linux-kernel - Re: [PATCH] tick/nohz_full: don't abuse smp_call_function_single() in tick_setup

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Date: Sat, 1 Jun 2024 16:03:22 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Ingo Molnar <mingo@...hat.com>, Nicholas Piggin <npiggin@...il.com>,
	Peter Zijlstra <peterz@...radead.org>, Phil Auld <pauld@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Chris von Recklinghausen <crecklin@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tick/nohz_full: don't abuse smp_call_function_single()
 in tick_setup_device()

Hi Frederic,

First of all, can we please make the additional changes you suggest on top of
this patch? I'd prefer to keep it as simple as possible, I will need to backport
it and I'd like to simplify the internal review.

On 05/30, Frederic Weisbecker wrote:
>
> And after all, pushing a bit further your subsequent patch, can we get rid of
> tick_do_timer_boot_cpu and ifdefery altogether? Such as:

Sure, I thought about this from the very beginning, see
https://lore.kernel.org/all/20240525135120.GA24152@redhat.com/
and the changelog in
[PATCH] tick/nohz_full: turn tick_do_timer_boot_cpu into boot_cpu_is_nohz_full
https://lore.kernel.org/all/20240530124032.GA26833@redhat.com/
on top of this patch.

And yes, in this case it is better to check that tick_do_timer_cpu != _NONE to
ensure that tick_nohz_full_cpu(tick_cpu) can't crash.

So I considered the change which is very close to yours, except

> +		} else if (timekeeper == TICK_DO_TIMER_NONE) {
> +			if (WARN_ON_ONCE(tick_nohz_full_enabled()))
> +				WRITE_ONCE(tick_do_timer_cpu, cpu);

I don't think we need to change tick_do_timer_cpu in this case.
And I am not sure we need to check tick_nohz_full_enabled() here.
IOW, I was thinking about

	if (!td->evtdev) {
		int 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 (!WARN_ON_ONCE(tick_cpu == TICK_DO_TIMER_NONE)) &&
			   tick_nohz_full_cpu(tick_cpu) &&
			   !tick_nohz_full_cpu(cpu)) {
			/*
			 * The boot CPU will stay in periodic (NOHZ disabled)
			 * mode until clocksource_done_booting() called after
			 * smp_init() selects a high resolution clocksource and
			 * timekeeping_notify() kicks the NOHZ stuff alive.
			 *
			 * So this WRITE_ONCE can only race with the READ_ONCE
			 * check in tick_periodic() but this race is harmless.
			 */
			WRITE_ONCE(tick_do_timer_cpu, cpu);
		}

But you know, somehow I like
[PATCH] tick/nohz_full: turn tick_do_timer_boot_cpu into boot_cpu_is_nohz_full
https://lore.kernel.org/all/20240530124032.GA26833@redhat.com/
a bit more, to me the code looks more understandable this way.

Note that this patch doesn't really need to keep #ifdef CONFIG_NO_HZ_FULL,

	if (!td->evtdev) {
		static bool boot_cpu_is_nohz_full;
		/*
		 * If no cpu took the do_timer update, assign it to
		 * this cpu:
		 */
		if (READ_ONCE(tick_do_timer_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.
			 */
			boot_cpu_is_nohz_full = tick_nohz_full_cpu(cpu);
		} else if (boot_cpu_is_nohz_full && !tick_nohz_full_cpu(cpu)) {
			boot_cpu_is_nohz_full = false;
			/*
			 * The boot CPU will stay in periodic (NOHZ disabled)
			 * mode until clocksource_done_booting() called after
			 * smp_init() selects a high resolution clocksource and
			 * timekeeping_notify() kicks the NOHZ stuff alive.
			 *
			 * So this WRITE_ONCE can only race with the READ_ONCE
			 * check in tick_periodic() but this race is harmless.
			 */
			WRITE_ONCE(tick_do_timer_cpu, cpu);
		}

should work without #ifdef.

In this case I don't think we need the _NONE check, tick_sched_do_timer() will
complain.

But I won't argue. I will be happy to make V2 which follows your recommendations
but again, can I do this on top of this patch?

Oleg.