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]; -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/