Re: CFS scheduler unfairly prefers pinned tasks

[Mike Galbraith <umgwanakikbuti@gmail.com>]

On Tue, 2015-10-06 at 04:45 +0200, Mike Galbraith wrote:
> On Tue, 2015-10-06 at 08:48 +1100, paul.szabo@sydney.edu.au wrote:
> > The Linux CFS scheduler prefers pinned tasks and unfairly
> > gives more CPU time to tasks that have set CPU affinity.
> > This effect is observed with or without CGROUP controls.
> > 
> > To demonstrate: on an otherwise idle machine, as some user
> > run several processes pinned to each CPU, one for each CPU
> > (as many as CPUs present in the system) e.g. for a quad-core
> > non-HyperThreaded machine:
> > 
> >   taskset -c 0 perl -e 'while(1){1}' &
> >   taskset -c 1 perl -e 'while(1){1}' &
> >   taskset -c 2 perl -e 'while(1){1}' &
> >   taskset -c 3 perl -e 'while(1){1}' &
> > 
> > and (as that same or some other user) run some without
> > pinning:
> > 
> >   perl -e 'while(1){1}' &
> >   perl -e 'while(1){1}' &
> > 
> > and use e.g.   top   to observe that the pinned processes get
> > more CPU time than "fair".

I see a fairness issue with pinned tasks and group scheduling, but one
opposite to your complaint.
 
Two task groups, one with 8 hogs (oink), one with 1 (pert), all are pinned.
  PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ P COMMAND
 3269 root      20   0    4060    724    648 R 100.0 0.004   1:00.02 1 oink
 3270 root      20   0    4060    652    576 R 100.0 0.004   0:59.84 2 oink
 3271 root      20   0    4060    692    616 R 100.0 0.004   0:59.95 3 oink
 3274 root      20   0    4060    608    532 R 100.0 0.004   1:00.01 6 oink
 3273 root      20   0    4060    728    652 R 99.90 0.005   0:59.98 5 oink
 3272 root      20   0    4060    644    568 R 99.51 0.004   0:59.80 4 oink
 3268 root      20   0    4060    612    536 R 99.41 0.004   0:59.67 0 oink
 3279 root      20   0    8312    804    708 R 88.83 0.005   0:53.06 7 pert
 3275 root      20   0    4060    656    580 R 11.07 0.004   0:06.98 7 oink
.
That group share math would make a huge compute group with progress
checkpoints sharing an SGI monster with one other hog amusing to watch.
  
	-Mike