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

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Date:   Tue, 25 Jun 2019 10:05:17 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        linux-kernel@...r.kernel.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

Qais Yousef's on June 24, 2019 8:57 pm:
> 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 :-)

That'll do it. Thanks for the report and investigation.

> 
> 
> 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
>> 
>