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From:   Valentin Schneider <vschneid@redhat.com>
To:     Phil Auld <pauld@redhat.com>
Cc:     linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] cpuhp: make target_store() a nop when target == state
In-Reply-To: <20220525133133.GA5500@pauld.bos.csb>
References: <20220523144728.32414-1-pauld@redhat.com>
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 <20220525133133.GA5500@pauld.bos.csb>
Date:   Wed, 25 May 2022 16:09:29 +0100
Message-ID: <xhsmhtu9dir86.mognet@vschneid.remote.csb>
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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

/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);
}