Re: [RFC 07/10] lib/cpumask: Introduce cpumask_not_dying_but()

[Pingfan Liu]

On Mon, Aug 22, 2022 at 07:15:45AM -0700, Yury Norov wrote:
> On Mon, Aug 22, 2022 at 10:15:17AM +0800, Pingfan Liu wrote:
> > During cpu hot-removing, the dying cpus are still in cpu_online_mask.
> > On the other hand, A subsystem will migrate its broker from the dying
> > cpu to a online cpu in its teardown cpuhp_step.
> > 
> > After enabling the teardown of cpus in parallel, cpu_online_mask can not
> > tell those dying from the real online.
> > 
> > Introducing a function cpumask_not_dying_but() to pick a real online
> > cpu.
> > 
> > Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
> > Cc: Yury Norov <yury.norov@xxxxxxxxx>
> > Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> > Cc: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: Steven Price <steven.price@xxxxxxx>
> > Cc: Mark Rutland <mark.rutland@xxxxxxx>
> > Cc: "Jason A. Donenfeld" <Jason@xxxxxxxxx>
> > Cc: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> > To: linux-kernel@xxxxxxxxxxxxxxx
> > ---
> >  include/linux/cpumask.h |  3 +++
> >  kernel/cpu.c            |  3 +++
> >  lib/cpumask.c           | 18 ++++++++++++++++++
> >  3 files changed, 24 insertions(+)
> > 
> > diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> > index 0d435d0edbcb..d2033a239a07 100644
> > --- a/include/linux/cpumask.h
> > +++ b/include/linux/cpumask.h
> > @@ -317,6 +317,9 @@ unsigned int cpumask_any_but(const struct cpumask *mask, unsigned int cpu)
> >  	return i;
> >  }
> >  
> > +/* for parallel kexec reboot */
> > +int cpumask_not_dying_but(const struct cpumask *mask, unsigned int cpu);
> > +
> >  #define CPU_BITS_NONE						\
> >  {								\
> >  	[0 ... BITS_TO_LONGS(NR_CPUS)-1] = 0UL			\
> > diff --git a/kernel/cpu.c b/kernel/cpu.c
> > index 90debbe28e85..771e344f8ff9 100644
> > --- a/kernel/cpu.c
> > +++ b/kernel/cpu.c
> > @@ -1282,6 +1282,9 @@ static void cpus_down_no_rollback(struct cpumask *cpus)
> >  	struct cpuhp_cpu_state *st;
> >  	unsigned int cpu;
> >  
> > +	for_each_cpu(cpu, cpus)
> > +		set_cpu_dying(cpu, true);
> > +
> >  	/* launch ap work one by one, but not wait for completion */
> >  	for_each_cpu(cpu, cpus) {
> >  		st = per_cpu_ptr(&cpuhp_state, cpu);
> > diff --git a/lib/cpumask.c b/lib/cpumask.c
> > index 8baeb37e23d3..6474f07ed87a 100644
> > --- a/lib/cpumask.c
> > +++ b/lib/cpumask.c
> > @@ -7,6 +7,24 @@
> >  #include <linux/memblock.h>
> >  #include <linux/numa.h>
> >  
> > +/* Used in parallel kexec-reboot cpuhp callbacks */
> > +int cpumask_not_dying_but(const struct cpumask *mask,
> > +					   unsigned int cpu)
> > +{
> > +	unsigned int i;
> > +
> > +	if (CONFIG_SHUTDOWN_NONBOOT_CPUS) {
> 
> Hmm... Would it even work? Anyways, the documentation says:
> Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
> symbol into a C boolean expression, and use it in a normal C conditional:
> 
> .. code-block:: c
> 
>         if (IS_ENABLED(CONFIG_SOMETHING)) {
>                 ...
>         }
> 

Yes, it shall be like you pointed out.

I changed the code from "#ifdef" style to "if (IS_ENABLED()" style just
before sending out the series. Sorry for the haste without compiling
check again.

> 
> > +		cpumask_check(cpu);
> > +		for_each_cpu(i, mask)
> > +			if (i != cpu && !cpumask_test_cpu(i, cpu_dying_mask))
> > +				break;
> > +		return i;
> > +	} else {
> > +		return cpumask_any_but(mask, cpu);
> > +	}
> > +}
> > +EXPORT_SYMBOL(cpumask_not_dying_but);
> 
> I don't like how you create a dedicated function for a random
> mask. Dying mask is nothing special, right? What you really

Yes, I agree.

> need is probably this:
>         cpumask_andnot_any_but(mask, cpu_dying_mask, cpu);
> 

That is it.

> Now, if you still think it's worth that, you can add a trivial wrapper
> for cpu_dying_mask. (But please pick some other name, because
> 'not dying but' sounds like a hangover description. :) )
> 

I think that since even if !IS_ENABLED(CONFIG_SHUTDOWN_NONBOOT_CPUS),
cpumask_andnot_any_but(mask, cpu_dying_mask, cpu) can work properly,
so replacing the callsite with "cpumask_andnot() + cpumask_any_but()"
will be a choice.

Appreciate for your help.


Thanks,

	Pingfan

> Thanks,
> Yury
> 
> > +
> >  /**
> >   * cpumask_next_wrap - helper to implement for_each_cpu_wrap
> >   * @n: the cpu prior to the place to search
> > -- 
> > 2.31.1