Similar to kstat_cpu_irqs_sum(), it counts the sum of all software
interrupts on a specified CPU.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
---
 include/linux/kernel_stat.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/include/linux/kernel_stat.h b/include/linux/kernel_stat.h
index ddb5a358fd829f4..61d427c1962bf1c 100644
--- 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;
+}
+
 /*
  * Number of interrupts per specific IRQ source, since bootup
  */
-- 
2.25.1

> 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.

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

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

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.

> 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.  ;-)

							Thanx, Paul

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