Re: [LKP] [mm] 3484b2de949: -46.2% aim7.jobs-per-min

Huang Ying
2015-02-28T07:30:25Z

On Sat, 2015-02-28 at 01:46 +0000, Mel Gorman wrote:
> On Fri, Feb 27, 2015 at 03:21:36PM +0800, Huang Ying wrote:
> > FYI, we noticed the below changes on
> >
> > git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> > commit 3484b2de9499df23c4604a513b36f96326ae81ad ("mm: rearrange zone fields into read-only, page alloc, statistics and page reclaim lines")
> >
> > The perf cpu-cycles for spinlock (zone->lock) increased a lot. I suspect there are some cache ping-pong or false sharing.
> >
>
> Are you sure about this result? I ran similar tests here and found that
> there was a major regression introduced near there but it was commit
> 05b843012335 ("mm: memcontrol: use root_mem_cgroup res_counter") that
> cause the problem and it was later reverted. On local tests on a 4-node
> machine, commit 3484b2de9499df23c4604a513b36f96326ae81ad was within 1%
> of the previous commit and well within the noise.

After applying the below debug patch, the performance regression
restored. So I think we can root cause this regression to be cache line
alignment related issue?

If my understanding were correct, after the 3484b2de94, lock and low
address area free_area are in the same cache line, so that the cache
line of the lock and the low address area of free_area will be switched
between MESI "E" and "S" state because it is written in one CPU (page
allocating with free_area) and frequently read (spinning on lock) in
another CPU.

Best Regards,
Huang, Ying

---
include/linux/mmzone.h | 2 ++
1 file changed, 2 insertions(+)

--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -468,6 +468,8 @@ struct zone {
/* Write-intensive fields used from the page allocator */
spinlock_t lock;

+ ZONE_PADDING(_pad_xx_)
+
/* free areas of different sizes */
struct free_area free_area[MAX_ORDER];