linux-kernel - [ISSUE] sched/cgroup: Does cpu-cgroup still works fine nowadays?

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Message-ID: <537192D3.5030907@linux.vnet.ibm.com>
Date:	Tue, 13 May 2014 11:34:43 +0800
From:	Michael wang <wangyun@...ux.vnet.ibm.com>
To:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <efault@....de>, Alex Shi <alex.shi@...aro.org>,
	Paul Turner <pjt@...gle.com>, Rik van Riel <riel@...hat.com>,
	Mel Gorman <mgorman@...e.de>, Paul Turner <pjt@...gle.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: [ISSUE] sched/cgroup: Does cpu-cgroup still works fine nowadays?

During our testing, we found that the cpu.shares doesn't work as
expected, the testing is:

X86 HOST:
	12 CPU
GUEST(KVM):
	6 VCPU

We create 3 GUEST, each with 1024 shares, the workload inside them is:

GUEST_1:
	dbench 6
GUEST_2:
	stress -c 6
GUEST_3:
	stress -c 6

So by theory, each GUEST will got (1024 / (3 * 1024)) * 1200% == 400%
according to the group share (3 groups are created by virtual manager on
same level, and they are the only groups heavily running in system).

Now if only GUEST_1 running, it got 300% CPU, which is 1/4 of the whole
CPU resource.

So when all 3 GUEST running concurrently, we expect:

		GUEST_1		GUEST_2		GUEST_3
CPU%		300%		450%		450%

That is the GUEST_1 got the 300% it required, and the unused 100% was
shared by the rest group.

But the result is:

		GUEST_1		GUEST_2		GUEST_3
CPU%		40%		580%		580%

GUEST_1 failed to gain the CPU it required, and the dbench inside it
dropped a lot on performance.

So is this results expected (I really do not think so...)?

Or that imply the cpu-cgroup got some issue to be fixed?

Any comments are welcomed :)

Regards,
Michael Wang

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