linux-kernel - Re: [PATCH v6 0/2] Make find_later_rq() choose a closer cpu in topology

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Date:   Fri, 18 Aug 2017 10:25:55 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     peterz@...radead.org, mingo@...nel.org
Cc:     linux-kernel@...r.kernel.org, juri.lelli@...il.com,
        rostedt@...dmis.org, kernel-team@....com
Subject: Re: [PATCH v6 0/2]  Make find_later_rq() choose a closer cpu in
 topology

On Mon, Aug 07, 2017 at 12:50:32PM +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.

Could you let me know your opinions about this?

> Change from v5
>    -. exclude two patches already picked up by peterz
>       (sched/deadline: Make find_later_rq() choose a closer cpu in topology)
>       (sched/deadline: Change return value of cpudl_find())
>    -. apply what peterz fixed for 'prefer sibling', into deadline and rt
> 
> Change from v4
>    -. remove a patch that might cause huge lock contention
>       (by spin lock(&cpudl.lock) in a hot path of scheduler)
> 
> Change from v3
>    -. rename closest_cpu to best_cpu so that it align with rt
>    -. protect referring cpudl.elements with cpudl.lock
>    -. change return value of cpudl_find() to bool
> 
> Change from v2
>    -. add support for SD_PREFER_SIBLING
> 
> Change from v1
>    -. clean up the patch
> 
> Byungchul Park (2):
>   sched/deadline: Add support for SD_PREFER_SIBLING on find_later_rq()
>   sched/rt: Add support for SD_PREFER_SIBLING on find_lowest_rq()
> 
>  kernel/sched/deadline.c | 46 +++++++++++++++++++++++++++++++++++++++++++---
>  kernel/sched/rt.c       | 47 ++++++++++++++++++++++++++++++++++++++++++++---
>  2 files changed, 87 insertions(+), 6 deletions(-)
> 
> -- 
> 1.9.1