lkml.org 
[lkml]   [2020]   [Aug]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH 2/2] powerpc/topology: Override cpu_smt_mask
Date

On 04/08/20 11:46, peterz@infradead.org wrote:
> On Tue, Aug 04, 2020 at 09:03:07AM +0530, Srikar Dronamraju wrote:
>> On Power9 a pair of cores can be presented by the firmware as a big-core
>> for backward compatibility reasons, with 4 threads per (small) core and 8
>> threads per big-core. cpu_smt_mask() should generally point to the cpu mask
>> of the (small)core.
>>
>> In order to maintain userspace backward compatibility (with Power8 chips in
>> case of Power9) in enterprise Linux systems, the topology_sibling_cpumask
>> has to be set to big-core. Hence override the default cpu_smt_mask() to be
>> powerpc specific allowing for better scheduling behaviour on Power.
>
> Why does Linux userspace care about this?

Ditto; from [1], a core contains CPUs that all share the same L1 (and capacity,
as per SD_SHARE_CPUCAPACITY). So IMO it makes perfect sense to have a first
domain spanning L1, and its parent spanning L2 - that means
topology_sibling_cpumask *itself* should span a single core rather than a
pair.

[1]: https://lkml.kernel.org/r/jhjr1sviswg.mognet@arm.com

\
 
 \ /
  Last update: 2020-08-04 13:04    [W:0.039 / U:0.420 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site