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

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Message-ID: <Y4a2MBVEYEY+alO8@yury-laptop>
Date:   Tue, 29 Nov 2022 17:47:28 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     Tariq Toukan <ttoukan.linux@...il.com>
Cc:     Valentin Schneider <vschneid@...hat.com>,
        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>,
        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 Mon, Nov 28, 2022 at 08:39:24AM +0200, Tariq Toukan wrote:
> 
> 
> On 11/17/2022 2:23 PM, Valentin Schneider wrote:
> > 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.
> > 
> 
> Good.
> Is a respin needed, to add the comment mentioned above?

If you think it's worth the effort.