linux-kernel - Re: [PATCH v6 1/9] smp: Run functions concurrently in smp_call_function_many

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Message-ID: <DB46F034-878B-448B-BDBF-C25056C6576C@vmware.com>
Date:   Mon, 1 Mar 2021 19:01:13 +0000
From:   Nadav Amit <namit@...are.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Rik van Riel <riel@...riel.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v6 1/9] smp: Run functions concurrently in
 smp_call_function_many_cond()



> On Mar 1, 2021, at 9:10 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Sat, Feb 20, 2021 at 03:17:04PM -0800, Nadav Amit 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.
>> +		 */
>> +		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);
> 
> I just ran into conflicts with another patch set, and noticed that the
> above should probably be:
> 
> 		if (nr_cpus == 1)
> 			send_call_function_single_ipi(last_cpu);
> 		else if (likely(nr_cpus > 1))
> 			arch_send_call_function_ipi_mask(cfd->cpumask_ipi);
> 
> Which will avoid the IPI when @last_cpu is idle.

Good point. Makes one wonder whether all these inter-core communication
(through cpu_tlbstate.is_lazy, csd->node.llist and ti->flags) are
really necessary or can be combined.

Well, that’s for later I presume.