linux-kernel - Re: [PATCH] cpuhp: make target_store() a nop when target == state

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Date:   Wed, 25 May 2022 11:11:52 -0400
From:   Phil Auld <pauld@...hat.com>
To:     Valentin Schneider <vschneid@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] cpuhp: make target_store() a nop when target == state

On Wed, May 25, 2022 at 04:09:29PM +0100 Valentin Schneider wrote:
> On 25/05/22 09:31, Phil Auld wrote:
> > On Wed, May 25, 2022 at 10:48:31AM +0100 Valentin Schneider wrote:
> >>
> >> Yeah it would be neater to not even enter cpu_{up, down}(), but my paranoia
> >> makes me think we need the comparison to happen with at least the
> >> cpu_add_remove_lock held to make sure st->state isn't moving under our
> >> feet, otherwise we may still end up with target == state in _cpu_down() and
> >> hit the bug you're describing.
> >>
> >
> > This is what I was originally doing before I tried to "optimize" it:
> >
> >          if (st->state < target)
> >                  ret = cpu_up(dev->id, target);
> >          else if (st->state > target)
> >                  ret = cpu_down(dev->id, target);
> >
> > This does the check under the lock and just falls through if state==target.
> > I think I'll go back to that version.
> >
> > I also noticed while testing that the boot cpu does not get its target set.
> > It's got state 233 but target 0.  So reading that out and writing it back
> > on offlines cpu0.   I'll try to find where that is not getting set.
> >
> 
> If I had to guess I'd say it's because the boot CPU doesn't go through the
> regular hotplug machinery and sets its state straight to CPUHP_ONLINE
>

Yes, that was my thought.


> /me digs
> 
> Maybe around this?
> 
> void __init boot_cpu_hotplug_init(void)
> {
>         this_cpu_write(cpuhp_state.booted_once, true);
>         this_cpu_write(cpuhp_state.state, CPUHP_ONLINE);
> }
> 

Right, just found that too. Probably should set the target there as well.


Cheers,
Phil

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