linux-kernel - Re: [PATCH v3 3/3] sched/rt: Add support for SD_PREFER_SIBLING on find_lowest

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Date:   Thu, 23 Mar 2017 07:08:24 -0700
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Byungchul Park <byungchul.park@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>, <mingo@...nel.org>,
        <linux-kernel@...r.kernel.org>, <juri.lelli@...il.com>,
        <kernel-team@....com>
Subject: Re: [PATCH v3 3/3] sched/rt: Add support for SD_PREFER_SIBLING on
 find_lowest_rq()

On Thu, 23 Mar 2017 19:36:51 +0900
Byungchul Park <byungchul.park@....com> wrote:

> On Thu, Mar 23, 2017 at 10:44:45AM +0100, Peter Zijlstra wrote:
> > On Thu, Mar 23, 2017 at 11:12:49AM +0900, Byungchul Park wrote:  
> > > It would be better to avoid pushing tasks to other cpu within
> > > a SD_PREFER_SIBLING domain, instead, get more chances to check
> > > other siblings.  
> > 
> > Did you forget to post the rt equivalent to patch 1?  
> 
> No. Fortunately, rt currently works as patch 1 does.

I'm thinking that the rt and deadline search for lowest rq functions
should be merged as one.

What they are doing is looking for the rq with the lowest priority.
deadline currently doesn't care if it picks an rq with an rt task
running on it, even when there's an rq with no rt tasks that the dl task
can migrate to. The same goes with rt. It could place an RT task on an
rq running a deadline task without knowing the rt task wont be able to
run on that cpu immediately.

-- Steve