linux-kernel - Re: [PATCH] cpu/hotplug: ensure the starting section runs fully regardless of target

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Message-ID: <z7nvvzu5lpdnul2m35lrsa7wo6plx7zsunhowtog3nqydmpene@tzvb2hofjeal>
Date: Mon, 9 Dec 2024 13:19:43 +0900
From: Koichiro Den <koichiro.den@...onical.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: linux-kernel@...r.kernel.org, peterz@...radead.org
Subject: Re: [PATCH] cpu/hotplug: ensure the starting section runs fully
 regardless of target

On Mon, Dec 09, 2024 at 09:12:44AM +0900, Koichiro Den wrote:
> On Sun, Dec 08, 2024 at 09:34:37PM +0100, Thomas Gleixner wrote:
> > On Sat, Dec 07 2024 at 23:47, Koichiro Den wrote:
> > >  static int take_cpu_down(void *_param)
> > >  {
> > >  	struct cpuhp_cpu_state *st = this_cpu_ptr(&cpuhp_state);
> > > -	enum cpuhp_state target = max((int)st->target, CPUHP_AP_OFFLINE);
> > >  	int err, cpu = smp_processor_id();
> > >  
> > >  	/* Ensure this CPU doesn't handle any more interrupts. */
> > > @@ -1285,8 +1284,9 @@ static int take_cpu_down(void *_param)
> > >  
> > >  	/*
> > >  	 * Invoke the former CPU_DYING callbacks. DYING must not fail!
> > > +	 * Regardless of st->target, it must run through to CPUHP_AP_OFFLINE.
> > >  	 */
> > > -	cpuhp_invoke_callback_range_nofail(false, cpu, st, target);
> > > +	cpuhp_invoke_callback_range_nofail(false, cpu, st, CPUHP_AP_OFFLINE);
> > 
> > This is really the wrong place. This want's to be enforced at the sysfs
> > interface already and reject writes which are between AP_OFFLINE and
> > AP_ONLINE.
> >   
> > It's utterly confusing to write a particular target and then magically
> > end up at some other state.
> 
> Ok, I'll send v2. Thanks for the review.

Now I'm wondering whether we should go further..
Because in PREPARE section, things are not fully symmetric, so
there is a problem like an example below:

    E.g.

    (1) writing <some state in between> into 'target' and then (2) bringing
    the the cpu fully online again by writing a large value leaves
    hrtimer_cpu_base's 'online' field at 0 because hrtimers:prepare does not
    re-run.

           - hrtimers:prepare (CPUHP_HRTIMERS_PREPARE)
           - ...
    ------ - <some state in between>
     ^  :  - ...
     :  :  - hrtimers:dying (CPUHP_AP_HRTIMERS_DYING)
    (1)(2)
     :  :
     :  v

While I understand this is still a debug option, it seems to me that there are
several approaches to consider here. I'm inclined toward (a).

(a). prohibit writing all halfway states in PREPARE+STARTING sections, i.e.

     --- a/kernel/cpu.c
     +++ b/kernel/cpu.c
     @@ -2759,7 +2759,8 @@ static ssize_t target_store(struct device *dev,
                     return ret;
     
      #ifdef CONFIG_CPU_HOTPLUG_STATE_CONTROL
     -       if (target < CPUHP_OFFLINE || target > CPUHP_ONLINE)
     +       if (target != CPUHP_OFFLINE || target > CPUHP_ONLINE ||
     +           target < CPUHP_AP_OFFLINE_IDLE)
                     return -EINVAL;
      #else
             if (target != CPUHP_OFFLINE && target != CPUHP_ONLINE)

(b). make all fully symmetric. (I'm not sure whether it could be possible)
(c). add all safety catch at sysfs interface. (For the above example, once
     it goes down to <some state in between>, disallow to go up without
     once going down to a state earlier than hrtimers:prepare beforehand.
     I guess this would mess up source code unnecessarily though.)
...

Let me know what you think.

Thanks,

-Koichiro Den


> 
> -Koichiro Den
> 
> > 
> > Thanks,
> > 
> >         tglx