linux-kernel - Re: [RFC PATCH 3/6] smp: Run functions concurrently in smp_call_function

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Message-ID: <57E94109-CBE8-43BA-98FF-646E8C9EE8A2@vmware.com>
Date:   Mon, 27 May 2019 17:39:35 +0000
From:   Nadav Amit <namit@...are.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Ingo Molnar <mingo@...hat.com>, Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Rik van Riel <riel@...riel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [RFC PATCH 3/6] smp: Run functions concurrently in
 smp_call_function_many()

> On May 27, 2019, at 2:15 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
>> +		/*
>> +		 * Choose the most efficient way to send an IPI. Note that the
>> +		 * number of CPUs might be zero due to concurrent changes to the
>> +		 * provided mask or cpu_online_mask.
>> +		 */
> 
> Since we have preemption disabled here, I don't think online mask can
> shrink, cpu-offline uses stop_machine().

Right. So I’ll update the comment, but IIUC the provided mask might still
change, so I’ll leave the rest of the comment and the code as is.

>> +		if (nr_cpus == 1)
>> +			arch_send_call_function_single_ipi(last_cpu);
>> +		else if (likely(nr_cpus > 1))
>> +			arch_send_call_function_ipi_mask(cfd->cpumask_ipi);
>> +	}