Received-SPF: pass (google.com: domain of linux-kernel+bounces-189377-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45e3:2400::1 as permitted sender) client-ip=2604:1380:45e3:2400::1;
Date: Sat, 25 May 2024 16:13:16 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Frederic Weisbecker <frederic@kernel.org>,
	Ingo Molnar <mingo@redhat.com>, Nicholas Piggin <npiggin@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>, Phil Auld <pauld@redhat.com>,
	Chris von Recklinghausen <crecklin@redhat.com>,
	linux-kernel@vger.kernel.org
Subject: Re: sched/isolation: tick_take_do_timer_from_boot() calls
 smp_call_function_single() with irqs disabled
Message-ID: <20240525141316.GB24152@redhat.com>
References: <20240522151742.GA10400@redhat.com>
 <20240523132358.GA1965@redhat.com>
 <87h6eneeu7.ffs@tglx>
 <ZlCwKk65-eL0FrKX@pavilion.home>
 <20240524183700.GA17065@redhat.com>
 <87v832dfw1.ffs@tglx>
 <20240525135120.GA24152@redhat.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20240525135120.GA24152@redhat.com>
User-Agent: Mutt/1.5.24 (2015-08-30)

On 05/25, Oleg Nesterov wrote:
>
> perhaps we can simply kill it along with #ifdef CONFIG_NO_HZ_FULL ?
>
> 	if (!td->evtdev) {
> 		tick_cpu = READ_ONCE(tick_do_timer_cpu);
> 		/*
> 		 * If no cpu took the do_timer update, assign it to
> 		 * this cpu:
> 		 */
> 		if (tick_cpu == TICK_DO_TIMER_BOOT) {
> 			WRITE_ONCE(tick_do_timer_cpu, cpu);
> 			tick_next_period = ktime_get();
> 			/*
> 			 * The boot CPU may be nohz_full, in which case the
> 			 * first housekeeping secondary will take do_timer
> 			 * from us.
> 			 */
> 		} else if (tick_nohz_full_cpu(tick_cpu) &&
> 			  !tick_nohz_full_cpu(cpu)) {
> 			WRITE_ONCE(tick_do_timer_cpu, cpu);
> 		}

although tick_nohz_full_cpu(tick_cpu) above depends on the fact that
tick_cpu = TICK_DO_TIMER_NONE should not be possible if
tick_nohz_full_enabled(), not good.

Oleg.