Re: [RFC PATCH] cpumask: Implement "random" version of cpumask_any_but()

[I Hsin Cheng]

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