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

Huang Ying
2015-03-05T05:35:24Z

Hi, Mel,

On Sat, 2015-02-28 at 15:30 +0800, Huang Ying wrote:
> 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.

What do you think about this?

Best Regards,
Huang, Ying

> 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];
>
>