linux-kernel - Re: [PATCH 1/2] sched/isolation: Merge individual nohz_full features into a common housekeeping flag

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Date:   Tue, 7 Feb 2023 12:49:41 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Michal Hocko <mhocko@...e.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Leonardo <leobras@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, cgroups@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 1/2] sched/isolation: Merge individual nohz_full features
 into a common housekeeping flag

On Mon, Feb 06, 2023 at 04:51:09PM +0100, Michal Koutný wrote:
> Hello Frederic.
> 
> On Sat, Feb 04, 2023 at 12:24:08AM +0100, Frederic Weisbecker <frederic@...nel.org> wrote:
> > The individual isolation features turned on by nohz_full were initially
> > split in order for each of them to be tunable through cpusets. However
> > plans have changed in favour of an interface (be it cpusets or sysctl)
> > grouping all these features to be turned on/off altogether.
> > Then should the need ever arise, the interface can still be expanded
> > to handle the individual isolation features.
> > 
> > Therefore the current isolation split between tick/timer/workqueue/rcu/
> > kthreads/misc doesn't make sense anymore.
> 
> Why it doesn't make sense? I think it's a useful annotation of
> respective operations wrt CPU isolation.

But what do we need these annotations for? The only outcome I've ever
seen with these is that it confuses everyone.

> 
> The grouping you did into HK_TYPE_KERNEL_NOISE (or even coarser) should
> IMO be done at the place where it'll be exposed into the favored
> interface (like it's with nohz_full=).

That being said I should reserve the grouping to HK_TYPE_KERNEL_NOISE when
I'll introduce the cpuset interface. This way I can add the support for
each part smoothly. For example first patch moves HK_TYPE_TIMER to
HK_TYPE_KERNEL_NOISE and unbound timers are supported by cpuset.kernel_noise,
second patch moves HK_TYPE_WQ to HK_TYPE_KERNEL_NOISE and unbound workqueues
are supported by cpuset.kernel_noise, etc until all of them turned by nohz_full=
are supported... This is what I'm doing in fact but I'm so slow to write this patchset...

Thanks.