linux-kernel - Re: [PATCH] proc/stat: Separate out individual irq counts into /proc/stat

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Date:   Thu, 19 Apr 2018 17:05:23 -0400
From:   Waiman Long <longman@...hat.com>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     linux-kernel@...r.kernel.org, rdunlap@...radead.org,
        akpm@...ux-foundation.org
Subject: Re: [PATCH] proc/stat: Separate out individual irq counts into
 /proc/stat_irqs

On 04/19/2018 04:39 PM, Alexey Dobriyan wrote:
>>
>> Yes, that can probably help.
>>
>> This is the data from the problematic skylake server:
>>
>> model name    : Intel(R) Xeon(R) Gold 6136 CPU @ 3.00GHz
>> 56 sosreport-carevalo.02076935-20180413085327/proc/stat
>> Interrupts: 5370
>> Interrupts without "0" entries: 1011
>>
>> There are still quite a large number of non-zero entries, though.
>>
>>> Or maintain array of registered irqs and iterate over them only.
>> Right, we can allocate a bitmap of used irqs to do that.
>>
>>> I have another idea.
>>>
>>> perf record shows mutex_lock/mutex_unlock at the top.
>>> Most of them are irq mutex not seqfile mutex as there are many more
>>> interrupts than reads. Take it once.
>>>
>> How many cpus are in your test system? In that skylake server, it was
>> the per-cpu summing operation of the irq counts that was consuming most
>> of the time for reading /proc/stat. I think we can certainly try to
>> optimize the lock taking.
> It's 16x(NR_IRQS: 4352, nr_irqs: 960, preallocated irqs: 16)
> Given that irq registering is rare operation, maintaining sorted array
> of irq should be the best option.
>

BTW, the skylake server is 2-socket 24-core 48-thread.

Cheers,
Longman