[PATCH] powerpc/smp: do not decrement idle task preempt count in CPU offline

[Nathan Lynch]

Valentin Schneider <valentin.schneider at arm.com> writes:

> On 15/10/21 09:55, Nathan Lynch wrote:
>> With PREEMPT_COUNT=y, when a CPU is offlined and then onlined again, we
>> get:
>>
>> BUG: scheduling while atomic: swapper/1/0/0x00000000
>> no locks held by swapper/1/0.
>> CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.15.0-rc2+ #100
>> Call Trace:
>>  dump_stack_lvl+0xac/0x108
>>  __schedule_bug+0xac/0xe0
>>  __schedule+0xcf8/0x10d0
>>  schedule_idle+0x3c/0x70
>>  do_idle+0x2d8/0x4a0
>>  cpu_startup_entry+0x38/0x40
>>  start_secondary+0x2ec/0x3a0
>>  start_secondary_prolog+0x10/0x14
>>
>> This is because powerpc's arch_cpu_idle_dead() decrements the idle task's
>> preempt count, for reasons explained in commit a7c2bb8279d2 ("powerpc:
>> Re-enable preemption before cpu_die()"), specifically "start_secondary()
>> expects a preempt_count() of 0."
>>
>> However, since commit 2c669ef6979c ("powerpc/preempt: Don't touch the idle
>> task's preempt_count during hotplug") and commit f1a0a376ca0c ("sched/core:
>> Initialize the idle task with preemption disabled"), that justification no
>> longer holds.
>>
>> The idle task isn't supposed to re-enable preemption, so remove the
>> vestigial preempt_enable() from the CPU offline path.
>>
>
> Humph, I got confused because 2c669ef6979c explicitly mentions hotplug,
> but that's the hotplug machinery which is already involved for bringing up
> the secondaries at boot time.
>
> IIUC your issue here is the preempt_count being messed up when
> hot-unplugging a CPU, which leads to fireworks during hotplug

That's right.

> (IOW I didn't
> test my last patch against hotplug - my bad!)
>
> Reviewed-by: Valentin Schneider <valentin.schneider at arm.com>

No worries and thank you for reviewing.


>> Tested with pseries and powernv in qemu, and pseries on PowerVM.
>>
>> Fixes: 2c669ef6979c ("powerpc/preempt: Don't touch the idle task's preempt_count during hotplug")
>> Fixes: f1a0a376ca0c ("sched/core: Initialize the idle task with preemption disabled")
>
> I think only the first Fixes: is needed.

OK, I'll re-send with that changed as well as your r-b. Thanks.