Re: [PATCH v2 3/4] sched/isolation: Add HK_TYPE_WQ to isolcpus=domain

[Frederic Weisbecker <frederic@kernel.org>]

On Fri, Oct 14, 2022 at 01:27:25PM -0300, Leonardo Brás wrote:
> Hello Frederic,
> 
> So, IIUC you are removing all flags composing nohz_full= parameter in favor of a
> unified NOHZ_FULL flag. 
> 
> I am very new to the code, and I am probably missing the whole picture, but I
> actually think it's a good approach to keep them split for a couple reasons:
> 1 - They are easier to understand in code (IMHO): 
> "This cpu should not do this, because it's not able to do WQ housekeeping" looks
> better than "because it's not in DOMAIN or NOHZ_FULL housekeeping"

A comment above each site may solve that.

> 
> 2 - They are simpler for using: 
> Suppose we have this function that should run at a WQ, but we want to keep them
> out of the isolated cpus. If we have the unified flags, we need to combine both
> DOMAIN and NOHZ_FULL bitmasks, and then combine it again with something like
> cpu_online_mask. It usually means allocating a new cpumask_t, and also freeing
> it afterwards.
> If we have a single WQ flag, we can avoid the allocation altogether by using
> for_each_cpu_and(), making the code much simpler.

I guess having a specific function for workqueues would arrange for it.

> 
> 3 - It makes easier to compose new isolation modes:
> In case the future requires a new isolation mode that also uses the types of
> isolation we currently have implemented, it would be much easier to just compose
> it with the current HK flags, instead of having to go through all usages and do
> a cpumask_and() there. Also, new isolation modes would make (2) worse.

Actually having a new feature merged in HK_NOHZ_FULL would make it easier to
handle as it avoids spreading cpumasks. I'm not sure I understand what you
mean.

Thanks.