netdev - Re: [RFC][Patch v1 1/3] sched/isolation: API to get num of hosekeeping CPUs

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Date:   Mon, 21 Sep 2020 23:16:51 -0400
From:   Nitesh Narayan Lal <nitesh@...hat.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        linux-pci@...r.kernel.org, mtosatti@...hat.com,
        sassmann@...hat.com, jeffrey.t.kirsher@...el.com,
        jacob.e.keller@...el.com, jlelli@...hat.com, hch@...radead.org,
        bhelgaas@...gle.com, mike.marciniszyn@...el.com,
        dennis.dalessandro@...el.com, thomas.lendacky@....com,
        jerinj@...vell.com, mathias.nyman@...el.com, jiri@...dia.com
Subject: Re: [RFC][Patch v1 1/3] sched/isolation: API to get num of
 hosekeeping CPUs


On 9/21/20 7:40 PM, Frederic Weisbecker wrote:
> On Wed, Sep 09, 2020 at 11:08:16AM -0400, Nitesh Narayan Lal wrote:
>> +/*
>> + * num_housekeeping_cpus() - Read the number of housekeeping CPUs.
>> + *
>> + * This function returns the number of available housekeeping CPUs
>> + * based on __num_housekeeping_cpus which is of type atomic_t
>> + * and is initialized at the time of the housekeeping setup.
>> + */
>> +unsigned int num_housekeeping_cpus(void)
>> +{
>> +	unsigned int cpus;
>> +
>> +	if (static_branch_unlikely(&housekeeping_overridden)) {
>> +		cpus = atomic_read(&__num_housekeeping_cpus);
>> +		/* We should always have at least one housekeeping CPU */
>> +		BUG_ON(!cpus);
>> +		return cpus;
>> +	}
>> +	return num_online_cpus();
>> +}
>> +EXPORT_SYMBOL_GPL(num_housekeeping_cpus);
>> +
>>  int housekeeping_any_cpu(enum hk_flags flags)
>>  {
>>  	int cpu;
>> @@ -131,6 +153,7 @@ static int __init housekeeping_setup(char *str, enum hk_flags flags)
>>  
>>  	housekeeping_flags |= flags;
>>  
>> +	atomic_set(&__num_housekeeping_cpus, cpumask_weight(housekeeping_mask));
> So the problem here is that it takes the whole cpumask weight but you're only
> interested in the housekeepers who take the managed irq duties I guess
> (HK_FLAG_MANAGED_IRQ ?).

IMHO we should also consider the cases where we only have nohz_full.
Otherwise, we may run into the same situation on those setups, do you agree?

>
>>  	free_bootmem_cpumask_var(non_housekeeping_mask);
>>  
>>  	return 1;
>> -- 
>> 2.27.0
>>
-- 
Thanks
Nitesh



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