everything.
- flushing just the right part makes it *faster*, which is a bit weird
  (but the results were consistent across several runs)

Anyway, at least this patch doesn't make it slower.

Change-Id: I3b263be1816de2dbb4aecb806501ad36e23f47fe