[PATCH v2] arch/arm64: Fix topology initialization for core scheduling

[PATCH v2] arch/arm64: Fix topology initialization for core scheduling

[Phil Auld]

Arm64 systems rely on store_cpu_topology() to call update_siblings_masks()
to transfer the toplogy to the various cpu masks. This needs to be done 
before the call to notify_cpu_starting() which tells the scheduler about 
each cpu found, otherwise the core scheduling data structures are setup 
in a way that does not match the actual topology.

Without this change stress-ng (which enables core scheduling in its prctl 
tests) causes a warning and then a crash (trimmed for legibility):

[ 1853.805168] ------------[ cut here ]------------
[ 1853.809784] task_rq(b)->core != rq->core
[ 1853.809792] WARNING: CPU: 117 PID: 0 at kernel/sched/fair.c:11102 cfs_prio_less+0x1b4/0x1c4
...
[ 1854.015210] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000010
...
[ 1854.231256] Call trace:
[ 1854.233689]  pick_next_task+0x3dc/0x81c
[ 1854.237512]  __schedule+0x10c/0x4cc
[ 1854.240988]  schedule_idle+0x34/0x54

Fixes: 9edeaea1bc45 ("sched: Core-wide rq->lock")
Signed-off-by: Phil Auld <pauld@redhat.com>
---
This is a similar issue to 
  f2703def339c ("MIPS: smp: fill in sibling and core maps earlier") 
which fixed it for MIPS. 

v2: Fixed the commit message. No code change.

 arch/arm64/kernel/smp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 27df5c1e6baa..3b46041f2b97 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -234,6 +234,7 @@ asmlinkage notrace void secondary_start_kernel(void)
 	 * Log the CPU info before it is marked online and might get read.
 	 */
 	cpuinfo_store_cpu();
+	store_cpu_topology(cpu);
 
 	/*
 	 * Enable GIC and timers.
@@ -242,7 +243,6 @@ asmlinkage notrace void secondary_start_kernel(void)
 
 	ipi_setup(cpu);
 
-	store_cpu_topology(cpu);
 	numa_add_cpu(cpu);
 
 	/*
-- 
2.18.0

[PATCH v2] arch/arm64: Fix topology initialization for core scheduling

[Phil Auld]

Arm64 systems rely on store_cpu_topology() to call update_siblings_masks()
to transfer the toplogy to the various cpu masks. This needs to be done 
before the call to notify_cpu_starting() which tells the scheduler about 
each cpu found, otherwise the core scheduling data structures are setup 
in a way that does not match the actual topology.

Without this change stress-ng (which enables core scheduling in its prctl 
tests) causes a warning and then a crash (trimmed for legibility):

[ 1853.805168] ------------[ cut here ]------------
[ 1853.809784] task_rq(b)->core != rq->core
[ 1853.809792] WARNING: CPU: 117 PID: 0 at kernel/sched/fair.c:11102 cfs_prio_less+0x1b4/0x1c4
...
[ 1854.015210] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000010
...
[ 1854.231256] Call trace:
[ 1854.233689]  pick_next_task+0x3dc/0x81c
[ 1854.237512]  __schedule+0x10c/0x4cc
[ 1854.240988]  schedule_idle+0x34/0x54

Fixes: 9edeaea1bc45 ("sched: Core-wide rq->lock")
Signed-off-by: Phil Auld <pauld@redhat.com>
---
This is a similar issue to 
  f2703def339c ("MIPS: smp: fill in sibling and core maps earlier") 
which fixed it for MIPS. 

v2: Fixed the commit message. No code change.

 arch/arm64/kernel/smp.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 27df5c1e6baa..3b46041f2b97 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -234,6 +234,7 @@ asmlinkage notrace void secondary_start_kernel(void)
 	 * Log the CPU info before it is marked online and might get read.
 	 */
 	cpuinfo_store_cpu();
+	store_cpu_topology(cpu);
 
 	/*
 	 * Enable GIC and timers.
@@ -242,7 +243,6 @@ asmlinkage notrace void secondary_start_kernel(void)
 
 	ipi_setup(cpu);
 
-	store_cpu_topology(cpu);
 	numa_add_cpu(cpu);
 
 	/*
-- 
2.18.0


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Re: [PATCH v2] arch/arm64: Fix topology initialization for core scheduling

[Dietmar Eggemann]

On 30/03/2022 17:56, Phil Auld wrote:
> Arm64 systems rely on store_cpu_topology() to call update_siblings_masks()
> to transfer the toplogy to the various cpu masks. This needs to be done 
> before the call to notify_cpu_starting() which tells the scheduler about 
> each cpu found, otherwise the core scheduling data structures are setup 
> in a way that does not match the actual topology.
> 
> Without this change stress-ng (which enables core scheduling in its prctl 
> tests) causes a warning and then a crash (trimmed for legibility):
> 
> [ 1853.805168] ------------[ cut here ]------------
> [ 1853.809784] task_rq(b)->core != rq->core
> [ 1853.809792] WARNING: CPU: 117 PID: 0 at kernel/sched/fair.c:11102 cfs_prio_less+0x1b4/0x1c4
> ...
> [ 1854.015210] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000010
> ...
> [ 1854.231256] Call trace:
> [ 1854.233689]  pick_next_task+0x3dc/0x81c
> [ 1854.237512]  __schedule+0x10c/0x4cc
> [ 1854.240988]  schedule_idle+0x34/0x54
> 
> Fixes: 9edeaea1bc45 ("sched: Core-wide rq->lock")
> Signed-off-by: Phil Auld <pauld@redhat.com>
> ---
> This is a similar issue to 
>   f2703def339c ("MIPS: smp: fill in sibling and core maps earlier") 
> which fixed it for MIPS. 
> 
> v2: Fixed the commit message. No code change.

Ah, the reason is that smt_mask is not correctly setup, so we bail on
`cpumask_weight(smt_mask) == 1` for !leaders in:

notify_cpu_starting()
  cpuhp_invoke_callback_range()
    sched_cpu_starting()
      sched_core_cpu_starting()

which leads to rq->core not being correctly set for !leader-rq's.

LGTM. Tested on: HPE Apollo 70 X1

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>

[...]

Re: [PATCH v2] arch/arm64: Fix topology initialization for core scheduling

[Dietmar Eggemann]

On 30/03/2022 17:56, Phil Auld wrote:
> Arm64 systems rely on store_cpu_topology() to call update_siblings_masks()
> to transfer the toplogy to the various cpu masks. This needs to be done 
> before the call to notify_cpu_starting() which tells the scheduler about 
> each cpu found, otherwise the core scheduling data structures are setup 
> in a way that does not match the actual topology.
> 
> Without this change stress-ng (which enables core scheduling in its prctl 
> tests) causes a warning and then a crash (trimmed for legibility):
> 
> [ 1853.805168] ------------[ cut here ]------------
> [ 1853.809784] task_rq(b)->core != rq->core
> [ 1853.809792] WARNING: CPU: 117 PID: 0 at kernel/sched/fair.c:11102 cfs_prio_less+0x1b4/0x1c4
> ...
> [ 1854.015210] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000010
> ...
> [ 1854.231256] Call trace:
> [ 1854.233689]  pick_next_task+0x3dc/0x81c
> [ 1854.237512]  __schedule+0x10c/0x4cc
> [ 1854.240988]  schedule_idle+0x34/0x54
> 
> Fixes: 9edeaea1bc45 ("sched: Core-wide rq->lock")
> Signed-off-by: Phil Auld <pauld@redhat.com>
> ---
> This is a similar issue to 
>   f2703def339c ("MIPS: smp: fill in sibling and core maps earlier") 
> which fixed it for MIPS. 
> 
> v2: Fixed the commit message. No code change.

Ah, the reason is that smt_mask is not correctly setup, so we bail on
`cpumask_weight(smt_mask) == 1` for !leaders in:

notify_cpu_starting()
  cpuhp_invoke_callback_range()
    sched_cpu_starting()
      sched_core_cpu_starting()

which leads to rq->core not being correctly set for !leader-rq's.

LGTM. Tested on: HPE Apollo 70 X1

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>

[...]


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Re: [PATCH v2] arch/arm64: Fix topology initialization for core scheduling

[Phil Auld]

On Thu, Mar 31, 2022 at 11:04:31AM +0200 Dietmar Eggemann wrote:
> On 30/03/2022 17:56, Phil Auld wrote:
> > Arm64 systems rely on store_cpu_topology() to call update_siblings_masks()
> > to transfer the toplogy to the various cpu masks. This needs to be done 
> > before the call to notify_cpu_starting() which tells the scheduler about 
> > each cpu found, otherwise the core scheduling data structures are setup 
> > in a way that does not match the actual topology.
> > 
> > Without this change stress-ng (which enables core scheduling in its prctl 
> > tests) causes a warning and then a crash (trimmed for legibility):
> > 
> > [ 1853.805168] ------------[ cut here ]------------
> > [ 1853.809784] task_rq(b)->core != rq->core
> > [ 1853.809792] WARNING: CPU: 117 PID: 0 at kernel/sched/fair.c:11102 cfs_prio_less+0x1b4/0x1c4
> > ...
> > [ 1854.015210] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000010
> > ...
> > [ 1854.231256] Call trace:
> > [ 1854.233689]  pick_next_task+0x3dc/0x81c
> > [ 1854.237512]  __schedule+0x10c/0x4cc
> > [ 1854.240988]  schedule_idle+0x34/0x54
> > 
> > Fixes: 9edeaea1bc45 ("sched: Core-wide rq->lock")
> > Signed-off-by: Phil Auld <pauld@redhat.com>
> > ---
> > This is a similar issue to 
> >   f2703def339c ("MIPS: smp: fill in sibling and core maps earlier") 
> > which fixed it for MIPS. 
> > 
> > v2: Fixed the commit message. No code change.
> 
> Ah, the reason is that smt_mask is not correctly setup, so we bail on
> `cpumask_weight(smt_mask) == 1` for !leaders in:
> 
> notify_cpu_starting()
>   cpuhp_invoke_callback_range()
>     sched_cpu_starting()
>       sched_core_cpu_starting()
> 
> which leads to rq->core not being correctly set for !leader-rq's.
> 

Exactly, sorry I was not clearer.  smt_mask must be setup correctly 
by the time sched_core_cpu_starting() is called. (Maybe I should crib
some of the above lines into the commit message?)


> LGTM. Tested on: HPE Apollo 70 X1
> 
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> 

Thanks!



Cheers,
Phil


> [...]
> 

-- 

Re: [PATCH v2] arch/arm64: Fix topology initialization for core scheduling

[Phil Auld]

On Thu, Mar 31, 2022 at 11:04:31AM +0200 Dietmar Eggemann wrote:
> On 30/03/2022 17:56, Phil Auld wrote:
> > Arm64 systems rely on store_cpu_topology() to call update_siblings_masks()
> > to transfer the toplogy to the various cpu masks. This needs to be done 
> > before the call to notify_cpu_starting() which tells the scheduler about 
> > each cpu found, otherwise the core scheduling data structures are setup 
> > in a way that does not match the actual topology.
> > 
> > Without this change stress-ng (which enables core scheduling in its prctl 
> > tests) causes a warning and then a crash (trimmed for legibility):
> > 
> > [ 1853.805168] ------------[ cut here ]------------
> > [ 1853.809784] task_rq(b)->core != rq->core
> > [ 1853.809792] WARNING: CPU: 117 PID: 0 at kernel/sched/fair.c:11102 cfs_prio_less+0x1b4/0x1c4
> > ...
> > [ 1854.015210] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000010
> > ...
> > [ 1854.231256] Call trace:
> > [ 1854.233689]  pick_next_task+0x3dc/0x81c
> > [ 1854.237512]  __schedule+0x10c/0x4cc
> > [ 1854.240988]  schedule_idle+0x34/0x54
> > 
> > Fixes: 9edeaea1bc45 ("sched: Core-wide rq->lock")
> > Signed-off-by: Phil Auld <pauld@redhat.com>
> > ---
> > This is a similar issue to 
> >   f2703def339c ("MIPS: smp: fill in sibling and core maps earlier") 
> > which fixed it for MIPS. 
> > 
> > v2: Fixed the commit message. No code change.
> 
> Ah, the reason is that smt_mask is not correctly setup, so we bail on
> `cpumask_weight(smt_mask) == 1` for !leaders in:
> 
> notify_cpu_starting()
>   cpuhp_invoke_callback_range()
>     sched_cpu_starting()
>       sched_core_cpu_starting()
> 
> which leads to rq->core not being correctly set for !leader-rq's.
> 

Exactly, sorry I was not clearer.  smt_mask must be setup correctly 
by the time sched_core_cpu_starting() is called. (Maybe I should crib
some of the above lines into the commit message?)


> LGTM. Tested on: HPE Apollo 70 X1
> 
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> 

Thanks!



Cheers,
Phil


> [...]
> 

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Re: [PATCH v2] arch/arm64: Fix topology initialization for core scheduling

[Dietmar Eggemann]

On 31/03/2022 15:21, Phil Auld wrote:
> On Thu, Mar 31, 2022 at 11:04:31AM +0200 Dietmar Eggemann wrote:
>> On 30/03/2022 17:56, Phil Auld wrote:

[...]

>> Ah, the reason is that smt_mask is not correctly setup, so we bail on
>> `cpumask_weight(smt_mask) == 1` for !leaders in:
>>
>> notify_cpu_starting()
>>   cpuhp_invoke_callback_range()
>>     sched_cpu_starting()
>>       sched_core_cpu_starting()
>>
>> which leads to rq->core not being correctly set for !leader-rq's.
>>
> 
> Exactly, sorry I was not clearer.  smt_mask must be setup correctly 
> by the time sched_core_cpu_starting() is called. (Maybe I should crib
> some of the above lines into the commit message?)

Yeah, maybe, it wouldn't hurt I guess. IMHO mentioning stress-ng's prctl
needs PR_SCHED_CORE support could also be handy since today's stress-ng
packages don't seem to have this yet.

Re: [PATCH v2] arch/arm64: Fix topology initialization for core scheduling

[Dietmar Eggemann]

On 31/03/2022 15:21, Phil Auld wrote:
> On Thu, Mar 31, 2022 at 11:04:31AM +0200 Dietmar Eggemann wrote:
>> On 30/03/2022 17:56, Phil Auld wrote:

[...]

>> Ah, the reason is that smt_mask is not correctly setup, so we bail on
>> `cpumask_weight(smt_mask) == 1` for !leaders in:
>>
>> notify_cpu_starting()
>>   cpuhp_invoke_callback_range()
>>     sched_cpu_starting()
>>       sched_core_cpu_starting()
>>
>> which leads to rq->core not being correctly set for !leader-rq's.
>>
> 
> Exactly, sorry I was not clearer.  smt_mask must be setup correctly 
> by the time sched_core_cpu_starting() is called. (Maybe I should crib
> some of the above lines into the commit message?)

Yeah, maybe, it wouldn't hurt I guess. IMHO mentioning stress-ng's prctl
needs PR_SCHED_CORE support could also be handy since today's stress-ng
packages don't seem to have this yet.

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Re: [PATCH v2] arch/arm64: Fix topology initialization for core scheduling

[Phil Auld]

On Thu, Mar 31, 2022 at 04:37:50PM +0200 Dietmar Eggemann wrote:
> On 31/03/2022 15:21, Phil Auld wrote:
> > On Thu, Mar 31, 2022 at 11:04:31AM +0200 Dietmar Eggemann wrote:
> >> On 30/03/2022 17:56, Phil Auld wrote:
> 
> [...]
> 
> >> Ah, the reason is that smt_mask is not correctly setup, so we bail on
> >> `cpumask_weight(smt_mask) == 1` for !leaders in:
> >>
> >> notify_cpu_starting()
> >>   cpuhp_invoke_callback_range()
> >>     sched_cpu_starting()
> >>       sched_core_cpu_starting()
> >>
> >> which leads to rq->core not being correctly set for !leader-rq's.
> >>
> > 
> > Exactly, sorry I was not clearer.  smt_mask must be setup correctly 
> > by the time sched_core_cpu_starting() is called. (Maybe I should crib
> > some of the above lines into the commit message?)
> 
> Yeah, maybe, it wouldn't hurt I guess. IMHO mentioning stress-ng's prctl
> needs PR_SCHED_CORE support could also be handy since today's stress-ng
> packages don't seem to have this yet.
> 

My scripts clone it so I did not realize that was not in prepackaged versions
yet.  But that said, that's really just a way to tickle the problem. Anyone
using core scheduling on such a system will hit this (at least the WARN part,
the actual crash was harder to create w/o all the threads and tasks stress-ng
uses). 

I can send a v3 with a further commit message update.


Cheers,
Phil

-- 

Re: [PATCH v2] arch/arm64: Fix topology initialization for core scheduling

[Phil Auld]

On Thu, Mar 31, 2022 at 04:37:50PM +0200 Dietmar Eggemann wrote:
> On 31/03/2022 15:21, Phil Auld wrote:
> > On Thu, Mar 31, 2022 at 11:04:31AM +0200 Dietmar Eggemann wrote:
> >> On 30/03/2022 17:56, Phil Auld wrote:
> 
> [...]
> 
> >> Ah, the reason is that smt_mask is not correctly setup, so we bail on
> >> `cpumask_weight(smt_mask) == 1` for !leaders in:
> >>
> >> notify_cpu_starting()
> >>   cpuhp_invoke_callback_range()
> >>     sched_cpu_starting()
> >>       sched_core_cpu_starting()
> >>
> >> which leads to rq->core not being correctly set for !leader-rq's.
> >>
> > 
> > Exactly, sorry I was not clearer.  smt_mask must be setup correctly 
> > by the time sched_core_cpu_starting() is called. (Maybe I should crib
> > some of the above lines into the commit message?)
> 
> Yeah, maybe, it wouldn't hurt I guess. IMHO mentioning stress-ng's prctl
> needs PR_SCHED_CORE support could also be handy since today's stress-ng
> packages don't seem to have this yet.
> 

My scripts clone it so I did not realize that was not in prepackaged versions
yet.  But that said, that's really just a way to tickle the problem. Anyone
using core scheduling on such a system will hit this (at least the WARN part,
the actual crash was harder to create w/o all the threads and tasks stress-ng
uses). 

I can send a v3 with a further commit message update.


Cheers,
Phil

-- 


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Re: [PATCH v2] arch/arm64: Fix topology initialization for core scheduling

[Dietmar Eggemann]

On 31/03/2022 16:53, Phil Auld wrote:
> On Thu, Mar 31, 2022 at 04:37:50PM +0200 Dietmar Eggemann wrote:
>> On 31/03/2022 15:21, Phil Auld wrote:
>>> On Thu, Mar 31, 2022 at 11:04:31AM +0200 Dietmar Eggemann wrote:
>>>> On 30/03/2022 17:56, Phil Auld wrote:

[...]

>> Yeah, maybe, it wouldn't hurt I guess. IMHO mentioning stress-ng's prctl
>> needs PR_SCHED_CORE support could also be handy since today's stress-ng
>> packages don't seem to have this yet.
>>
> 
> My scripts clone it so I did not realize that was not in prepackaged versions
> yet.  But that said, that's really just a way to tickle the problem. Anyone
> using core scheduling on such a system will hit this (at least the WARN part,
> the actual crash was harder to create w/o all the threads and tasks stress-ng
> uses).

OK.

I just saw that there is even a kselftest for this:

9f2699007493 - kselftest: Add test for core sched prctl interface
(2021-05-12 Chris Hyser)

But it doesn't trigger the issue.

> I can send a v3 with a further commit message update.

Sounds good.

Re: [PATCH v2] arch/arm64: Fix topology initialization for core scheduling

[Dietmar Eggemann]

On 31/03/2022 16:53, Phil Auld wrote:
> On Thu, Mar 31, 2022 at 04:37:50PM +0200 Dietmar Eggemann wrote:
>> On 31/03/2022 15:21, Phil Auld wrote:
>>> On Thu, Mar 31, 2022 at 11:04:31AM +0200 Dietmar Eggemann wrote:
>>>> On 30/03/2022 17:56, Phil Auld wrote:

[...]

>> Yeah, maybe, it wouldn't hurt I guess. IMHO mentioning stress-ng's prctl
>> needs PR_SCHED_CORE support could also be handy since today's stress-ng
>> packages don't seem to have this yet.
>>
> 
> My scripts clone it so I did not realize that was not in prepackaged versions
> yet.  But that said, that's really just a way to tickle the problem. Anyone
> using core scheduling on such a system will hit this (at least the WARN part,
> the actual crash was harder to create w/o all the threads and tasks stress-ng
> uses).

OK.

I just saw that there is even a kselftest for this:

9f2699007493 - kselftest: Add test for core sched prctl interface
(2021-05-12 Chris Hyser)

But it doesn't trigger the issue.

> I can send a v3 with a further commit message update.

Sounds good.

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