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

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Message-ID: <ABD10CF3-0FD9-47E8-BC80-9A3733DADC52@vmware.com>
Date:   Thu, 25 Jul 2019 19:10:57 +0000
From:   Nadav Amit <namit@...are.com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     Peter Zijlstra <peterz@...radead.org>,
        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>,
        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 25, 2019, at 5:36 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> On Mon, 22 Jul 2019, Nadav Amit wrote:
>>> On Jul 22, 2019, at 11:51 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
>>> void on_each_cpu(void (*func) (void *info), void *info, int wait)
>>> {
>>>       unsigned long flags;
>>> 
>>>       preempt_disable();
>>> 	smp_call_function(func, info, wait);
>>> 
>>> smp_call_function() has another preempt_disable as it can be called
>>> separately and it does:
>>> 
>>>       preempt_disable();
>>>       smp_call_function_many(cpu_online_mask, func, info, wait);
>>> 
>>> Your new on_each_cpu() implementation does not. So there is a
>>> difference. Whether it matters or not is a different question, but that
>>> needs to be explained and documented.
>> 
>> Thanks for explaining - so your concern is for CPUs being offlined.
>> 
>> But unless I am missing something: on_each_cpu() calls __on_each_cpu_mask(),
>> which disables preemption and calls __smp_call_function_many().
>> 
>> Then  __smp_call_function_many() runs:
>> 
>> 	cpumask_and(cfd->cpumask, mask, cpu_online_mask);
>> 
>> … before choosing which remote CPUs should run the function. So the only
>> case that I was missing is if the current CPU goes away and the function is
>> called locally.
>> 
>> Can it happen? I can add documentation and a debug assertion for this case.
> 
> I don't think it can happen:
> 
>  on_each_cpu()
>    on_each_cpu_mask(....)
>      preempt_disable()
>        __smp_call_function_many()
> 
> So if a CPU goes offline between on_each_cpu() and preempt_disable() then
> there is no damage. After the preempt_disable() it can't go away anymore
> and the task executing this cannot be migrated either.
> 
> So yes, it's safe, but please add a big fat comment so future readers won't
> be puzzled.

I will do. I will need some more time to respin the next version. I see that
what I build on top of it might require some changes, and I want to minimize
them.