linux-kernel - softirq under reporting with CONFIG_NO_HZ

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Date:	Mon, 04 May 2015 23:09:16 -0400
From:	Jason Baron <jbaron@...mai.com>
To:	open list <linux-kernel@...r.kernel.org>
Subject: softirq under reporting with CONFIG_NO_HZ_* and under syn flood ?

Hi,

Creating a synflood via: 'hping3 -V -S  -M 0 -p 80 --flood -y <ip addr>', when
I have:

CONFIG_NO_HZ_IDLE=y or
CONFIG_NO_HZ_FULL=y set, and

either:

CONFIG_VIRT_CPU_ACCOUNTING_GEN=y or
CONFIG_TICK_CPU_ACCOUNTING=y

shows almost 0% cpu usage via top (or /proc/stat), whereas if I have turbostat
running there is substantial cpu usage....

I'm just wondering if anybody else has seen/can reproduce this, as
it seems quite strange.

The under-reporting here also seems specific to this workload- Other network
loads that I generate seem to properly account cpu time.

If I switch to CONFIG_HZ_PERIODIC=y or set
CONFIG_IRQ_TIME_ACCOUNTING=y, I again see substantial softirq usage
as would be expected here.

I'm attaching one complete .config that exhibits the issue.

Thanks,

-Jason

View attachment "config" of type "text/plain" (96024 bytes)