linux-kernel - Re: [PATCH v3] genirq: Machine-parsable version of /proc/interrupts

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Date:   Thu, 8 Sep 2016 18:15:11 -0400
From:   Craig Gallek <kraigatgoog@...il.com>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        David Decotigny <decot@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] genirq: Machine-parsable version of /proc/interrupts

On Thu, Sep 8, 2016 at 6:00 PM, Randy Dunlap <rdunlap@...radead.org> wrote:
> On 09/08/16 13:25, Craig Gallek wrote:
>> From: Craig Gallek <kraig@...gle.com>
>>
>> Add struct kobject to struct irq_desc to allow for easy export
>> to sysfs.  This allows for much simpler userspace-parsing of
>> the information contained in struct irq_desc.
>>
>> Note that sysfs is not available at the time of early irq initialization.
>> These interrupts are accounted for using a postcore_initcall callback.
>>
>> Examples:
>>   /sys/kernel/irq/18/actions: i801_smbus,ehci_hcd:usb1,uhci_hcd:usb7
>>   /sys/kernel/irq/18/chip_name:       IR-IO-APIC
>>   /sys/kernel/irq/18/hwirq:           18
>>   /sys/kernel/irq/18/name:            fasteoi
>>   /sys/kernel/irq/18/per_cpu_count:   0,0
>>   /sys/kernel/irq/18/type:            level
>>
>>   /sys/kernel/irq/25/actions: ahci0
>>   /sys/kernel/irq/25/chip_name:       IR-PCI-MSI
>>   /sys/kernel/irq/25/hwirq:           512000
>>   /sys/kernel/irq/25/name:            edge
>>   /sys/kernel/irq/25/per_cpu_count:   29036,0
>>   /sys/kernel/irq/25/type:            edge
>
> Thanks for the update.
>
> One concern:
>
> This per_cpu_count is for online CPUs only.
> How does this help when the online CPUs change?
>
> E.g., above could be for CPUs 1 and 5.
> The next time that it is read it could be for CPUs 0 and 3.
> Seems that it could be confusing even for software reading it.

Thanks for the feedback. I imagine most use cases for this will simply
add up all the values to obtain a total.  There's not a lot of use for
the individual elements unless you additionally know something about
the CPU id layout, interrupt pinning, and/or CPU online state.  The
/proc/interrupts interface has this same issue, but additionally uses
column headers.  There's really know way to give a similar consistent
view of all of this data using multiple sysfs files.  Given this lack
of atomicity, across files, I don't think it's unreasonable for the
counter order to change when the system's online CPUs change.

The only alternative I could imagine would be to include a header row
in this file listing the CPU ids as a parallel list.  I think this
goes against the standard sysfs file format, though...