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

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Date:	Tue, 26 Aug 2008 07:36:57 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
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

Nick Piggin wrote:
> 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.
>   

I had a feeling you may object to this patch based on your comments on
the first one.  Thats why I CC'd you so you wouldnt think I was trying
to sneak something past ;)

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

The problem I was seeing was that even moving 2 was too many in the
ftraces traces I looked at.  I think the idea of making a variable limit
(set via a sysctl, etc) here is a good one, but I would recommend we
have the default be "1" for CONFIG_PREEMPT (or at least
CONFIG_PREEMPT_RT) based on what I know right now.   I know last time
you objected to any kind of special cases for the preemptible kernels,
but I think this is a good compromise.  Would this be acceptable?

Thanks Nick,

-Greg

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