linux-kernel - Re: sched/isolation: tick_take_do_timer_from_boot() calls smp_call_function

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Date: Sat, 25 May 2024 16:13:16 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Frederic Weisbecker <frederic@...nel.org>,
	Ingo Molnar <mingo@...hat.com>, Nicholas Piggin <npiggin@...il.com>,
	Peter Zijlstra <peterz@...radead.org>, Phil Auld <pauld@...hat.com>,
	Chris von Recklinghausen <crecklin@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: sched/isolation: tick_take_do_timer_from_boot() calls
 smp_call_function_single() with irqs disabled

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.