Re: [PATCHSET for-4.11] cgroup: implement cgroup v2 thread mode

[Mike Galbraith <efault@gmx.de>]

On Sun, 2017-02-12 at 14:05 +0900, Tejun Heo wrote:

> > I think cgroup tree depth is a more significant issue; because of
> > hierarchy we often do tree walks (uo-to-root or down-to-task).
> > 
> > So creating elaborate trees is something I try not to do.
> 
> So, as long as the depth stays reasonable (single digit or lower),
> what we try to do is keeping tree traversal operations aggregated or
> located on slow paths.  There still are places that this overhead
> shows up (e.g. the block controllers aren't too optimized) but it
> isn't particularly difficult to make a handful of layers not matter at
> all.

A handful of cpu bean counting layers stings considerably.

homer:/abuild # pipe-test 1                          
2.010057 usecs/loop -- avg 2.010057 995.0 KHz
2.006630 usecs/loop -- avg 2.009714 995.2 KHz
2.127118 usecs/loop -- avg 2.021455 989.4 KHz
2.256244 usecs/loop -- avg 2.044934 978.0 KHz
1.993693 usecs/loop -- avg 2.039810 980.5 KHz
^C
homer:/abuild # cgexec -g cpu:hurt pipe-test 1
2.771641 usecs/loop -- avg 2.771641 721.6 KHz
2.432333 usecs/loop -- avg 2.737710 730.5 KHz
2.750493 usecs/loop -- avg 2.738988 730.2 KHz
2.663203 usecs/loop -- avg 2.731410 732.2 KHz
2.762564 usecs/loop -- avg 2.734525 731.4 KHz
^C
homer:/abuild # cgexec -g cpu:hurt/pain pipe-test 1
2.967201 usecs/loop -- avg 2.967201 674.0 KHz
3.049012 usecs/loop -- avg 2.975382 672.2 KHz
3.031226 usecs/loop -- avg 2.980966 670.9 KHz
2.954259 usecs/loop -- avg 2.978296 671.5 KHz
2.933432 usecs/loop -- avg 2.973809 672.5 KHz
^C
...
homer:/abuild # cgexec -g cpu:hurt/pain/ouch/moan/groan pipe-test 1
4.417044 usecs/loop -- avg 4.417044 452.8 KHz
4.494913 usecs/loop -- avg 4.424831 452.0 KHz
4.253861 usecs/loop -- avg 4.407734 453.7 KHz
4.378059 usecs/loop -- avg 4.404766 454.1 KHz
4.179895 usecs/loop -- avg 4.382279 456.4 KHz