linux-kernel - Re: [PATCH] kernel/isolation: Asset that a housekeeping CPU comes up at boot time

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Message-ID: <20190624105729.3isejrp4455suxaz@e107158-lin.cambridge.arm.com>
Date:   Mon, 24 Jun 2019 11:57:36 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Frederic Weisbecker <frederic@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] kernel/isolation: Asset that a housekeeping CPU comes up
 at boot time

On 06/01/19 21:39, Nicholas Piggin wrote:
> With the change to allow the boot CPU0 to be isolated, it is possible
> to specify command line options that result in no housekeeping CPU
> online at boot.
> 
> An 8 CPU system booted with "nohz_full=0-6 maxcpus=4", for example.
> 
> It is not easily possible at housekeeping init time to know all the
> various SMP options that will result in an invalid configuration, so
> this patch adds a sanity check after SMP init, to ensure that a
> housekeeping CPU has been onlined.
> 
> The panic is undesirable, but it's better than the alternative of an
> obscure non deterministic failure. The panic will reliably happen
> when advanced parameters are used incorrectly.
> 
> Signed-off-by: Nicholas Piggin <npiggin@...il.com>
> ---
>  kernel/sched/isolation.c | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
> index 123ea07a3f3b..7b9e1e0d4ec3 100644
> --- a/kernel/sched/isolation.c
> +++ b/kernel/sched/isolation.c
> @@ -63,6 +63,29 @@ void __init housekeeping_init(void)
>  	WARN_ON_ONCE(cpumask_empty(housekeeping_mask));
>  }
>  
> +static int __init housekeeping_verify_smp(void)
> +{
> +	int cpu;
> +
> +	/*
> +	 * Early housekeeping setup is done before CPUs come up, and there are
> +	 * a range of options scattered around that can restrict which CPUs
> +	 * come up. It is possible to pass in a combination of housekeeping
> +	 * and SMP arguments that result in housekeeping assigned to an
> +	 * offline CPU.
> +	 *
> +	 * Check that condition here after SMP comes up, and give a useful
> +	 * error message rather than an obscure non deterministic crash or
> +	 * hang later.
> +	 */
> +	for_each_online_cpu(cpu) {
> +		if (cpumask_test_cpu(cpu, housekeeping_mask))
> +			return 0;
> +	}
> +	panic("Housekeeping: nohz_full= or isolcpus= resulted in no online CPUs for housekeeping.\n");

I am hitting this panic when I boot my juno board.


I have CONFIG_CPU_ISOLATION=y but I don't pass nohuz_full nor isolcpus in the
commandline. I think what's going on is that housekeeping_setup() doesn't get
called and hence housekeeping_mask isn't initialized in my case, causing this
check to fail and trigger the panic.

The below seems to 'fix' it though not sure if it's the right way forward.
A revert obviously fixes it too but I doubt we want that :-)


diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index 7b9e1e0d4ec3..a9ca8628c1a2 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -67,6 +67,9 @@ static int __init housekeeping_verify_smp(void)
 {
 	int cpu;
 
+	if (!housekeeping_flags)
+		return 0;
+
 	/*
 	 * Early housekeeping setup is done before CPUs come up, and there are
 	 * a range of options scattered around that can restrict which CPUs


Cheers

--
Qais Yousef


> +}
> +core_initcall(housekeeping_verify_smp);
> +
>  static int __init housekeeping_setup(char *str, enum hk_flags flags)
>  {
>  	cpumask_var_t non_housekeeping_mask;
> -- 
> 2.20.1
>