linux-kernel - Re: [PATCH v2] sched/deadline: Make find_later

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Message-ID: <20170322232145.GO11100@X58A-UD3R>
Date:   Thu, 23 Mar 2017 08:21:46 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org,
        juri.lelli@...il.com, rostedt@...dmis.org, kernel-team@....com
Subject: Re: [PATCH v2] sched/deadline: Make find_later_rq() choose a closer
 cpu in topology

On Wed, Mar 22, 2017 at 01:37:28PM +0100, Peter Zijlstra wrote:
> On Tue, Mar 21, 2017 at 04:52:24PM +0900, Byungchul Park wrote:
> > When cpudl_find() returns any among free_cpus, the cpu might not be
> > closer than others, considering sched domain. For example:
> > 
> >    this_cpu: 15
> >    free_cpus: 0, 1,..., 14 (== later_mask)
> >    best_cpu: 0
> > 
> >    topology:
> > 
> >    0 --+
> >        +--+
> >    1 --+  |
> >           +-- ... --+
> >    2 --+  |         |
> >        +--+         |
> >    3 --+            |
> > 
> >    ...             ...
> > 
> >    12 --+           |
> >         +--+        |
> >    13 --+  |        |
> >            +-- ... -+
> >    14 --+  |
> >         +--+
> >    15 --+
> > 
> > In this case, it would be best to select 14 since it's a free cpu and
> > closest to 15(this_cpu). However, currently the code select 0(best_cpu)
> > even though that's just any among free_cpus. Fix it.
> 
> This would result in picking the HT sibling, if available. Which is
> typically the worst possible pick.
> 
> If you add support for SD_PREFER_SIBLING, which denotes a preference for
> any other sibling domain above this one, this might work.

Sure. I will add that support as well. Thank you.