netdev - Re: [PATCH v2 0/4] cpumask: improve on cpumask_local

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Date:   Thu, 17 Nov 2022 12:23:45 +0000
From:   Valentin Schneider <vschneid@...hat.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Barry Song <baohua@...nel.org>,
        Ben Segall <bsegall@...gle.com>,
        haniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Gal Pressman <gal@...dia.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Ingo Molnar <mingo@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Mel Gorman <mgorman@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Saeed Mahameed <saeedm@...dia.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tariq Toukan <tariqt@...dia.com>,
        Tariq Toukan <ttoukan.linux@...il.com>,
        Tony Luck <tony.luck@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-crypto@...r.kernel.org, netdev@...r.kernel.org,
        linux-rdma@...r.kernel.org
Subject: Re: [PATCH v2 0/4] cpumask: improve on cpumask_local_spread() locality

On 15/11/22 10:32, Yury Norov wrote:
> On Tue, Nov 15, 2022 at 05:24:56PM +0000, Valentin Schneider wrote:
>>
>> Is this meant as a replacement for [1]?
>
> No. Your series adds an iterator, and in my experience the code that
> uses iterators of that sort is almost always better and easier to
> understand than cpumask_nth() or cpumask_next()-like users.
>
> My series has the only advantage that it allows keep existing codebase
> untouched.
>

Right

>> I like that this is changing an existing interface so that all current
>> users directly benefit from the change. Now, about half of the users of
>> cpumask_local_spread() use it in a loop with incremental @i parameter,
>> which makes the repeated bsearch a bit of a shame, but then I'm tempted to
>> say the first point makes it worth it.
>>
>> [1]: https://lore.kernel.org/all/20221028164959.1367250-1-vschneid@redhat.com/
>
> In terms of very common case of sequential invocation of local_spread()
> for cpus from 0 to nr_cpu_ids, the complexity of my approach is n * log n,
> and your approach is amortized O(n), which is better. Not a big deal _now_,
> as you mentioned in the other email. But we never know how things will
> evolve, right?
>
> So, I would take both and maybe in comment to cpumask_local_spread()
> mention that there's a better alternative for those who call the
> function for all CPUs incrementally.
>

Ack, sounds good.

> Thanks,
> Yury