linux-kernel - Re: [PATCH] softirq: let ksoftirqd do its job

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Date:   Thu, 1 Sep 2016 15:00:10 +0200
From:   Hannes Frederic Sowa <hannes@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        David Miller <davem@...emloft.net>,
        Rik van Riel <riel@...hat.com>,
        Paolo Abeni <pabeni@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>, Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH] softirq: let ksoftirqd do its job

On 01.09.2016 14:57, Eric Dumazet wrote:
> On Thu, 2016-09-01 at 14:38 +0200, Jesper Dangaard Brouer wrote:
> 
>> Correction, on the server-under-test, I'm actually running RHEL7.2
>>
>>
>>> How do I verify/check if I have enabled a cpu-cgroup?
>>
>> Hannes says I can look in "/proc/self/cgroup"
>>
>>  $ cat /proc/self/cgroup
>>  7:net_cls:/
>>  6:blkio:/
>>  5:devices:/
>>  4:perf_event:/
>>  3:cpu,cpuacct:/
>>  2:cpuset:/
>>  1:name=systemd:/user.slice/user-1000.slice/session-c1.scope
>>  
>> And that "/" indicate I've not enabled cgroups, right?
>>
> 
> In my experience, I found that times displayed by top are often off for
> softirq processing.
> 
> Before applying my patch, top shows very small amount of cpu time for
> udp_rcv and ksoftirqd/0 , while obviously cpu 0 is completely busy.
> 
> Make sure to try latest Linus tree, as I did yesterday, because
> apparently things are better than a few weeks back.
> 
> BTW, even 'perf top' has sometimes problems showing me cycles spent in
> softirq. I need to make sure the cpu processing NIC interrupts also
> spend cycles in some user space program to get meaningful results.

I think that ksoftirqd time is actually accounted to system:

excerpt from irqtime_account_process_tick in kernel/sched/cputime.c

	if (this_cpu_ksoftirqd() == p) {
		/*
		 * ksoftirqd time do not get accounted in cpu_softirq_time.
		 * So, we have to handle it separately here.
		 * Also, p->stime needs to be updated for ksoftirqd.
		 */
		__account_system_time(p, cputime, scaled, CPUTIME_SOFTIRQ);
	} else if (user_tick) {