Re: [PATCH v2 0/4] cpumask: improve on cpumask_local_spread() locality

From: Tariq Toukan <ttoukan.linux@gmail.com>
To: Valentin Schneider <vschneid@redhat.com>,
	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>, 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
Date: Mon, 28 Nov 2022 08:39:24 +0200	[thread overview]
Message-ID: <665b6081-be55-de9a-1f7f-70a143df329d@gmail.com> (raw)
In-Reply-To: <xhsmho7t5ydke.mognet@vschneid.remote.csb>

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?