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

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Message-ID: <CF32049E-D6AA-4AD5-A276-0CEC84B6DB11@vmware.com>
Date:   Mon, 22 Jul 2019 18:41:44 +0000
From:   Nadav Amit <namit@...are.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <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 Jul 22, 2019, at 11:16 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> 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.

But then smpcfd_dead_cpu() will not respect the “cpu” argument. Do you still
prefer it this way (instead of the current preempt_enable() /
preempt_disable())?