Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:8 as permitted sender) client-ip=2620:137:e000::3:8;
From:   Valentin Schneider <vschneid@redhat.com>
To:     Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
Cc:     linux-kernel@vger.kernel.org, Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 3/3] sched/nohz: Remove weird error handling from
 find_new_ilb()
In-Reply-To: <ZR/o8+EJGLMi3uPr@gmail.com>
References: <20231006102518.2452758-1-mingo@kernel.org>
 <20231006102518.2452758-4-mingo@kernel.org>
 <20231006103858.GB36277@noisy.programming.kicks-ass.net>
 <ZR/o8+EJGLMi3uPr@gmail.com>
Date:   Mon, 09 Oct 2023 17:11:53 +0200
Message-ID: <xhsmhh6mzomo6.mognet@vschneid.remote.csb>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

On 06/10/23 13:01, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@infradead.org> wrote:
>
>> On Fri, Oct 06, 2023 at 12:25:18PM +0200, Ingo Molnar wrote:
>> > find_new_ilb() returns nr_cpu_ids on failure - which is a weird
>> > choice in itself: not only is it a global variable, it is
>> > a +1 out of bounds CPU index...
>> 
>> FWIW this is what all the cpumask bitops return when they've exhausted
>> the mask. Eg. no bits left set etc..
>
> yeah, which then results in type-forcing uglies like:
>
>    kernel/events/core.c:	if ((unsigned)cpu >= nr_cpu_ids) {
>    kernel/events/core.c:   if ((unsigned)cpu >= nr_cpu_ids) {
>    kernel/smp.c:	   if ((unsigned)cpu >= nr_cpu_ids || !cpu_online(cpu)) {

I can't see why we'd want smp_call_function_single*() /
generic_exec_single() to take a signed int as input, shouldn't this just be
unsigned?

The perf thing does look like it wants signed though...

>
> :-/
>
> So I don't think this is a particularly well thought-out interface.
>
> Thanks,
>
> 	Ingo