Re: hackbench regression since 2.6.25-rc

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 13 Mar 2008 15:46:57 +0800 "Zhang, Yanmin" <yanmin_zhang@linux.intel.com> wrote:

> Comparing with 2.6.24, on my 16-core tigerton, hackbench process mode has about
> 40% regression with 2.6.25-rc1, and more than 20% regression with kernel
> 2.6.25-rc4, because rc4 includes the reverting patch of scheduler load balance.
> 
> Command to start it.
> #hackbench 100 process 2000
> I ran it for 3 times and sum the values.
> 
> I tried to investiagte it by bisect.
> Kernel up to tag 0f4dafc0563c6c49e17fe14b3f5f356e4c4b8806 has the 20% regression.
> Kernel up to tag 6e90aa972dda8ef86155eefcdbdc8d34165b9f39 hasn't regression.
> 
> Any bisect between above 2 tags cause kernel hang. I tried to checkout to a point between
> these 2 tags for many times manually and kernel always paniced.
> 
> All patches between the 2 tags are on kobject restructure. I guess such restructure
> creates more cache miss on the 16-core tigerton.
> 

That's pretty surprising - hackbench spends most of its time in userspace
and zeroing out anonymous pages.  It shouldn't be fiddling with kobjects
much at all.

Some kernel profiling might be needed here..