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linux/kernel/sched
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Odin Ugedal 0258bdfaff sched/fair: Fix unfairness caused by missing load decay 
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 3d30544f02 ("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 039ae8bcf7 ("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: 3d30544f02 ("sched/fair: Apply more PELT fixes")
Fixes: 039ae8bcf7 ("sched/fair: Fix O(nr_cgroups) in the load balancing path")
Signed-off-by: Odin Ugedal <odin@uged.al>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/20210501141950.23622-2-odin@uged.al
2021-05-06 15:33:27 +02:00
..
autogroup.c sched/autogroup: Make autogroup_path() always available 2019-06-24 19:23:40 +02:00
autogroup.h
clock.c sched: Fix various typos 2021-03-22 00:11:52 +01:00
completion.c completion: Use lockdep_assert_RT_in_threaded_ctx() in complete_all() 2020-03-23 18:40:25 +01:00
core.c sched: Fix out-of-bound access in uclamp 2021-05-06 15:33:26 +02:00
cpuacct.c sched: Fix various typos 2021-03-22 00:11:52 +01:00
cpudeadline.c sched,rt: Use the full cpumask for balancing 2020-11-10 18:39:00 +01:00
cpudeadline.h
cpufreq_schedutil.c Scheduler updates for this cycle are: 2021-04-28 13:33:57 -07:00
cpufreq.c cpufreq: Avoid leaving stale IRQ work items during CPU offline 2019-12-12 17:59:43 +01:00
cpupri.c sched: Fix various typos 2021-03-22 00:11:52 +01:00
cpupri.h sched/cpupri: Add CPUPRI_HIGHER 2020-10-29 11:00:30 +01:00
cputime.c Scheduler updates for this cycle are: 2021-04-28 13:33:57 -07:00
deadline.c sched: Fix various typos 2021-03-22 00:11:52 +01:00
debug.c sched/debug: Fix cgroup_path[] serialization 2021-04-21 13:55:42 +02:00
fair.c sched/fair: Fix unfairness caused by missing load decay 2021-05-06 15:33:27 +02:00
features.h sched: Warn on long periods of pending need_resched 2021-04-21 13:55:41 +02:00
idle.c sched: Fix various typos 2021-03-22 00:11:52 +01:00
isolation.c isolcpus: Affine unbound kernel threads to housekeeping cpus 2020-06-15 14:10:03 +02:00
loadavg.c sched: Fix various typos 2021-03-22 00:11:52 +01:00
Makefile kcsan: Improve various small stylistic details 2019-11-20 10:47:23 +01:00
membarrier.c sched/membarrier: fix missing local execution of ipi_sync_rq_state() 2021-03-06 12:40:21 +01:00
pelt.c sched: Fix various typos 2021-03-22 00:11:52 +01:00
pelt.h sched: Fix various typos 2021-03-22 00:11:52 +01:00
psi.c psi: Fix psi state corruption when schedule() races with cgroup move 2021-05-06 15:33:26 +02:00
rt.c sched: Fix various typos 2021-03-22 00:11:52 +01:00
sched-pelt.h sched/fair: Fix "runnable_avg_yN_inv" not used warnings 2019-06-17 12:15:58 +02:00
sched.h sched: Warn on long periods of pending need_resched 2021-04-21 13:55:41 +02:00
smp.h sched/headers: Split out open-coded prototypes into kernel/sched/smp.h 2020-05-28 11:03:20 +02:00
stats.c sched: Fix various typos 2021-03-22 00:11:52 +01:00
stats.h psi: Optimize task switch inside shared cgroups 2021-03-06 12:40:23 +01:00
stop_task.c sched: Remove select_task_rq()'s sd_flag parameter 2020-11-10 18:39:06 +01:00
swait.c sched/swait: Prepare usage in completions 2020-03-21 16:00:23 +01:00
topology.c sched/debug: Rename the sched_debug parameter to sched_verbose 2021-04-17 13:22:44 +02:00
wait_bit.c sched/wait: fix ___wait_var_event(exclusive) 2019-12-17 13:32:50 +01:00
wait.c sched/wait: Add add_wait_queue_priority() 2020-11-15 09:49:09 -05:00