linux-kernel - Re: [PATCH v2 1/3] sched: Add helper kstat_cpu_softirqs

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Date:   Fri, 28 Oct 2022 10:38:15 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     "Elliott, Robert (Servers)" <elliott@....com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        "Neeraj Upadhyay" <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] sched: Add helper kstat_cpu_softirqs_sum()



On 2022/10/28 3:04, Elliott, Robert (Servers) wrote:
> 
>> Similar to kstat_cpu_irqs_sum(), it counts the sum of all software
>> interrupts on a specified CPU.
>>
>> diff --git a/include/linux/kernel_stat.h b/include/linux/kernel_stat.h
>> @@ -67,6 +67,17 @@ static inline unsigned int kstat_softirqs_cpu(unsigned int irq, int cpu)
>>         return kstat_cpu(cpu).softirqs[irq];
>>  }
>>
>> +static inline unsigned int kstat_cpu_softirqs_sum(int cpu)
>> +{
>> +	int i;
>> +	unsigned int sum = 0;
>> +
>> +	for (i = 0; i < NR_SOFTIRQS; i++)
>> +		sum += kstat_softirqs_cpu(i, cpu);
>> +
>> +	return sum;
>> +}
> 
> In the function upon which this is based:
> 
> struct kernel_stat {
>         unsigned long irqs_sum;
>         unsigned int softirqs[NR_SOFTIRQS];
> };
> 
> static inline unsigned int kstat_cpu_irqs_sum(unsigned int cpu)
> {
>         return kstat_cpu(cpu).irqs_sum;
> }
> 
> kstat_cpu_irqs_sum returns an unsigned long as an unsigned int, which
> could cause large values to be truncated. Should that return
> unsigned long? The only existing caller is fs/proc/stat.c which

This should be a mistake on:
commit f2c66cd8eeddedb4 ("/proc/stat: scalability of irq num per cpu")

I'll correct it to "unsigned long" in the next version. Thanks.

> puts it into a u64:
>         u64 sum = 0;
>         ...
>         sum             += kstat_cpu_irqs_sum(i);
> 
> The softirqs field is an unsigned int, so the new function doesn't have
> this inconsistency.

OK.

To be honest, I did the math. CONFIG_HZ=250
2^32 / 250 / 3600 / 24 / 365 = 0.545 < 1 year

So, in theory, for those 32-bit processors, we should use "unsigned long long".
Of course, from a programming point of view, 64-bit consists of two 32-bits,
and there is an atomicity problem. I think that's probably why members of
struct kernel_stat don't use u64.

However, it seems that the type of member softirqs can currently be changed to
unsigned long. So, at least on a 64-bit processor, it won't have a count
overflow problem.

> 
> .
> 

-- 
Regards,
  Zhen Lei