[PATCH v3 0/2] cpuhp: fix some st->target issues

[PATCH v3 0/2] cpuhp: fix some st->target issues

[Phil Auld]

Several small fixes that clean up some cpuhp inconsistencies.
The first prevents target_store() from calling cpu_down() when
target == state which prevents the cpu being incorrectly marked
as dying.  The second just makes the boot cpu have a valid cpuhp
target rather than 0 (CPU_OFFLINE) while being in state
CPU_ONLINE.

A further issue which these two patches don't address is that
the cpuX/online file looks at the device->offline state and can
thus get out of sync with the actual cpuhp state if the cpuhp
target is used to change state.

v3: Added code to make sure st->target == target in the nop case.

Phil Auld (2):
  cpuhp: make target_store() a nop when target == state
  cpuhp: Set cpuhp target for boot cpu

 kernel/cpu.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

-- 
2.31.1

[PATCH v3 1/2] cpuhp: make target_store() a nop when target == state

[Phil Auld]

writing the current state back in hotplug/target calls cpu_down()
which will set cpu dying even when it isn't and then nothing will
ever clear it. A stress test that reads values and writes them back
for all cpu device files in sysfs will trigger the BUG() in
select_fallback_rq once all cpus are marked as dying.

kernel/cpu.c::target_store()
	...
        if (st->state < target)
                ret = cpu_up(dev->id, target);
        else
                ret = cpu_down(dev->id, target);

cpu_down() -> cpu_set_state()
	 bool bringup = st->state < target;
	 ...
	 if (cpu_dying(cpu) != !bringup)
		set_cpu_dying(cpu, !bringup);

Fix this by letting state==target fall through in the target_store()
conditional. Also make sure st->target == target in that case.

Signed-off-by: Phil Auld <pauld@redhat.com>
---
 kernel/cpu.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index bbad5e375d3b..305694a2ca26 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2326,8 +2326,10 @@ static ssize_t target_store(struct device *dev, struct device_attribute *attr,
 
 	if (st->state < target)
 		ret = cpu_up(dev->id, target);
-	else
+	else if (st->state > target)
 		ret = cpu_down(dev->id, target);
+	else if (st->target != target)
+		st->target = target;
 out:
 	unlock_device_hotplug();
 	return ret ? ret : count;
-- 
2.31.1

[PATCH v3 2/2] cpuhp: Set cpuhp target for boot cpu

[Phil Auld]

Since the boot cpu does not go through the hotplug process it ends
up with state == CPUHP_ONLINE but target == CPUHP_OFFLINE.
Set the target to match in boot_cpu_hotplug_init().

Signed-off-by: Phil Auld <pauld@redhat.com>
---
 kernel/cpu.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 305694a2ca26..104f43a6f4ad 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2690,6 +2690,7 @@ void __init boot_cpu_hotplug_init(void)
 	cpumask_set_cpu(smp_processor_id(), &cpus_booted_once_mask);
 #endif
 	this_cpu_write(cpuhp_state.state, CPUHP_ONLINE);
+	this_cpu_write(cpuhp_state.target, CPUHP_ONLINE);
 }
 
 /*
-- 
2.31.1

Re: [PATCH v3 1/2] cpuhp: make target_store() a nop when target == state

[Valentin Schneider]

On 11/07/22 17:16, Phil Auld wrote:
> writing the current state back in hotplug/target calls cpu_down()
> which will set cpu dying even when it isn't and then nothing will
> ever clear it. A stress test that reads values and writes them back
> for all cpu device files in sysfs will trigger the BUG() in
> select_fallback_rq once all cpus are marked as dying.
>
> kernel/cpu.c::target_store()
>       ...
>         if (st->state < target)
>                 ret = cpu_up(dev->id, target);
>         else
>                 ret = cpu_down(dev->id, target);
>
> cpu_down() -> cpu_set_state()
>        bool bringup = st->state < target;
>        ...
>        if (cpu_dying(cpu) != !bringup)
>               set_cpu_dying(cpu, !bringup);
>
> Fix this by letting state==target fall through in the target_store()
> conditional. Also make sure st->target == target in that case.
>
> Signed-off-by: Phil Auld <pauld@redhat.com>

One nit below, otherwise:
Reviewed-by: Valentin Schneider <vschneid@redhat.com>

> ---
>  kernel/cpu.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index bbad5e375d3b..305694a2ca26 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -2326,8 +2326,10 @@ static ssize_t target_store(struct device *dev, struct device_attribute *attr,
>
>       if (st->state < target)
>               ret = cpu_up(dev->id, target);
> -	else
> +	else if (st->state > target)
>               ret = cpu_down(dev->id, target);
> +	else if (st->target != target)

Should we make this:

        else if (WARN(st->target != target))

> +		st->target = target;
>  out:
>       unlock_device_hotplug();
>       return ret ? ret : count;
> --
> 2.31.1

Re: [PATCH v3 2/2] cpuhp: Set cpuhp target for boot cpu

[Valentin Schneider]

On 11/07/22 17:16, Phil Auld wrote:
> Since the boot cpu does not go through the hotplug process it ends
> up with state == CPUHP_ONLINE but target == CPUHP_OFFLINE.
> Set the target to match in boot_cpu_hotplug_init().
>
> Signed-off-by: Phil Auld <pauld@redhat.com>

Reviewed-by: Valentin Schneider <vschneid@redhat.com>

Re: [PATCH v3 1/2] cpuhp: make target_store() a nop when target == state

[Phil Auld]

On Tue, Jul 19, 2022 at 03:34:03PM +0100 Valentin Schneider wrote:
> On 11/07/22 17:16, Phil Auld wrote:
> > writing the current state back in hotplug/target calls cpu_down()
> > which will set cpu dying even when it isn't and then nothing will
> > ever clear it. A stress test that reads values and writes them back
> > for all cpu device files in sysfs will trigger the BUG() in
> > select_fallback_rq once all cpus are marked as dying.
> >
> > kernel/cpu.c::target_store()
> >       ...
> >         if (st->state < target)
> >                 ret = cpu_up(dev->id, target);
> >         else
> >                 ret = cpu_down(dev->id, target);
> >
> > cpu_down() -> cpu_set_state()
> >        bool bringup = st->state < target;
> >        ...
> >        if (cpu_dying(cpu) != !bringup)
> >               set_cpu_dying(cpu, !bringup);
> >
> > Fix this by letting state==target fall through in the target_store()
> > conditional. Also make sure st->target == target in that case.
> >
> > Signed-off-by: Phil Auld <pauld@redhat.com>
> 
> One nit below, otherwise:
> Reviewed-by: Valentin Schneider <vschneid@redhat.com>
>

Thanks!

> > ---
> >  kernel/cpu.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/cpu.c b/kernel/cpu.c
> > index bbad5e375d3b..305694a2ca26 100644
> > --- a/kernel/cpu.c
> > +++ b/kernel/cpu.c
> > @@ -2326,8 +2326,10 @@ static ssize_t target_store(struct device *dev, struct device_attribute *attr,
> >
> >       if (st->state < target)
> >               ret = cpu_up(dev->id, target);
> > -	else
> > +	else if (st->state > target)
> >               ret = cpu_down(dev->id, target);
> > +	else if (st->target != target)
> 
> Should we make this:
> 
>         else if (WARN(st->target != target))
>

If you think that's important I can make it a WARN, sure.

I'll try to remember to keep your Reviewed-bys this time if that's okay.



Cheers,
Phil

> > +		st->target = target;
> >  out:
> >       unlock_device_hotplug();
> >       return ret ? ret : count;
> > --
> > 2.31.1
> 

-- 

Re: [PATCH v3 0/2] cpuhp: fix some st->target issues

[Phil Auld]

On Mon, Jul 11, 2022 at 05:16:17PM -0400 Phil Auld wrote:
> Several small fixes that clean up some cpuhp inconsistencies.
> The first prevents target_store() from calling cpu_down() when
> target == state which prevents the cpu being incorrectly marked
> as dying.  The second just makes the boot cpu have a valid cpuhp
> target rather than 0 (CPU_OFFLINE) while being in state
> CPU_ONLINE.
> 
> A further issue which these two patches don't address is that
> the cpuX/online file looks at the device->offline state and can
> thus get out of sync with the actual cpuhp state if the cpuhp
> target is used to change state.
> 
> v3: Added code to make sure st->target == target in the nop case.
> 
> Phil Auld (2):
>   cpuhp: make target_store() a nop when target == state
>   cpuhp: Set cpuhp target for boot cpu
> 
>  kernel/cpu.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> -- 
> 2.31.1
> 

Ping?

Any thoughts on this? I'm not sure what tree it needs to go in but
getting a cpu shutdown by writing the current target/state back
in here seems wrong.


Thanks,
Phil

--