linux-kernel - Re: Nohz_full on boot CPU is broken (was: Re: [PATCH v2 1/1] wq: Avoid using isolated cpus' timers on queue_delayed

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Date: Tue, 9 Apr 2024 15:07:28 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Nick Piggin <npiggin@...il.com>, Tejun Heo <tj@...nel.org>,
	Leonardo Bras <leobras@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	linux-kernel@...r.kernel.org, Junyao Zhao <junzhao@...hat.com>,
	Chris von Recklinghausen <crecklin@...hat.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)

On 04/09, Frederic Weisbecker wrote:
>
> Le Sun, Apr 07, 2024 at 03:09:14PM +0200, Oleg Nesterov a écrit :
> > 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.
>
> Right but there are many possible calls to housekeeping on boot before the
> first housekeeper becomes online.

Well, it seems that other callers are more or less fine in this respect...
At least the kernel boots fine with that commit reverted.

But a) I didn't try to really check, and b) this doesn't matter.

I agree, and that is why I never blamed this change in queue_delayed_work().

OK, you seem to agree with the patch below, I'll write the changelog/comment
and send it "officially".

Or did I misunderstand you?

Oleg.


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();