Received-SPF: pass (google.com: domain of linux-crypto-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   Valentin Schneider <vschneid@redhat.com>
To:     Yury Norov <yury.norov@gmail.com>
Cc:     linux-kernel@vger.kernel.org,
        "David S. Miller" <davem@davemloft.net>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Barry Song <baohua@kernel.org>,
        Ben Segall <bsegall@google.com>,
        haniel Bristot de Oliveira <bristot@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Gal Pressman <gal@nvidia.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Heiko Carstens <hca@linux.ibm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Jesse Brandeburg <jesse.brandeburg@intel.com>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Leon Romanovsky <leonro@nvidia.com>,
        Mel Gorman <mgorman@suse.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Saeed Mahameed <saeedm@nvidia.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Tariq Toukan <tariqt@nvidia.com>,
        Tariq Toukan <ttoukan.linux@gmail.com>,
        Tony Luck <tony.luck@intel.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        linux-crypto@vger.kernel.org, netdev@vger.kernel.org,
        linux-rdma@vger.kernel.org
Subject: Re: [PATCH v2 0/4] cpumask: improve on cpumask_local_spread() locality
In-Reply-To: <Y3PXw8Hqn+RCMg2J@yury-laptop>
References: <20221112190946.728270-1-yury.norov@gmail.com>
 <xhsmh7czwyvtj.mognet@vschneid.remote.csb> <Y3PXw8Hqn+RCMg2J@yury-laptop>
Date:   Thu, 17 Nov 2022 12:23:45 +0000
Message-ID: <xhsmho7t5ydke.mognet@vschneid.remote.csb>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

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