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

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Date:   Thu, 27 Oct 2022 19:04:53 +0000
From:   "Elliott, Robert (Servers)" <elliott@....com>
To:     Zhen Lei <thunder.leizhen@...wei.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()


> 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:

irqs_sumstruct 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
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.