Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:2 as permitted sender) client-ip=2620:137:e000::3:2;
Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Fri, 6 Oct 2023 13:01:07 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     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>,
        Valentin Schneider <vschneid@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()
Message-ID: <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>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20231006103858.GB36277@noisy.programming.kicks-ass.net>
Precedence: bulk


* 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)) {

:-/

So I don't think this is a particularly well thought-out interface.

Thanks,

	Ingo