From: Frederic Weisbecker <frederic@kernel.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Nick Piggin <npiggin@gmail.com>, 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: Re: Nohz_full on boot CPU is broken (was: Re: [PATCH v2 1/1] wq: Avoid using isolated cpus' timers on queue_delayed_work)
Date: Tue, 9 Apr 2024 14:05:40 +0200 [thread overview]
Message-ID: <ZhUvFFeKfFtJGSY8@localhost.localdomain> (raw)
In-Reply-To: <20240407135248.GB10796@redhat.com>
Le Sun, Apr 07, 2024 at 03:52:48PM +0200, Oleg Nesterov a écrit :
> On 04/07, Oleg Nesterov wrote:
> >
> > On 04/05, Frederic Weisbecker wrote:
> > >
> > > +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)
> >
> > Thanks... So this is intentional. I was confused by
> >
> > 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.
> >
> > from Documentation/timers/no_hz.rst
> >
> > > 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?
> >
> > Well, the changelog says
> >
> > nohz_full has been trialed at a large supercomputer site and found to
> > significantly reduce jitter. In order to deploy it in production, they
> > need CPU0 to be nohz_full
> >
> > so I guess this feature has users.
> >
> > But after the commit aae17ebb53cd3da ("workqueue: Avoid using isolated cpus'
> > timers on queue_delayed_work") the kernel will crash at boot time if the boot
> > CPU is nohz_full.
> >
> > So we need a workaround at least. I am starting to think I will send a trivial
> > patch which changes __queue_delayed_work() to validate the cpu returned by
> > housekeeping_any_cpu(HK_TYPE_TIMER).
> >
> > But perhaps something like below makes more sense as a (stupid) workaround?
>
> Or simply
>
>
> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> index 373d42c707bc..e912555c6fc8 100644
> --- a/kernel/sched/isolation.c
> +++ b/kernel/sched/isolation.c
> @@ -46,7 +46,11 @@ int housekeeping_any_cpu(enum hk_type type)
> if (cpu < nr_cpu_ids)
> return cpu;
>
> - return cpumask_any_and(housekeeping.cpumasks[type], cpu_online_mask);
> + cpu = cpumask_any_and(housekeeping.cpumasks[type], cpu_online_mask);
> + if (cpu < nr_cpu_ids)
> + return cpu;
> +
> + WARN_ON_ONCE(system_state == SYSTEM_RUNNING);
> }
> }
> return smp_processor_id();
Yep, looks good!
Thanks.
next prev parent reply other threads:[~2024-04-09 12:05 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 ` Nohz_full on boot CPU is broken (was: Re: [PATCH v2 1/1] wq: Avoid using isolated cpus' timers on queue_delayed_work) Frederic Weisbecker
2024-04-07 13:09 ` Oleg Nesterov
2024-04-07 13:52 ` Oleg Nesterov
2024-04-09 12:05 ` Frederic Weisbecker [this message]
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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