linux-kernel - Re: [PATCHv9 2/3] irq: use a struct for the kstat

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Message-ID: <3f4b7dbe-93c1-4cb0-8233-18e8432409f8@linux.alibaba.com>
Date: Fri, 23 Feb 2024 15:18:48 +0800
From: Bitao Hu <yaoma@...ux.alibaba.com>
To: Thomas Gleixner <tglx@...utronix.de>, dianders@...omium.org,
 akpm@...ux-foundation.org, liusong@...ux.alibaba.com, pmladek@...e.com,
 kernelfans@...il.com, deller@....de, npiggin@...il.com,
 tsbogend@...ha.franken.de, James.Bottomley@...senPartnership.com,
 jan.kiszka@...mens.com
Cc: linux-kernel@...r.kernel.org, linux-mips@...r.kernel.org,
 linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
 yaoma@...ux.alibaba.com
Subject: Re: [PATCHv9 2/3] irq: use a struct for the kstat_irqs in the
 interrupt descriptor

Hi,

On 2024/2/22 21:22, Thomas Gleixner wrote:
> On Thu, Feb 22 2024 at 17:34, Bitao Hu wrote:
> 
> First of all the subsystem prefix is 'genirq:'. 'git log kernel/irq/'
> gives you a pretty good hint. It's documented....
> 
> Secondly the subject line does not match what this patch is about. It's
> not about using a struct, it's about providing a snapshot mechanism, no?
> 
>> The current implementation uses an int for the kstat_irqs in the
>> interrupt descriptor.
>>
>> However, we need to know the number of interrupts which happened
>> since softlockup detection took a snapshot in order to analyze
>> the problem caused by an interrupt storm.
>>
>> Replacing an int with a struct and providing sensible interfaces
>> for the watchdog code can keep it self contained to the interrupt
>> core code.
> 
> So something like this makes a useful change log for this:
> 
>   Subject: genirq: Provide a snapshot mechanism for interrupt statistics
> 
>   The soft lockup detector lacks a mechanism to identify interrupt storms
>   as root cause of a lockup. To enable this the detector needs a
>   mechanism to snapshot the interrupt count statistics on a CPU when the
>   detector observes a potential lockup scenario and compare that against
>   the interrupt count when it warns about the lockup later on. The number
>   of interrupts in that period give a hint whether the lockup might be
>   caused by an interrupt storm.
> 
>   Instead of having extra storage in the lockup detector and accessing
>   the internals of the interrupt descriptor directly, convert the per CPU
>   irq_desc::kstat_irq member to a data structure which contains the
>   counter plus a snapshot member and provide interfaces to take a
>   snapshot of all interrupts on the current CPU and to retrieve the delta
>   of a specific interrupt later on.
> 
Thanks, the changelog you wrote very clearly articulates the purpose of
this patch.

> Hmm?
> 
>> Signed-off-by: Bitao Hu <yaoma@...ux.alibaba.com>
> 
> Interesting. You fully authored the patch?
> 
> That's not how it works. You cannot take work from others and claim that
> it is yours. The minimal courtesy is to add a 'Originally-by:' tag.
> 
I'm very sorry, the majority of this patch is your work, I will add an
'Originally-by:' tag.

>> diff --git a/kernel/irq/proc.c b/kernel/irq/proc.c
>> index 623b8136e9af..3ad40cf30c66 100644
>> --- a/kernel/irq/proc.c
>> +++ b/kernel/irq/proc.c
>> @@ -488,18 +488,15 @@ int show_interrupts(struct seq_file *p, void *v)
>>   	if (!desc || irq_settings_is_hidden(desc))
>>   		goto outsparse;
>>   
>> -	if (desc->kstat_irqs) {
>> -		for_each_online_cpu(j)
>> -			any_count |= data_race(*per_cpu_ptr(desc->kstat_irqs, j));
>> -	}
>> +	if (desc->kstat_irqs)
>> +		any_count = data_race(desc->tot_count);
> 
> This is an unrelated change and needs to be split out into a separate
> patch with a proper changelog which explains why this is equivalent.
> 

Alright, I will remove this change witch is not related to the purpose
of this patch.

I guess that the purpose of suggesting this change in your V1 response
was to speedup the 'show_interrupts'. However, after reviewing the
usage of 'desc->tot_count' in 'unsigned int kstat_irqs(unsigned int 
irq)', I think the change might be as follows:

diff --git a/kernel/irq/proc.c b/kernel/irq/proc.c
index 623b8136e9af..53b8d6edd7ac 100644
--- a/kernel/irq/proc.c
+++ b/kernel/irq/proc.c
@@ -489,8 +489,13 @@ int show_interrupts(struct seq_file *p, void *v)
                 goto outsparse;

         if (desc->kstat_irqs) {
-               for_each_online_cpu(j)
-                       any_count |= 
data_race(per_cpu(desc->kstat_irqs->cnt, j));
+               if (!irq_settings_is_per_cpu_devid(desc) &&
+                   !irq_settings_is_per_cpu(desc) &&
+                   !irq_is_nmi(desc))
+                       any_count = data_race(desc->tot_count);
+               else
+                       for_each_online_cpu(j)
+                               any_count |= 
data_race(per_cpu(desc->kstat_irqs->cnt, j));
         }

         if ((!desc->action || irq_desc_is_chained(desc)) && !any_count)

Is my idea correct?

Best Regards,
	Bitao