From: Oleg Nesterov <oleg@redhat.com>
To: Tejun Heo <tj@kernel.org>
Cc: 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>,
Frederic Weisbecker <frederic@kernel.org>
Subject: Re: [PATCH v2 1/1] wq: Avoid using isolated cpus' timers on queue_delayed_work
Date: Wed, 3 Apr 2024 22:38:14 +0200 [thread overview]
Message-ID: <20240403203814.GD31764@redhat.com> (raw)
In-Reply-To: <Zg2qFinSkAOmRHcM@slm.duckdns.org>
Hi Tejun,
On 04/03, Tejun Heo wrote:
>
> (cc'ing Frederic and quoting whole body)
>
> Hello, Oleg.
>
> On Tue, Apr 02, 2024 at 12:58:47PM +0200, Oleg Nesterov wrote:
> > This patch was applied as aae17ebb53cd3da but as Chris reports with this
> > commit the kernel can crash at boot time because __queue_delayed_work()
> > doesn't check that housekeeping_any_cpu() returns a valid cpu < nr_cpu_ids.
> >
> > Just boot the kernel with nohz_full=mask which includes the boot cpu, say
> > nohz_full=0-6 on a machine with 8 CPUs. __queue_delayed_work() will use
> > add_timer_on(timer, NR_CPUS /* returned by housekeeping_any_cpu */) until
> > start_secondary() brings CPU 7 up.
> >
> > The problem is simple, but I do not know what should we do, I know nothing
> > about CPU isolation.
> >
> > We can fix __queue_delayed_work(), this is simple, but other callers of
> > housekeeping_any_cpu() seem to assume it must always return a valid CPU
> > too. So perhaps we should change housekeeping_any_cpu()
>
> Yeah, patching this up from wq side is easy but housekeeping_any_cpu()
> always being able to pick a housekeeping CPU would be better.
>
> > - 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;
> >
> > ?
> >
> > But I'm afraid this can hide other problems. May be
> >
> > - 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(system_state > SYSTEM_BOOTING);
> >
> > ?
> >
> > -------------------------------------------------------------------------------
> > 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.
Oleg.
> > So it seems that we should fix housekeeping_setup() ? see the patch below.
> >
> > In any case the usage of cpu_present_mask doesn't look right to me.
> >
> > Oleg.
> >
> > --- a/kernel/sched/isolation.c
> > +++ b/kernel/sched/isolation.c
> > @@ -129,7 +154,7 @@ static int __init housekeeping_setup(char *str, unsigned long flags)
> > cpumask_andnot(housekeeping_staging,
> > cpu_possible_mask, non_housekeeping_mask);
> >
> > - if (!cpumask_intersects(cpu_present_mask, housekeeping_staging)) {
> > + if (!cpumask_test_cpu(smp_processor_id(), housekeeping_staging)) {
> > __cpumask_set_cpu(smp_processor_id(), housekeeping_staging);
> > __cpumask_clear_cpu(smp_processor_id(), non_housekeeping_mask);
> > if (!housekeeping.flags) {
> >
next prev parent reply other threads:[~2024-04-03 20:39 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 [this message]
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
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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