linux-kernel - CFS scheduler unfairly prefers pinned tasks

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Message-Id: <201510052148.t95LmQm6018585@como.maths.usyd.edu.au>
Date:	Tue, 6 Oct 2015 08:48:26 +1100
From:	paul.szabo@...ney.edu.au
To:	linux-kernel@...r.kernel.org
Subject: CFS scheduler unfairly prefers pinned tasks

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".

Fairness is obtained when either:
 - there are as many un-pinned processes as CPUs; or
 - with CGROUP controls and the two kinds of processes run by
   different users, when there is just one un-pinned process; or
 - if the pinning is turned off for these processes (or they
   are started without).

Any insight is welcome!

---

I would appreciate replies direct to me as I am not subscribed to the
linux-kernel mailing list (but will try to watch the archives).

This bug is also reported to Debian, please see
  http://bugs.debian.org/800945

I use Debian with the 3.16 kernel, have not yet tried 4.* kernels.

Thanks, Paul

Paul Szabo   psz@...hs.usyd.edu.au   http://www.maths.usyd.edu.au/u/psz/
School of Mathematics and Statistics   University of Sydney    Australia
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