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

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Date:   Sat, 29 Oct 2022 17:51:51 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     <paulmck@...nel.org>
CC:     "Elliott, Robert (Servers)" <elliott@....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 2022/10/29 6:35, Paul E. McKenney wrote:
> On Fri, Oct 28, 2022 at 10:38:15AM +0800, Leizhen (ThunderTown) wrote:
>>
>>
>> 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
> 
> For this to be a problem, our RCU CPU stall warning would have to be
> for a months-long grace period, even on systems with HZ=1000.  In almost
> all cases, the system would have OOMed long before then.

Yes.

> 
>> 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.
> 
> An unsigned long should suffice.  ;-)

include/linux/irqdesc.h:58:     unsigned int __percpu   *kstat_irqs;

I found another place where the hard interrupt count was stored with type "unsigned int",
it's used by "/proc/interrupts". Maybe the user-mode program gets it periodically and
accumulates it to a 64-bit value. Of course, maybe half a year later, no one cares about
the specific interrupts count anymore.

So, apart from what Elliott mentioned, I won't change the rest.

> 
> 							Thanx, Paul
> .
> 

-- 
Regards,
  Zhen Lei