Re: [PATCH 1/1] sched/core: Fix stuck on completion for affine_move_task() when stopper disable

[Kuyo Chang (張建文)]

On Wed, 2023-09-27 at 10:08 +0200, Peter Zijlstra wrote:
>  	 
> External email : Please do not click links or open attachments until
> you have verified the sender or the content.
>  On Wed, Sep 27, 2023 at 11:34:28AM +0800, Kuyo Chang wrote:
> > From: kuyo chang <kuyo.chang@xxxxxxxxxxxx>
> > 
> > [Syndrome] hung detect shows below warning msg
> > [ 4320.666557] [   T56] khungtaskd: [name:hung_task&]INFO: task
> stressapptest:17803 blocked for more than 3600 seconds.
> > [ 4320.666589] [   T56] khungtaskd:
> [name:core&]task:stressapptest   state:D stack:0     pid:17803
> ppid:17579  flags:0x04000008
> > [ 4320.666601] [   T56] khungtaskd: Call trace:
> > [ 4320.666607] [   T56] khungtaskd:  __switch_to+0x17c/0x338
> > [ 4320.666642] [   T56] khungtaskd:  __schedule+0x54c/0x8ec
> > [ 4320.666651] [   T56] khungtaskd:  schedule+0x74/0xd4
> > [ 4320.666656] [   T56] khungtaskd:  schedule_timeout+0x34/0x108
> > [ 4320.666672] [   T56] khungtaskd:  do_wait_for_common+0xe0/0x154
> > [ 4320.666678] [   T56] khungtaskd:  wait_for_completion+0x44/0x58
> > [ 4320.666681] [   T56]
> khungtaskd:  __set_cpus_allowed_ptr_locked+0x344/0x730
> > [ 4320.666702] [   T56]
> khungtaskd:  __sched_setaffinity+0x118/0x160
> > [ 4320.666709] [   T56] khungtaskd:  sched_setaffinity+0x10c/0x248
> > [ 4320.666715] [   T56]
> khungtaskd:  __arm64_sys_sched_setaffinity+0x15c/0x1c0
> > [ 4320.666719] [   T56] khungtaskd:  invoke_syscall+0x3c/0xf8
> > [ 4320.666743] [   T56] khungtaskd:  el0_svc_common+0xb0/0xe8
> > [ 4320.666749] [   T56] khungtaskd:  do_el0_svc+0x28/0xa8
> > [ 4320.666755] [   T56] khungtaskd:  el0_svc+0x28/0x9c
> > [ 4320.666761] [   T56] khungtaskd:  el0t_64_sync_handler+0x7c/0xe4
> > [ 4320.666766] [   T56] khungtaskd:  el0t_64_sync+0x18c/0x190
> > 
> > [Analysis]
> > 
> > After add some debug footprint massage, this issue happened at
> stopper
> > disable case.
> > It cannot exec migration_cpu_stop fun to complete migration.
> > This will cause stuck on wait_for_completion.
> 
> How did you get in this situation?
> 

This issue occurs at CPU hotplug/set_affinity stress test.
The reproduce ratio is very low(about once a week).

So I add/record some debug message to snapshot the task status while it
stuck on wait_for_completion.

Below is the snapshot status while issue happened:

cpu_active_mask is 0xFC
new_mask is 0x8
pending->arg.dest_cpu is 0x3
task_on_cpu(rq,p) is 1
task_cpu is 0x2
p__state = TASK_RUNNING
flag is SCA_CHACK|SCA_USER
stop_one_cpu_nowait(stopper->enabled) return value is false.

I also record the footprint at migration_cpu_stop.
It shows the migration_cpu_stop is not execute.


> > Signed-off-by: kuyo chang <kuyo.chang@xxxxxxxxxxxx>
> > ---
> >  kernel/sched/core.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 1dc0b0287e30..98c217a1caa0 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -3041,8 +3041,9 @@ static int affine_move_task(struct rq *rq,
> struct task_struct *p, struct rq_flag
> >  task_rq_unlock(rq, p, rf);
> >  
> >  if (!stop_pending) {
> > -stop_one_cpu_nowait(cpu_of(rq), migration_cpu_stop,
> > -    &pending->arg, &pending->stop_work);
> > +if (!stop_one_cpu_nowait(cpu_of(rq), migration_cpu_stop,
> > +    &pending->arg, &pending->stop_work))
> > +return -ENOENT;
> 
> And -ENOENT is the right return code for when the target CPU is not
> available?
> 
> I suspect you're missing more than halp the picture and this is a
> band-aid solution at best. Please try harder.
> 

I think -ENOENT means stopper is not execute? 
Perhaps the error code is abused, or could you kindly give me some
suggestions?

Thanks,
Kuyo

> >  }
> >  
> >  if (flags & SCA_MIGRATE_ENABLE)
> > -- 
> > 2.18.0
> >