linux-kernel - Re: [PATCH] arm64: defconfig: enable fine-grained task level IRQ time accounting

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Date:   Wed, 2 Aug 2017 15:33:33 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Gregory CLEMENT <gregory.clement@...e-electrons.com>
Cc:     will.deacon@....com, Arnd Bergmann <arnd@...db.de>,
        Olof Johansson <olof@...om.net>,
        thomas.petazzoni@...e-electrons.com, andrew@...n.ch,
        jaz@...ihalf.com, linux-kernel@...r.kernel.org, nadavh@...vell.com,
        neta@...vell.com, tn@...ihalf.com, Marcin Wojtas <mw@...ihalf.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm64: defconfig: enable fine-grained task level IRQ
 time accounting

On Wed, Aug 02, 2017 at 03:11:43PM +0200, Gregory CLEMENT wrote:
>  On lun., juil. 31 2017, Marcin Wojtas <mw@...ihalf.com> wrote:
> > Tests showed, that under certain conditions, the summary number of jiffies
> > spent on softirq/idle, which are counted by system statistics can be even
> > below 10% of expected value, resulting in false load presentation.
> >
> > The issue was observed on the quad-core Marvell Armada 8k SoC, whose two
> > 10G ports were bound into L2 bridge. Load was controlled by bidirectional
> > UDP traffic, produced by a packet generator. Under such condition,
> > the dominant load is softirq. With 100% single CPU occupation or without
> > any activity (all CPUs 100% idle), total number of jiffies is 10000 (2500
> > per each core) in 10s interval. Also with other kind of load this was
> > true.
> >
> > However below a saturation threshold it was observed, that with CPU which
> > was occupied almost by softirqs only, the statistic were awkward. See
> > the mpstat output:
> >
> > CPU %usr %nice %sys %iowait %irq %soft %steal %guest %gnice %idle
> > all 0.00  0.00 0.13    0.00 0.00  0.55   0.00   0.00   0.00 99.32
> >   0 0.00  0.00 0.00    0.00 0.00 23.08   0.00   0.00   0.00 76.92
> >   1 0.00  0.00 0.40    0.00 0.00  0.00   0.00   0.00   0.00 99.60
> >   2 0.00  0.00 0.00    0.00 0.00  0.00   0.00   0.00   0.00 100.00
> >   3 0.00  0.00 0.00    0.00 0.00  0.00   0.00   0.00   0.00 100.00
> >
> > Above would mean basically no total load, debug CPU0 occupied in 25%.
> > Raw statistics, printed every 10s from /proc/stat unveiled a root
> > cause - summary idle/softirq jiffies on loaded CPU were below 200,
> > i.e. over 90% samples lost. All problems were gone after enabling
> > fine granulity IRQ time accounting.
> >
> > This patch fixes possible wrong statistics processing by enabling
> > CONFIG_IRQ_TIME_ACCOUNTING for arm64 platfroms, which is by
> > default done on other architectures, e.g. x86 and arm. Tests
> > showed no noticeable performance penalty, nor stability impact.
> 
> Who should take this patch?
> 
> I think that all the defconfig under arm64 are merged through the
> arm-soc subsystem, but this one is not really specific to a
> SoC. However, as it was experimented on an mvebu SoC, if you agree I
> can take it.

It's fine by me to go via arm-soc.

-- 
Catalin