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

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Date:   Fri, 24 Mar 2017 07:45:59 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Steven Rostedt <rostedt@...dmis.org>
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, Mar 23, 2017 at 07:08:24AM -0700, Steven Rostedt wrote:
> 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.

I also think so. And IMHO there are more things to care wrt rt/dl
migration. As you said, the cases should be considered and fixed. I
wonder if the rt and dl seartch should be merged as one though..

Thank you,
Byungchul