linux-kernel - Re: [PATCH 08/16] smp: optimize smp_call_function_many

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Date:   Wed, 20 Jul 2022 10:06:24 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        Alexander Lobakin <alexandr.lobakin@...el.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Alexey Klimov <aklimov@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Ben Segall <bsegall@...gle.com>,
        Christoph Lameter <cl@...ux.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dennis Zhou <dennis@...nel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Eric Dumazet <edumazet@...gle.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Ingo Molnar <mingo@...hat.com>,
        Isabella Basso <isabbasso@...eup.net>,
        John Fastabend <john.fastabend@...il.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Juergen Gross <jgross@...e.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        KP Singh <kpsingh@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Martin KaFai Lau <kafai@...com>,
        Mel Gorman <mgorman@...e.de>, Miroslav Benes <mbenes@...e.cz>,
        Nathan Chancellor <nathan@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Song Liu <songliubraving@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tejun Heo <tj@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Valentin Schneider <vschneid@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Vlastimil Babka <vbabka@...e.cz>, Yonghong Song <yhs@...com>,
        linux-mm@...ck.org, netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH 08/16] smp: optimize smp_call_function_many_cond() for
 more

On Mon, Jul 18, 2022 at 11:29:06PM +0200, Peter Zijlstra wrote:
> On Mon, Jul 18, 2022 at 12:28:36PM -0700, Yury Norov wrote:
> 
> > ---
> >  kernel/smp.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/smp.c b/kernel/smp.c
> > index 7ed2b9b12f74..f96fdf944b4a 100644
> > --- a/kernel/smp.c
> > +++ b/kernel/smp.c
> > @@ -942,7 +942,11 @@ static void smp_call_function_many_cond(const struct cpumask *mask,
> >  
> >  	if (run_remote) {
> >  		cfd = this_cpu_ptr(&cfd_data);
> > -		cpumask_and(cfd->cpumask, mask, cpu_online_mask);
> > +		if (mask == cpu_online_mask)
> > +			cpumask_copy(cfd->cpumask, cpu_online_mask);
> > +		else
> > +			cpumask_and(cfd->cpumask, mask, cpu_online_mask);
> > +
> 
> Or... you could optimize cpumask_and() to detect the src1p == src2p case?

This is not what I would consider as optimization. For vast majority
of users this check is useless because they know for sure that
cpumasks are different.

For this case I can invent something like cpumask_and_check_eq(), so
that there'll be minimal impact on user code. (Suggestions for a better
name are very welcome.)

Thanks,
Yury