Re: PowerPC guest getting "BUG: scheduling while atomic" on linux-next-20210623 during secondary CPUs bringup

Re: PowerPC guest getting "BUG: scheduling while atomic" on linux-next-20210623 during secondary CPUs bringup

[Srikar Dronamraju]

* Bharata B Rao <bharata@linux.ibm.com> [2021-06-24 21:25:09]:

> A PowerPC KVM guest gets the following BUG message when booting
> linux-next-20210623:
> 
> smp: Bringing up secondary CPUs ...
> BUG: scheduling while atomic: swapper/1/0/0x00000000
> no locks held by swapper/1/0.
> Modules linked in:
> CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.13.0-rc7-next-20210623
> Call Trace:
> [c00000000ae5bc20] [c000000000badc64] dump_stack_lvl+0x98/0xe0 (unreliable)
> [c00000000ae5bc60] [c000000000210200] __schedule_bug+0xb0/0xe0
> [c00000000ae5bcd0] [c000000001609e28] __schedule+0x1788/0x1c70
> [c00000000ae5be20] [c00000000160a8cc] schedule_idle+0x3c/0x70
> [c00000000ae5be50] [c00000000022984c] do_idle+0x2bc/0x420
> [c00000000ae5bf00] [c000000000229d88] cpu_startup_entry+0x38/0x40
> [c00000000ae5bf30] [c0000000000666c0] start_secondary+0x290/0x2a0
> [c00000000ae5bf90] [c00000000000be54] start_secondary_prolog+0x10/0x14
> 
> <The above repeats for all the secondary CPUs>
> 
> smp: Brought up 2 nodes, 16 CPUs
> numa: Node 0 CPUs: 0-7
> numa: Node 1 CPUs: 8-15
> 
> This seems to have started from next-20210521 and isn't seen on
> next-20210511.
> 

Bharata,

I think the regression is due to Commit f1a0a376ca0c ("sched/core:
Initialize the idle task with preemption disabled")

Can you please try with the above commit reverted?

-- 
Thanks and Regards
Srikar Dronamraju

Re: PowerPC guest getting "BUG: scheduling while atomic" on linux-next-20210623 during secondary CPUs bringup

[Bharata B Rao]

On Fri, Jun 25, 2021 at 11:16:08AM +0530, Srikar Dronamraju wrote:
> * Bharata B Rao <bharata@linux.ibm.com> [2021-06-24 21:25:09]:
> 
> > A PowerPC KVM guest gets the following BUG message when booting
> > linux-next-20210623:
> > 
> > smp: Bringing up secondary CPUs ...
> > BUG: scheduling while atomic: swapper/1/0/0x00000000
> > no locks held by swapper/1/0.
> > Modules linked in:
> > CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.13.0-rc7-next-20210623
> > Call Trace:
> > [c00000000ae5bc20] [c000000000badc64] dump_stack_lvl+0x98/0xe0 (unreliable)
> > [c00000000ae5bc60] [c000000000210200] __schedule_bug+0xb0/0xe0
> > [c00000000ae5bcd0] [c000000001609e28] __schedule+0x1788/0x1c70
> > [c00000000ae5be20] [c00000000160a8cc] schedule_idle+0x3c/0x70
> > [c00000000ae5be50] [c00000000022984c] do_idle+0x2bc/0x420
> > [c00000000ae5bf00] [c000000000229d88] cpu_startup_entry+0x38/0x40
> > [c00000000ae5bf30] [c0000000000666c0] start_secondary+0x290/0x2a0
> > [c00000000ae5bf90] [c00000000000be54] start_secondary_prolog+0x10/0x14
> > 
> > <The above repeats for all the secondary CPUs>
> > 
> > smp: Brought up 2 nodes, 16 CPUs
> > numa: Node 0 CPUs: 0-7
> > numa: Node 1 CPUs: 8-15
> > 
> > This seems to have started from next-20210521 and isn't seen on
> > next-20210511.
> > 
> 
> Bharata,
> 
> I think the regression is due to Commit f1a0a376ca0c ("sched/core:
> Initialize the idle task with preemption disabled")
> 
> Can you please try with the above commit reverted?

Yes, reverting that commit helps.

Regards,
Bharata.

Re: PowerPC guest getting "BUG: scheduling while atomic" on linux-next-20210623 during secondary CPUs bringup

[Peter Zijlstra]

On Fri, Jun 25, 2021 at 11:16:08AM +0530, Srikar Dronamraju wrote:
> * Bharata B Rao <bharata@linux.ibm.com> [2021-06-24 21:25:09]:
> 
> > A PowerPC KVM guest gets the following BUG message when booting
> > linux-next-20210623:
> > 
> > smp: Bringing up secondary CPUs ...
> > BUG: scheduling while atomic: swapper/1/0/0x00000000

'funny', your preempt_count is actually too low. The check here is for
preempt_count() == DISABLE_OFFSET (aka. 1 when PREEMPT=y), but you have
0.

> > no locks held by swapper/1/0.
> > Modules linked in:
> > CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.13.0-rc7-next-20210623
> > Call Trace:
> > [c00000000ae5bc20] [c000000000badc64] dump_stack_lvl+0x98/0xe0 (unreliable)
> > [c00000000ae5bc60] [c000000000210200] __schedule_bug+0xb0/0xe0
> > [c00000000ae5bcd0] [c000000001609e28] __schedule+0x1788/0x1c70
> > [c00000000ae5be20] [c00000000160a8cc] schedule_idle+0x3c/0x70
> > [c00000000ae5be50] [c00000000022984c] do_idle+0x2bc/0x420
> > [c00000000ae5bf00] [c000000000229d88] cpu_startup_entry+0x38/0x40
> > [c00000000ae5bf30] [c0000000000666c0] start_secondary+0x290/0x2a0
> > [c00000000ae5bf90] [c00000000000be54] start_secondary_prolog+0x10/0x14
> > 
> > <The above repeats for all the secondary CPUs>
> > 
> > smp: Brought up 2 nodes, 16 CPUs
> > numa: Node 0 CPUs: 0-7
> > numa: Node 1 CPUs: 8-15
> > 
> > This seems to have started from next-20210521 and isn't seen on
> > next-20210511.
> > 
> 
> Bharata,
> 
> I think the regression is due to Commit f1a0a376ca0c ("sched/core:
> Initialize the idle task with preemption disabled")

So that extra preempt_disable() that got removed would've incremented it
to 1 and then things would've been fine.

Except.. Valentin changed things such that preempt_count() should've
been inittialized to 1, instead of 0, but for some raisin that didn't
stick.. what gives.

So we have init_idle(p) -> init_idle_preempt_count(p) ->
task_thread_info(p)->preempt_count = PREEMPT_DISABLED;

But somehow, by the time you're running start_secondary(), that's gotten
to be 0 again. Does DEBUG_PREEMPT give more clues?

Re: PowerPC guest getting "BUG: scheduling while atomic" on linux-next-20210623 during secondary CPUs bringup

[Bharata B Rao]

On Fri, Jun 25, 2021 at 09:28:09AM +0200, Peter Zijlstra wrote:
> On Fri, Jun 25, 2021 at 11:16:08AM +0530, Srikar Dronamraju wrote:
> > * Bharata B Rao <bharata@linux.ibm.com> [2021-06-24 21:25:09]:
> > 
> > > A PowerPC KVM guest gets the following BUG message when booting
> > > linux-next-20210623:
> > > 
> > > smp: Bringing up secondary CPUs ...
> > > BUG: scheduling while atomic: swapper/1/0/0x00000000
> 
> 'funny', your preempt_count is actually too low. The check here is for
> preempt_count() == DISABLE_OFFSET (aka. 1 when PREEMPT=y), but you have
> 0.
> 
> > > no locks held by swapper/1/0.
> > > Modules linked in:
> > > CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.13.0-rc7-next-20210623
> > > Call Trace:
> > > [c00000000ae5bc20] [c000000000badc64] dump_stack_lvl+0x98/0xe0 (unreliable)
> > > [c00000000ae5bc60] [c000000000210200] __schedule_bug+0xb0/0xe0
> > > [c00000000ae5bcd0] [c000000001609e28] __schedule+0x1788/0x1c70
> > > [c00000000ae5be20] [c00000000160a8cc] schedule_idle+0x3c/0x70
> > > [c00000000ae5be50] [c00000000022984c] do_idle+0x2bc/0x420
> > > [c00000000ae5bf00] [c000000000229d88] cpu_startup_entry+0x38/0x40
> > > [c00000000ae5bf30] [c0000000000666c0] start_secondary+0x290/0x2a0
> > > [c00000000ae5bf90] [c00000000000be54] start_secondary_prolog+0x10/0x14
> > > 
> > > <The above repeats for all the secondary CPUs>
> > > 
> > > smp: Brought up 2 nodes, 16 CPUs
> > > numa: Node 0 CPUs: 0-7
> > > numa: Node 1 CPUs: 8-15
> > > 
> > > This seems to have started from next-20210521 and isn't seen on
> > > next-20210511.
> > > 
> > 
> > Bharata,
> > 
> > I think the regression is due to Commit f1a0a376ca0c ("sched/core:
> > Initialize the idle task with preemption disabled")
> 
> So that extra preempt_disable() that got removed would've incremented it
> to 1 and then things would've been fine.
> 
> Except.. Valentin changed things such that preempt_count() should've
> been inittialized to 1, instead of 0, but for some raisin that didn't
> stick.. what gives.
> 
> So we have init_idle(p) -> init_idle_preempt_count(p) ->
> task_thread_info(p)->preempt_count = PREEMPT_DISABLED;
> 
> But somehow, by the time you're running start_secondary(), that's gotten
> to be 0 again. Does DEBUG_PREEMPT give more clues?

PREEMPTION is off here.

Regards,
Bharata.

Re: PowerPC guest getting "BUG: scheduling while atomic" on linux-next-20210623 during secondary CPUs bringup

[Valentin Schneider]

On 25/06/21 09:28, Peter Zijlstra wrote:
> On Fri, Jun 25, 2021 at 11:16:08AM +0530, Srikar Dronamraju wrote:
>> Bharata,
>>
>> I think the regression is due to Commit f1a0a376ca0c ("sched/core:
>> Initialize the idle task with preemption disabled")
>
> So that extra preempt_disable() that got removed would've incremented it
> to 1 and then things would've been fine.
>
> Except.. Valentin changed things such that preempt_count() should've
> been inittialized to 1, instead of 0, but for some raisin that didn't
> stick.. what gives.
>
> So we have init_idle(p) -> init_idle_preempt_count(p) ->
> task_thread_info(p)->preempt_count = PREEMPT_DISABLED;
>
> But somehow, by the time you're running start_secondary(), that's gotten
> to be 0 again. Does DEBUG_PREEMPT give more clues?

Given the preempt_count isn't reset between hotplugs anymore, you might be
able to find the culprit with a hotplug cycle and ftrace with
trace_prempt_off and trace_preempt_on events (requires PREEMPT_TRACER
IIRC).

It's doable at boot time too, but that will mean sifting through many more
events than you'd like...

Re: PowerPC guest getting "BUG: scheduling while atomic" on linux-next-20210623 during secondary CPUs bringup

[Peter Zijlstra]

On Fri, Jun 25, 2021 at 02:23:16PM +0530, Bharata B Rao wrote:
> On Fri, Jun 25, 2021 at 09:28:09AM +0200, Peter Zijlstra wrote:
> > On Fri, Jun 25, 2021 at 11:16:08AM +0530, Srikar Dronamraju wrote:
> > > * Bharata B Rao <bharata@linux.ibm.com> [2021-06-24 21:25:09]:
> > > 
> > > > A PowerPC KVM guest gets the following BUG message when booting
> > > > linux-next-20210623:
> > > > 
> > > > smp: Bringing up secondary CPUs ...
> > > > BUG: scheduling while atomic: swapper/1/0/0x00000000
> > 
> > 'funny', your preempt_count is actually too low. The check here is for
> > preempt_count() == DISABLE_OFFSET (aka. 1 when PREEMPT=y), but you have
> > 0.
> > 
> > > > no locks held by swapper/1/0.
> > > > Modules linked in:
> > > > CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.13.0-rc7-next-20210623
> > > > Call Trace:
> > > > [c00000000ae5bc20] [c000000000badc64] dump_stack_lvl+0x98/0xe0 (unreliable)
> > > > [c00000000ae5bc60] [c000000000210200] __schedule_bug+0xb0/0xe0
> > > > [c00000000ae5bcd0] [c000000001609e28] __schedule+0x1788/0x1c70
> > > > [c00000000ae5be20] [c00000000160a8cc] schedule_idle+0x3c/0x70
> > > > [c00000000ae5be50] [c00000000022984c] do_idle+0x2bc/0x420
> > > > [c00000000ae5bf00] [c000000000229d88] cpu_startup_entry+0x38/0x40
> > > > [c00000000ae5bf30] [c0000000000666c0] start_secondary+0x290/0x2a0
> > > > [c00000000ae5bf90] [c00000000000be54] start_secondary_prolog+0x10/0x14
> > > > 
> > > > <The above repeats for all the secondary CPUs>
> > > > 
> > > > smp: Brought up 2 nodes, 16 CPUs
> > > > numa: Node 0 CPUs: 0-7
> > > > numa: Node 1 CPUs: 8-15
> > > > 
> > > > This seems to have started from next-20210521 and isn't seen on
> > > > next-20210511.
> > > > 
> > > 
> > > Bharata,
> > > 
> > > I think the regression is due to Commit f1a0a376ca0c ("sched/core:
> > > Initialize the idle task with preemption disabled")
> > 
> > So that extra preempt_disable() that got removed would've incremented it
> > to 1 and then things would've been fine.
> > 
> > Except.. Valentin changed things such that preempt_count() should've
> > been inittialized to 1, instead of 0, but for some raisin that didn't
> > stick.. what gives.
> > 
> > So we have init_idle(p) -> init_idle_preempt_count(p) ->
> > task_thread_info(p)->preempt_count = PREEMPT_DISABLED;
> > 
> > But somehow, by the time you're running start_secondary(), that's gotten
> > to be 0 again. Does DEBUG_PREEMPT give more clues?
> 
> PREEMPTION is off here.

You mean: CONFIG_PREEMPTION=n, what about CONFIG_PREEMPT_COUNT?

Because if both are =n, then I don't see how that warning could trigger.
in_atomic_preempt_off() would then result in prempt_count() == 0, and
per the print above, it *is* 0.

Re: PowerPC guest getting "BUG: scheduling while atomic" on linux-next-20210623 during secondary CPUs bringup

[Bharata B Rao]

On Fri, Jun 25, 2021 at 12:16:52PM +0200, Peter Zijlstra wrote:
> You mean: CONFIG_PREEMPTION=n, what about CONFIG_PREEMPT_COUNT?
> 
> Because if both are =n, then I don't see how that warning could trigger.
> in_atomic_preempt_off() would then result in prempt_count() == 0, and
> per the print above, it *is* 0.

CONFIG_PREEMPTION isn't set.

Also other PREEMPT related options are as under:

# CONFIG_PREEMPT_NONE is not set
CONFIG_PREEMPT_VOLUNTARY=y
# CONFIG_PREEMPT is not set
CONFIG_PREEMPT_COUNT=y
CONFIG_PREEMPT_NOTIFIERS=y
CONFIG_PREEMPTIRQ_TRACEPOINTS=y
# CONFIG_PREEMPTIRQ_DELAY_TEST is not set

Regards,
Bharata.