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

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Date:   Wed, 22 Mar 2017 13:37:28 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Byungchul Park <byungchul.park@....com>
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 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.