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, 04 Jun 2012 16:41:35 +0200
From:	Mike Galbraith <efault@....de>
To:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
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

On Mon, 2012-06-04 at 20:08 +0530, Srivatsa Vaddagiri wrote: 
> * Mike Galbraith <efault@....de> [2012-06-04 16:30:34]:
> 
> > Yeah, this is true, it is a latency source and a fairness violation.
> > Slow path balance consideration does make some sense to me.
> > 
> > But, if you have an RT requirement, you can't afford to mix unknown
> > entities, nor over-commit etc.  A realtime application will assign all
> > resources, so the load balancer becomes essentially unemployed.  No?
> > Non critical worker-bees may be allowed to bounce around in say a
> > cpuset, but none of the CPUs which do critical work will ever be
> > over-committed, else application just lost the war.  In that regard,
> > twiddling the load balancer to accommodate strange sounding case still
> > seems wrong to me.
> 
> Btw the patch should help non-rt case as well (where a high
> priority SCHED_OTHER is hogging cpu while low-priority SCHED_OTHER task
> on that same cpu suffers as we choose not to move it to another
> cpu (because of the way balance_cpu based load balance is written).

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

-Mike

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