linux-kernel - Re: [PATCH] cpu/hotplug: Fix rollback during error-out in takedown

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Date:   Thu, 6 Sep 2018 14:30:41 +0530
From:   Neeraj Upadhyay <neeraju@...eaurora.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     josh@...htriplett.org, peterz@...radead.org, mingo@...nel.org,
        jiangshanlai@...il.com, dzickus@...hat.com,
        brendan.jackman@....com, malat@...ian.org,
        linux-kernel@...r.kernel.org, sramana@...eaurora.org,
        linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] cpu/hotplug: Fix rollback during error-out in
 takedown_cpu()



On 09/06/2018 01:48 PM, Thomas Gleixner wrote:
> On Thu, 6 Sep 2018, Neeraj Upadhyay wrote:
>> On 09/05/2018 06:47 PM, Thomas Gleixner wrote:
>>> On Wed, 5 Sep 2018, Neeraj Upadhyay wrote:
>>>> On 09/05/2018 05:53 PM, Thomas Gleixner wrote:
>>>>> And looking closer this is a general issue. Just that the TEARDOWN state
>>>>> makes it simple to observe. It's universaly broken, when the first
>>>>> teardown
>>>>> callback fails because, st->state is only decremented _AFTER_ the
>>>>> callback
>>>>> returns success, but undo_cpu_down() increments unconditionally.
>>>>>
>>>> As per my understanding, there are 2 problems here; one is fixed with your
>>>> patch, and other is cpuhp_reset_state() is used during rollback from
>>>> non-AP to
>>>> AP state, which seem to result in 2 increments of st->state (one increment
>>>> done by cpuhp_reset_state() and another by cpu_thread_fun()) .
>>> And how did your hack fix that up magically? I'll have a look later today.
>>>
>>> Thanks,
>>>
>>> 	tglx
>>
>> The hack fixes it by not calling cpuhp_reset_state() and doing rollback state
>> reset inline in  _cpu_down().
> 
> And how is that any different from the proper patch I sent? It ends up in
> the same state. So I have a hard time to understand your blurb above where
> you claim that my patch just solves one of two problems?
> 
> Thanks,
> 
> 	tglx
>

Yes, your patch solves the problem related to smpboot unparking being 
skipped during rollback and with the hack we end up in same state. The 
second thing, which I am referring to, is that there is one additional 
state increment. I missed the part that, it could be required, so that 
we reach CPUHP_AP_ONLINE_IDLE before calling __cpuhp_kick_ap(). So, it's 
not a problem.

* 	cpuhp_reset_state() does one state increment and we reach 
CPUHP_AP_ONLINE_IDLE.

	if (ret && st->state > CPUHP_TEARDOWN_CPU && st->state < prev_state) {
		cpuhp_reset_state(st, prev_state);
		__cpuhp_kick_ap(st);
	}

         CPUHP_TEARDOWN_CPU,
         CPUHP_AP_ONLINE_IDLE,
         CPUHP_AP_SMPBOOT_THREADS,


* cpuhp_thread_fun() does one more increment before invoking state 
callback (so, we skip CPUHP_AP_ONLINE_IDLE) and we reach 
CPUHP_AP_SMPBOOT_THREADS:

	static void cpuhp_thread_fun(unsigned int cpu)
	<snip>
		if (bringup) {
			st->state++;
			state = st->state;
	<snip>


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