linux-kernel - Re: [PATCH] sched: balance_cpu to consider other cpus in its group as target of (pinned) task migration

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Date:	Mon, 4 Jun 2012 20:30:40 +0530
From:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
To:	Mike Galbraith <efault@....de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Prashanth Nageshappa <prashanth@...ux.vnet.ibm.com>,
	mingo@...nel.org, LKML <linux-kernel@...r.kernel.org>,
	roland@...nel.org, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] sched: balance_cpu to consider other cpus in its group
 as target of (pinned) task migration

* Mike Galbraith <efault@....de> [2012-06-04 16:41:35]:

> But high priority SCHED_OTHER tasks do not hog the CPU, they get their
> fair share as defined by the user.

Consider this case. System with 2 cores (each with 2 thread) and 3
cgroups :

	A (1024) -> has 2 tasks (A0, A1)
	B (2048) -> has 2 tasks (B0, B1)
	C (1024) -> has 1 tasks (C0 - pinned to CPUs 1,2)

(B0, B1) collectively are eligible to consume 2 full cpus worth of
bandwidth, (A0, A1) together are eligible to consume 1 full-cpu
worth of bandwidth and finally C0 is eligible to get 1 full-cpu worth of
bandwidth. 

Currently C0 is sleeping as a result of which tasks could be spread as:

	CPU0 -> A0
	CPU1 -> A1

	CPU2 -> B0
	CPU3 -> B1

Now C0 wakes up and lands on CPU2 (which was its prev_cpu).

	CPU0 -> A0
	CPU1 -> A1

	CPU2 -> B0, C0
	CPU3 -> B1

Ideally CPU1 needs to pull it C0 to itself (while A1 moves to CPU0). Do
you agree to that? I doubt that happens because of how CPU0 does load
balance on behalf of itself and CPU1 (and thus fails to pull C0 to its
core).

- vatsa

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