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Message-ID: <20240525135120.GA24152@redhat.com>
Date: Sat, 25 May 2024 15:51:20 +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
Thomas, thanks a lot!
Let me grep a bit more to better understand your explanations.
Just one note for now.
On 05/25, Thomas Gleixner wrote:
>
> --- a/kernel/time/tick-common.c
> +++ b/kernel/time/tick-common.c
> @@ -229,11 +209,9 @@ static void tick_setup_device(struct tick_device *td,
> if (tick_nohz_full_cpu(cpu))
> tick_do_timer_boot_cpu = cpu;
>
> - } else if (tick_do_timer_boot_cpu != -1 &&
> - !tick_nohz_full_cpu(cpu)) {
> - tick_take_do_timer_from_boot();
> + } else if (tick_do_timer_boot_cpu != -1 && !tick_nohz_full_cpu(cpu)) {
> + WRITE_ONCE(tick_do_timer_cpu, cpu);
> tick_do_timer_boot_cpu = -1;
> - WARN_ON(READ_ONCE(tick_do_timer_cpu) != cpu);
> #endif
> }
>
> along with the removal of the SMP function call voodoo programming gunk,
Yes,
> Changing the horribly lazy and incomprehensible '-1' to an actual
> meaningful define, e.g. TICK_DO_TIMER_NONE, would definitely help along
> with renaming the variable to tick_do_timer_nohz_full_boot_cpu.
Better yet, we can make it a boolean, we do not need cpu number. And
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);
}
Oleg.
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