linux-kernel - Re: [PATCH v3 1/9] smp: Run functions concurrently in smp_call_function

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Date:   Mon, 22 Jul 2019 20:16:58 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Nadav Amit <namit@...are.com>, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Rik van Riel <riel@...riel.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v3 1/9] smp: Run functions concurrently in
 smp_call_function_many()

On Fri, Jul 19, 2019 at 11:23:06AM -0700, Dave Hansen wrote:
> On 7/18/19 5:58 PM, Nadav Amit wrote:
> > @@ -624,16 +622,11 @@ EXPORT_SYMBOL(on_each_cpu);
> >  void on_each_cpu_mask(const struct cpumask *mask, smp_call_func_t func,
> >  			void *info, bool wait)
> >  {
> > -	int cpu = get_cpu();
> > +	preempt_disable();
> >  
> > -	smp_call_function_many(mask, func, info, wait);
> > -	if (cpumask_test_cpu(cpu, mask)) {
> > -		unsigned long flags;
> > -		local_irq_save(flags);
> > -		func(info);
> > -		local_irq_restore(flags);
> > -	}
> > -	put_cpu();
> > +	__smp_call_function_many(mask, func, func, info, wait);
> > +
> > +	preempt_enable();
> >  }
> 
> The get_cpu() was missing it too, but it would be nice to add some
> comments about why preempt needs to be off.  I was also thinking it
> might make sense to do:
> 
> 	cfd = get_cpu_var(cfd_data);
> 	__smp_call_function_many(cfd, ...);
> 	put_cpu_var(cfd_data);
> 	
> instead of the explicit preempt_enable/disable(), but I don't feel too
> strongly about it.

It is also required for cpu hotplug.