linux-kernel - Re: [PATCH 2/5] sched: pull only one task during NEWIDLE balancing to limit critical section

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Message-Id: <200808261621.33810.nickpiggin@yahoo.com.au>
Date:	Tue, 26 Aug 2008 16:21:33 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	mingo@...e.hu, srostedt@...hat.com, peterz@...radead.org,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
	npiggin@...e.de, gregory.haskins@...il.com
Subject: Re: [PATCH 2/5] sched: pull only one task during NEWIDLE balancing to limit critical section

On Tuesday 26 August 2008 06:15, Gregory Haskins wrote:
> git-id c4acb2c0669c5c5c9b28e9d02a34b5c67edf7092 attempted to limit
> newidle critical section length by stopping after at least one task
> was moved.  Further investigation has shown that there are other
> paths nested further inside the algorithm which still remain that allow
> long latencies to occur with newidle balancing.  This patch applies
> the same technique inside balance_tasks() to limit the duration of
> this optional balancing operation.
>
> Signed-off-by: Gregory Haskins <ghaskins@...ell.com>
> CC: Nick Piggin <npiggin@...e.de>

Hmm, this (andc4acb2c0669c5c5c9b28e9d02a34b5c67edf7092) still could
increase the amount of work to do significantly for workloads where
the CPU is going idle and pulling tasks over frequently. I don't
really like either of them too much.

Maybe increasing the limit would effectively amortize most of the
problem (say, limit to move 16 tasks at most).
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