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Message-Id: <1501532531-4499-1-git-send-email-mw@semihalf.com>
Date: Mon, 31 Jul 2017 22:22:11 +0200
From: Marcin Wojtas <mw@...ihalf.com>
To: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Cc: catalin.marinas@....com, will.deacon@....com, andrew@...n.ch,
thomas.petazzoni@...e-electrons.com,
gregory.clement@...e-electrons.com, nadavh@...vell.com,
neta@...vell.com, mw@...ihalf.com, jaz@...ihalf.com,
tn@...ihalf.com
Subject: [PATCH] arm64: defconfig: enable fine-grained task level IRQ time accounting
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.
Signed-off-by: Marcin Wojtas <mw@...ihalf.com>
---
arch/arm64/configs/defconfig | 1 +
1 file changed, 1 insertion(+)
diff --git a/arch/arm64/configs/defconfig b/arch/arm64/configs/defconfig
index 44423e6..ed51ac6 100644
--- a/arch/arm64/configs/defconfig
+++ b/arch/arm64/configs/defconfig
@@ -3,6 +3,7 @@ CONFIG_POSIX_MQUEUE=y
CONFIG_AUDIT=y
CONFIG_NO_HZ_IDLE=y
CONFIG_HIGH_RES_TIMERS=y
+CONFIG_IRQ_TIME_ACCOUNTING=y
CONFIG_BSD_PROCESS_ACCT=y
CONFIG_BSD_PROCESS_ACCT_V3=y
CONFIG_TASKSTATS=y
--
1.8.3.1
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