Re: [PATCH v2 1/1] sched/fair: Fix unfairness caused by missing load decay

[Peter Zijlstra]

On Tue, May 04, 2021 at 09:14:12AM +0200, Vincent Guittot wrote:
> On Sat, 1 May 2021 at 16:22, Odin Ugedal <odin@xxxxxxx> wrote:
> >
> > This fixes an issue where old load on a cfs_rq is not properly decayed,
> > resulting in strange behavior where fairness can decrease drastically.
> > Real workloads with equally weighted control groups have ended up
> > getting a respective 99% and 1%(!!) of cpu time.
> >
> > When an idle task is attached to a cfs_rq by attaching a pid to a cgroup,
> > the old load of the task is attached to the new cfs_rq and sched_entity by
> > attach_entity_cfs_rq. If the task is then moved to another cpu (and
> > therefore cfs_rq) before being enqueued/woken up, the load will be moved
> > to cfs_rq->removed from the sched_entity. Such a move will happen when
> > enforcing a cpuset on the task (eg. via a cgroup) that force it to move.
> >
> > The load will however not be removed from the task_group itself, making
> > it look like there is a constant load on that cfs_rq. This causes the
> > vruntime of tasks on other sibling cfs_rq's to increase faster than they
> > are supposed to; causing severe fairness issues. If no other task is
> > started on the given cfs_rq, and due to the cpuset it would not happen,
> > this load would never be properly unloaded. With this patch the load
> > will be properly removed inside update_blocked_averages. This also
> > applies to tasks moved to the fair scheduling class and moved to another
> > cpu, and this path will also fix that. For fork, the entity is queued
> > right away, so this problem does not affect that.
> >
> > This applies to cases where the new process is the first in the cfs_rq,
> > issue introduced 3d30544f0212 ("sched/fair: Apply more PELT fixes"), and
> > when there has previously been load on the cgroup but the cgroup was
> > removed from the leaflist due to having null PELT load, indroduced
> > in 039ae8bcf7a5 ("sched/fair: Fix O(nr_cgroups) in the load balancing
> > path").
> >
> > For a simple cgroup hierarchy (as seen below) with two equally weighted
> > groups, that in theory should get 50/50 of cpu time each, it often leads
> > to a load of 60/40 or 70/30.
> >
> > parent/
> >   cg-1/
> >     cpu.weight: 100
> >     cpuset.cpus: 1
> >   cg-2/
> >     cpu.weight: 100
> >     cpuset.cpus: 1
> >
> > If the hierarchy is deeper (as seen below), while keeping cg-1 and cg-2
> > equally weighted, they should still get a 50/50 balance of cpu time.
> > This however sometimes results in a balance of 10/90 or 1/99(!!) between
> > the task groups.
> >
> > $ ps u -C stress
> > USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
> > root       18568  1.1  0.0   3684   100 pts/12   R+   13:36   0:00 stress --cpu 1
> > root       18580 99.3  0.0   3684   100 pts/12   R+   13:36   0:09 stress --cpu 1
> >
> > parent/
> >   cg-1/
> >     cpu.weight: 100
> >     sub-group/
> >       cpu.weight: 1
> >       cpuset.cpus: 1
> >   cg-2/
> >     cpu.weight: 100
> >     sub-group/
> >       cpu.weight: 10000
> >       cpuset.cpus: 1
> >
> > This can be reproduced by attaching an idle process to a cgroup and
> > moving it to a given cpuset before it wakes up. The issue is evident in
> > many (if not most) container runtimes, and has been reproduced
> > with both crun and runc (and therefore docker and all its "derivatives"),
> > and with both cgroup v1 and v2.
> >
> > Fixes: 3d30544f0212 ("sched/fair: Apply more PELT fixes")
> > Fixes: 039ae8bcf7a5 ("sched/fair: Fix O(nr_cgroups) in the load balancing path")
> > Signed-off-by: Odin Ugedal <odin@xxxxxxx>
> 
> Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

Thanks!