Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Mon, 18 Jul 2022 23:26:48 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Yury Norov <yury.norov@gmail.com>
Cc:     linux-kernel@vger.kernel.org,
        Alexander Lobakin <alexandr.lobakin@intel.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Alexey Klimov <aklimov@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Ben Segall <bsegall@google.com>,
        Christoph Lameter <cl@linux.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Dennis Zhou <dennis@kernel.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Eric Dumazet <edumazet@google.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Guenter Roeck <linux@roeck-us.net>,
        Ingo Molnar <mingo@redhat.com>,
        Isabella Basso <isabbasso@riseup.net>,
        John Fastabend <john.fastabend@gmail.com>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        Juergen Gross <jgross@suse.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        KP Singh <kpsingh@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Mel Gorman <mgorman@suse.de>, Miroslav Benes <mbenes@suse.cz>,
        Nathan Chancellor <nathan@kernel.org>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Song Liu <songliubraving@fb.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Tejun Heo <tj@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Valentin Schneider <vschneid@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Vlastimil Babka <vbabka@suse.cz>, Yonghong Song <yhs@fb.com>,
        linux-mm@kvack.org, netdev@vger.kernel.org, bpf@vger.kernel.org
Subject: Re: [PATCH 07/16] smp: optimize smp_call_function_many_cond()
Message-ID: <YtXQGMijINmEoiVi@worktop.programming.kicks-ass.net>
References: <20220718192844.1805158-1-yury.norov@gmail.com>
 <20220718192844.1805158-8-yury.norov@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20220718192844.1805158-8-yury.norov@gmail.com>
Precedence: bulk

On Mon, Jul 18, 2022 at 12:28:35PM -0700, Yury Norov wrote:

> diff --git a/kernel/smp.c b/kernel/smp.c
> index dd215f439426..7ed2b9b12f74 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -880,6 +880,28 @@ EXPORT_SYMBOL_GPL(smp_call_function_any);
>  #define SCF_WAIT	(1U << 0)
>  #define SCF_RUN_LOCAL	(1U << 1)
>  
> +/* Check if we need remote execution, i.e., any CPU excluding this one. */
> +static inline bool __need_remote_exec(const struct cpumask *mask, unsigned int this_cpu)
> +{
> +	unsigned int cpu;
> +
> +	switch (num_online_cpus()) {
> +	case 0:
> +		return false;
> +	case 1:
> +		return cpu_online(this_cpu) ? false : true;
> +	default:
> +		if (mask == cpu_online_mask)
> +			return true;
> +	}
> +
> +	cpu = cpumask_first_and(mask, cpu_online_mask);
> +	if (cpu == this_cpu)
> +		cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
> +
> +	return cpu < nr_cpu_ids;
> +}
> +
>  static void smp_call_function_many_cond(const struct cpumask *mask,
>  					smp_call_func_t func, void *info,
>  					unsigned int scf_flags,
> @@ -916,12 +938,7 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
>  	if ((scf_flags & SCF_RUN_LOCAL) && cpumask_test_cpu(this_cpu, mask))
>  		run_local = true;
>  
> -	/* Check if we need remote execution, i.e., any CPU excluding this one. */
> -	cpu = cpumask_first_and(mask, cpu_online_mask);
> -	if (cpu == this_cpu)
> -		cpu = cpumask_next_and(cpu, mask, cpu_online_mask);
> -	if (cpu < nr_cpu_ids)
> -		run_remote = true;
> +	run_remote = __need_remote_exec(mask, this_cpu);
>  
>  	if (run_remote) {
>  		cfd = this_cpu_ptr(&cfd_data);

This is more complex code for, very little to no gain. Why ?!