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

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Message-ID: <20221028175709.GP5600@paulmck-ThinkPad-P17-Gen-1>
Date:   Fri, 28 Oct 2022 10:57:09 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Elliott, Robert (Servers)" <elliott@....com>
Cc:     Zhen Lei <thunder.leizhen@...wei.com>,
        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 Thu, Oct 27, 2022 at 07:04:53PM +0000, 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:
> 
> 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.

Good point!

Zhen Lei, thoughts?

							Thanx, Paul