[PATCH V3 10/10] sched/deadline: Prevent CPU hotplug operation if DL task on CPU

Mathieu Poirier
2018-02-13T20:33:37Z

When a DL task is assigned a CPU the "utilisation" (this_bw) and the
"active utilisation" (running_bw) fields of rq->dl are incremented
accordingly. If the CPU is hotplugged out the DL task is transferred
to another CPU but the task's contribution to this_bw and running_bw
isn't substracted from the outgoing CPU's rq nor added to the newly
appointed CPU.

In this example (where a kernel has been instrumented to output the
relevant information) we have a 4 CPU system with one 6:10 DL task that
has been assigned to CPU 2:

root@dragon:/home/linaro/# cat /proc/rq_debug

dl_rq[0]:
.online : yes
.dl_nr_running : 0
.running_bw : 0
.this_bw : 0
.rd->span : 0-3
.dl_nr_migratory : 0
.rd->dl_bw->bw : 3984588
.rd->dl_bw->total_bw : 629145

dl_rq[1]:
.online : yes
.dl_nr_running : 0
.running_bw : 0
.this_bw : 0
.rd->span: : 0-3
.dl_nr_migratory : 0
.rd->dl_bw->bw : 3984588 <-- RD capacity for 4 CPUs
.rd->dl_bw->total_bw : 629145

dl_rq[2]:
.online : yes
.dl_nr_running : 1 <-- One task running
.running_bw : 629145 <-- Normal behavior
.this_bw : 629145 <-- Normal behavior
.rd->span : 0-3
.dl_nr_migratory : 1
.rd->dl_bw->bw : 3984588
.rd->dl_bw->total_bw : 629145

dl_rq[3]:
.online : yes
.dl_nr_running : 0
.running_bw : 0
.this_bw : 0
.rd->span : 0-3
.dl_nr_migratory : 0
.rd->dl_bw->bw : 3984588
.rd->dl_bw->total_bw : 629145

At this point we hotplug out CPU2 and list the status again:

root@dragon:/home/linaro/# echo 0 > /sys/devices/system/cpu/cpu2/online
root@dragon:/home/linaro/# cat /proc/rq_debug

dl_rq[0]:
.online : yes
.dl_nr_running : 1 <-- DL task was moved here
.running_bw : 0 <-- Contribution not added
.this_bw : 0 <-- Contribution not added
.rd->span : 0-1,3
.dl_nr_migratory : 1
.rd->dl_bw->bw : 2988441 <-- RD capacity updated
.rd->dl_bw->total_bw : 629145

dl_rq[1]:
.online : yes
.dl_nr_running : 0
.running_bw : 0
.this_bw : 0
.rd->span : 0-1,3
.dl_nr_migratory : 0
.rd->dl_bw->bw : 2988441
.rd->dl_bw->total_bw : 629145

dl_rq[2]:
.online : no <-- runqueue no longer online
.dl_nr_running : 0 <-- DL task was moved
.running_bw : 629145 <-- Contribution not substracted
.this_bw : 629145 <-- Contribution not substracted
.rd->span : 2
.dl_nr_migratory : 0
.rd->dl_bw->bw : 996147
.rd->dl_bw->total_bw : 0

dl_rq[3]:
.online : yes
.dl_nr_running : 0
.running_bw : 0
.this_bw : 0
.rd->span : 0-1,3
.dl_nr_migratory : 0
.rd->dl_bw->bw : 2988441
.rd->dl_bw->total_bw: 629145

Upon rebooting the system a splat is also produced:

[ 578.184789] ------------[ cut here ]------------
[ 578.184813] dl_rq->running_bw > old
[ 578.184838] WARNING: CPU: 0 PID: 4076 at /home/mpoirier/work/linaro/deadline/kernel/kernel/sched/deadline.c:98 dequeue_task_dl+0x128/0x168
[ 578.191693] Modules linked in:
[ 578.191705] CPU: 0 PID: 4076 Comm: burn Not tainted 4.15.0-00009-gf597fc1e5764-dirty #259
[ 578.191708] Hardware name: Qualcomm Technologies, Inc. APQ 8016 SBC (DT)
[ 578.191713] pstate: 60000085 (nZCv daIf -PAN -UAO)
[ 578.191718] pc : dequeue_task_dl+0x128/0x168
[ 578.191722] lr : dequeue_task_dl+0x128/0x168
[ 578.191724] sp : ffff8000383ebbf0
[ 578.191727] x29: ffff8000383ebbf0 x28: ffff800038288000
[ 578.191733] x27: 0000000000000009 x26: ffff800038890000
[ 578.191739] x25: ffff800038994e60 x24: ffff800038994e00
[ 578.191744] x23: 0000000000000000 x22: 0000000000000000
[ 578.191749] x21: 000000000000000e x20: ffff800038288000
[ 578.191755] x19: ffff80003d950aa8 x18: 0000000000000010
[ 578.191761] x17: 0000000000000001 x16: 0000000000002710
[ 578.191766] x15: 0000000000000006 x14: ffff0000892ed37f
[ 578.191772] x13: ffff0000092ed38d x12: 0000000000000000
[ 578.191778] x11: ffff8000383eb840 x10: 0000000005f5e0ff
[ 578.191784] x9 : 0000000000000034 x8 : 625f676e696e6e75
[ 578.191794] x7 : 723e2d71725f6c64 x6 : 000000000000016c
[ 578.191800] x5 : 0000000000000000 x4 : 0000000000000000
[ 578.191806] x3 : ffffffffffffffff x2 : 000080003480f000
[ 578.191812] x1 : ffff800038288000 x0 : 0000000000000017
[ 578.191818] Call trace:
[ 578.191824] dequeue_task_dl+0x128/0x168
[ 578.191830] sched_move_task+0xa8/0x150
[ 578.191837] sched_autogroup_exit_task+0x20/0x30
[ 578.191843] do_exit+0x2c4/0x9f8
[ 578.191847] do_group_exit+0x3c/0xa0
[ 578.191853] get_signal+0x2a4/0x568
[ 578.191860] do_signal+0x70/0x210
[ 578.191866] do_notify_resume+0xe0/0x138
[ 578.191870] work_pending+0x8/0x10
[ 578.191874] ---[ end trace 345388d10dc698fe ]---

As a stop-gap measure before the real solution is available this patch
prevents users from carrying out a CPU hotplug operation if a DL task is
running (or suspended) on said CPU.

Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
---
kernel/sched/deadline.c | 5 +++++
1 file changed, 5 insertions(+)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 8eb508cf1990..c46aaa7c3569 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2741,11 +2741,16 @@ bool dl_cpu_busy(unsigned int cpu)
int cpus;

rcu_read_lock_sched();
+ overflow = !!(cpu_rq(cpu)->dl.this_bw);
+ if (overflow)
+ goto out;
+
dl_b = dl_bw_of(cpu);
raw_spin_lock_irqsave(&dl_b->lock, flags);
cpus = dl_bw_cpus(cpu);
overflow = __dl_overflow(dl_b, cpus, 0, 0);
raw_spin_unlock_irqrestore(&dl_b->lock, flags);
+out:
rcu_read_unlock_sched();
return overflow;
}
--
2.7.4