linux-kernel - Re: [RFC PATCH] cpumask: Implement "random" version of cpumask_any

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <Z4TqjoBAOXZTKYcs@vaxr-BM6660-BM6360>
Date: Mon, 13 Jan 2025 18:27:26 +0800
From: I Hsin Cheng <richard120310@...il.com>
To: Kuan-Wei Chiu <visitorckw@...il.com>
Cc: yury.norov@...il.com, linux@...musvillemoes.dk, jserv@...s.ncku.edu.tw,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] cpumask: Implement "random" version of
 cpumask_any_but()

On Mon, Jan 13, 2025 at 06:13:07PM +0800, Kuan-Wei Chiu wrote:
> Hi I Hsin,
> 
> On Mon, Jan 13, 2025 at 02:18:39PM +0800, I Hsin Cheng wrote:
> > Original implementation of "cpumask_any_but()" isn't actually random as
> > the comment claims itself to be. It's behavior is in fact to select the
> > first cpu in "mask" which isn't equal to "cpu".
> > 
> > Re-implement the function so we can choose a random cpu by randomly
> > select the value of "n" and choose the nth cpu in "mask"
> > 
> This patch may slow down the efficiency of cpumask_any_but(). Are there
> any in-tree users of cpumask_any_but() that require it to return a
> truly random id, or benefit from such behavior?

Hi Kuan-Wei,

Thanks for your review ! Yes indeed it may slow down the efficiency
abit.

However there are some use cases I think randomness is important
such as "dl_task_offline_migration()" under kernel/sched/deadline.c ,
where the operation of cpu picking shouldn't favor certain cpu too much.

Also "select_task_rq()" utitlize "cpumask_any()" to pick cpu, it doesn't
need to be perfectly random, but neither should it only favor certain
cpu.

What do you think?

Best regards,
I Hsin