linux-kernel - Re: [PATCH v4 0/2] Improving topology_span

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Message-ID: <5291cd17-cdf5-4d66-8109-be27a4d2048a@linux.ibm.com>
Date: Fri, 7 Mar 2025 15:36:13 +0530
From: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
To: Steve Wahl <steve.wahl@....com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>,
        linux-kernel@...r.kernel.org, Naman Jain <namjain@...ux.microsoft.com>,
        Saurabh Singh Sengar <ssengar@...ux.microsoft.com>,
        srivatsa@...il.mit.edu, Michael Kelley <mhklinux@...look.com>,
        Russ Anderson <rja@....com>, Dimitri Sivanich <sivanich@....com>,
        K Prateek Nayak
 <kprateek.nayak@....com>,
        Vishal Chourasia <vishalc@...ux.ibm.com>, samir <samir@...ux.ibm.com>,
        Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
Subject: Re: [PATCH v4 0/2] Improving topology_span_sane

Hi Steve,

On 04/03/25 21:38, Steve Wahl wrote:
> toplogy_span_sane() has an O(N^2) algorithm that takes an inordinate
> amount of time on systems with a large number of cpus.
> 
> The first patch in this series replaces the algorithm used with a O(N)
> method that should exactly duplicate the previous code's results.
> 
> The second patch simplifies the first, taking a similar amount of time
> to run, but potentially has different results than previous code under
> situations believed to not truly exist, like a CPU not being included
> in its own span.

I have reviewed the proposed approach for the topology sanity check and
it looks good to me.

I have also tested the patch on a Power10 system with 12 cores (96 CPUs).
The average CPU hotplug latency decreased by around 10%.

Therefore,

Reviewed-by: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
Tested-by: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>

Thanks,
Madadi Vineeth Reddy

> 
> Version 1:
>   * Original patch
> 
> Version 2:
> 
>   * Adopted simplifications from K Prateek Nayak,and fixed use of
>     num_possible_cpus().
> 
> Version 3:
> 
>   * Undid the simplifications from version 2 when noticed that results
>     could differ from original code; kept num_possible_cpus() fix.
> 
> Version 4:
> 
>   * Turned the patch into a series of 2, the second re-introduces the
>     simplifications, and includes further simplification suggested by
>     Valentin Schneider in the discussion for Version 2.
> 
> Steve Wahl (2):
>   sched/topology: improve topology_span_sane speed
>   sched/topology: Refinement to topology_span_sane speedup
> 
>  kernel/sched/topology.c | 73 +++++++++++++++++++++++++++--------------
>  1 file changed, 48 insertions(+), 25 deletions(-)
>