Re: [PATCH 3/3] sched: Use cpu_dying() to fix balance_push vs hotplug-rollback

[Valentin Schneider <valentin.schneider@arm.com>]

On 20/04/21 16:58, Peter Zijlstra wrote:
> On Tue, Apr 20, 2021 at 04:39:04PM +0200, Peter Zijlstra wrote:
>> On Tue, Apr 20, 2021 at 04:20:56PM +0200, Peter Zijlstra wrote:
>> > On Tue, Apr 20, 2021 at 10:46:33AM +0100, Vincent Donnefort wrote:
>> >
>> > > Found the issue:
>> > >
>> > > $ cat hotplug/states:
>> > > 219: sched:active
>> > > 220: online
>> > >
>> > > CPU0:
>> > >
>> > > $ echo 219 > hotplug/fail
>> > > $ echo 0 > online
>> > >
>> > > => cpu_active = 1 cpu_dying = 1
>> > >
>> > > which means that later on, for another CPU hotunplug, in
>> > > __balance_push_cpu_stop(), the fallback rq for a kthread can select that
>> > > CPU0, but __migrate_task() would fail and we end-up in an infinite loop,
>> > > trying to migrate that task to CPU0.
>> > >
>> > > The problem is that for a failure in sched:active, as "online" has no callback,
>> > > there will be no call to cpuhp_invoke_callback(). Hence, the cpu_dying bit would
>> > > not be reset.
>> >
>> > Urgh! Good find.
>
>> I seem to have triggered the BUG() in select_fallback_rq() with your recipie.
>> Have cpu0 fail on sched:active, then offline all other CPUs.
>>
>> Now lemme add that patch.
>
> (which obviously didn't actually build) seems to fix it.
>

Moving the cpu_dying_mask update from cpuhp_invoke_callback() to
cpuhp_{set, reset}_state() means we lose an update in cpuhp_issue_call(),
but AFAICT that wasn't required (this doesn't actually change a CPU's
hotplug state, rather executes some newly installed/removed callbacks whose
state maps below the CPU's current hp state).

Actually staring at it some more, it might have caused bugs: if a
cpuhp_setup_state() fails, we can end up in cpuhp_rollback_install() which
will end up calling cpuhp_invoke_callback(bringup=false) and mess with the
dying mask.

FWIW:

Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>