From: Frederic Weisbecker <frederic@kernel.org>
To: Oleg Nesterov <oleg@redhat.com>, Nick Piggin <npiggin@gmail.com>
Cc: Tejun Heo <tj@kernel.org>, Leonardo Bras <leobras@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
Lai Jiangshan <jiangshanlai@gmail.com>,
linux-kernel@vger.kernel.org, Junyao Zhao <junzhao@redhat.com>,
Chris von Recklinghausen <crecklin@redhat.com>
Subject: Nohz_full on boot CPU is broken (was: Re: [PATCH v2 1/1] wq: Avoid using isolated cpus' timers on queue_delayed_work)
Date: Fri, 5 Apr 2024 23:52:03 +0200 [thread overview]
Message-ID: <ZhByg-xQv6_PC3Pd@localhost.localdomain> (raw)
In-Reply-To: <20240405140449.GB22839@redhat.com>
+Cc Nick
Le Fri, Apr 05, 2024 at 04:04:49PM +0200, Oleg Nesterov a écrit :
> On 04/03, Oleg Nesterov wrote:
> >
> > > > OTOH, Documentation/timers/no_hz.rst says
> > > >
> > > > Therefore, the
> > > > boot CPU is prohibited from entering adaptive-ticks mode. Specifying a
> > > > "nohz_full=" mask that includes the boot CPU will result in a boot-time
> > > > error message, and the boot CPU will be removed from the mask.
> > > >
> > > > and this doesn't match the reality.
> > >
> > > Don't some archs allow the boot CPU to go down too tho? If so, this doesn't
> > > really solve the problem, right?
> >
> > I do not know. But I thought about this too.
> >
> > In the context of this discussion we do not care if the boot CPU goes down.
> > But we need at least one housekeeping CPU after cpu_down(). The comment in
> > cpu_down_maps_locked() says
> >
> > Also keep at least one housekeeping cpu onlined
> >
> > but it checks HK_TYPE_DOMAIN, and I do not know (and it is too late for me
> > to try to read the code ;) if housekeeping.cpumasks[HK_TYPE_TIMER] can get
> > empty or not.
>
> This nearly killed me, but I managed to convince myself we shouldn't worry
> about cpu_down().
>
> HK_FLAG_TIMER implies HK_FLAG_TICK.
>
> HK_FLAG_TICK implies tick_nohz_full_setup() which sets
> tick_nohz_full_mask = non_housekeeping_mask.
>
> When tick_setup_device() is called on a housekeeping CPU it does
>
> else if (tick_do_timer_boot_cpu != -1 &&
> !tick_nohz_full_cpu(cpu)) {
> tick_take_do_timer_from_boot();
> tick_do_timer_boot_cpu = -1;
>
>
> and this sets tick_do_timer_cpu = first-housekeeping-cpu.
>
> cpu_down(tick_do_timer_cpu) will fail, tick_nohz_cpu_down() will nack it.
>
> So cpu_down() can't make housekeeping.cpumasks[HK_FLAG_TIMER] empty and I
> still think that the change below is the right approach.
>
> But probably WARN_ON() in housekeeping_any_cpu() makes sense anyway.
>
> What do you think?
Good analysis on this nasty housekeeping VS tick code. I promised so many
times to cleanup this mess but things keep piling up.
It is indeed possible for the boot CPU to be a nohz_full CPU and as
you can see, it's only half-working. This is so ever since:
08ae95f4fd3b (nohz_full: Allow the boot CPU to be nohz_full)
I wish I had nacked it before it got merged, especially as the changelog mentions
that the user could have solved this with modifying its setup... I would love
to revert that now but I don't know if anyone uses this and have it working
by chance somewhere... Should we continue to support a broken feature? Can we
break user ABI if it's already half-broken?
Anyway so during boot it's possible to have an empty
housekeeping_mask(HK_TYPE_TIMER) & cpu_online_mask. After boot though (provided
any CPU from the housekeeping_mask(HK_TYPE_TIMER) has actually booted, which
isn't even guaranteed if maxcpus= is passed...) the first online housekeeping can't
go down like you spotted.
Thanks.
next prev parent reply other threads:[~2024-04-05 21:52 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-30 1:00 [PATCH v2 1/1] wq: Avoid using isolated cpus' timers on queue_delayed_work Leonardo Bras
2024-01-30 1:22 ` Tejun Heo
2024-01-30 2:58 ` Leonardo Bras
2024-04-02 10:58 ` Oleg Nesterov
2024-04-03 19:12 ` Tejun Heo
2024-04-03 20:38 ` Oleg Nesterov
2024-04-05 14:04 ` Oleg Nesterov
2024-04-05 15:38 ` Tejun Heo
2024-04-05 22:03 ` Frederic Weisbecker
2024-04-05 21:52 ` Frederic Weisbecker [this message]
2024-04-07 13:09 ` Nohz_full on boot CPU is broken (was: Re: [PATCH v2 1/1] wq: Avoid using isolated cpus' timers on queue_delayed_work) Oleg Nesterov
2024-04-07 13:52 ` Oleg Nesterov
2024-04-09 12:05 ` Frederic Weisbecker
2024-04-09 12:04 ` Frederic Weisbecker
2024-04-09 13:07 ` Oleg Nesterov
2024-04-09 13:59 ` Frederic Weisbecker
2024-04-10 4:26 ` Nicholas Piggin
2024-04-10 13:55 ` Oleg Nesterov
2024-04-11 13:41 ` Oleg Nesterov
2024-04-11 14:39 ` [PATCH] sched/isolation: fix boot crash when the boot CPU is nohz_full Oleg Nesterov
2024-04-11 16:59 ` Oleg Nesterov
2024-04-13 14:17 ` [PATCH] sched/isolation: fix boot crash when maxcpus < first-housekeeping-cpu Oleg Nesterov
2024-04-18 14:54 ` Phil Auld
2024-04-18 15:40 ` Frederic Weisbecker
2024-04-24 20:05 ` [tip: sched/urgent] sched/isolation: Fix boot crash when maxcpus < first housekeeping CPU tip-bot2 for Oleg Nesterov
2024-04-28 8:13 ` Ingo Molnar
2024-04-28 13:16 ` Oleg Nesterov
2024-04-28 8:24 ` tip-bot2 for Oleg Nesterov
2024-04-15 21:37 ` [PATCH] sched/isolation: fix boot crash when the boot CPU is nohz_full Frederic Weisbecker
2024-04-18 14:50 ` Phil Auld
2024-04-22 18:50 ` Oleg Nesterov
2024-04-24 14:42 ` Phil Auld
2024-04-24 20:05 ` [tip: sched/urgent] sched/isolation: {revent " tip-bot2 for Oleg Nesterov
2024-04-24 20:41 ` Phil Auld
2024-04-28 8:14 ` Ingo Molnar
2024-04-29 11:50 ` Phil Auld
2024-04-28 8:24 ` [tip: sched/urgent] sched/isolation: Prevent " tip-bot2 for Oleg Nesterov
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